Types, Values, and Variables

Links

Explicit Conversions

The simplest way to perform an explicit type conversion is to use the Boolean(), Number(), and String() functions.

Any value other than null or undefined has a toString() method.

Conversion Idioms

Destructuring Assignment

Destructuring assignment makes it easy to work with functions that return arrays of values:

function toPolar(x, y) {
  return [Math.sqrt(x*x+y*y), Math.atan2(y,x)];
}

Variable destructuring in loops:

for(const [name, value] of Object.entries(o)) {
  console.log(name, value); // Prints "x 1" and "y 2"
}

Note: The Object.entries() method returns an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. (The only important difference is that a for...in loop enumerates properties in the prototype chain as well).

The list of variables on the left can include extra commas to skip certain values on the right

Note: the last comma does not stand for a value.

To collect all unused or remaining values into a single variable when destructuring an array, use three dots (...) before the last variable name on the left-hand side

Destructuring assignment can also be performed when the righthand side is an object value.

Expressions and Operators

In JavaScript, the values null and undefined are the only two values that do not have properties. In a regular property access expression using . or [], you get a TypeError if the expression on the left evaluates to null or undefined. You can use ?. and ?.[] syntax to guard against errors of this  type.

You can also invoke a function using ?.() instead of ().

With the new ?.() invocation syntax, if the expression to the left of the ?. evaluates to null or undefined, then the entire invocation expression evaluates to undefined and no exception is thrown.

Write the function invocation using ?.(), knowing that invocation will only happen if there is actually a value to be invoked

function square(x, log) { 
  log?.(x); // Call the function if there is one
  return x * x;
}

Note that expression x++ is not always the same as x = x + 1.The ++ operator never performs string concatenation: it always converts its operand to a number and increments it. If x is the string “1”, ++x is the number 2, but x + 1 is the string “11”.

JavaScript objects are compared by reference, not by value. An object is equal to itself, but not to any other object. If two distinct objects have the same number of properties, with the same names and values, they are still not equal. Similarly, two arrays that have the same elements in the same order are not equal to each other.

NaN value is never equal to any other value, including itself! To check whether a value x is NaN, use x !== , or the global isNaN() function.

If both values refer to the same object, array, or function, they are equal. If they refer to different objects, they are not equal, even if both objects have identical properties.

Evaluating Expressions

JavaScript has the ability to interpret strings of JavaScript source code, evaluating them to produce a value.

Because of security issues, some web servers use the HTTP “Content-Security-Policy” header to disable eval() for an entire website.

First-Defined (??)

The first-defined operator ?? evaluates to its first defined operand: if its left operand is not null and not undefined, it returns that value.

a ?? b is equivalent to (a !== null && a !== undefined) ? a : b

?? is a useful alternative to ||.  The problem with this idiomatic use is that zero, the empty string, and false are all falsy values that may be perfectly valid in some circumstances. In this code example, if maxWidth is zero, that value will be ignored. But if we change the || operator to ??, we end up with an expression where zero is a valid value.

delete Operator

Deleting an array element leaves a “hole” in the array and does not change the array’s length. The resulting array is sparse.

void Operator

Using the void operator makes sense only if the operand has side effects.

Statements

Expressions are evaluated to produce a value, but statements are executed to make something happen.

Expressions with side effects, such as assignments and function invocations, can stand alone as statements, and when used this way are known as expression statements.

A similar category of statements are the declaration statements that declare new variables and define new functions.

If a function does not have any side effects, there is no sense in calling it, unless it is part of a larger expression or an assignment statement.

for/of

The for/of loop works with iterable objects. Arrays, strings, sets, and maps are iterable.

let data = [1, 2, 3, 4, 5, 6, 7, 8, 9], sum = 0;
for(let element of data) {
  sum += element;
}

let text = "Na na na na na na na na";
let wordSet = new Set(text.split(" "));
let unique = [];
for(let word of wordSet) {
  unique.push(word);
}

let frequency = {};
for(let letter of "mississippi") {
  if (frequency[letter]) {
    frequency[letter]++;
  } 
  else {
    frequency[letter] = 1;
  }
}

let m = new Map([[1, "one"]]);
for(let [key, value] of m) {
  key // => 1
  value // => "one"
}

Objects are not (by default) iterable. Attempting to use for/of on a regular object throws a TypeError at runtime.

If you want to iterate through the properties of an object, you can use the for/in loop.

Note: for/of can be used on objects with Object.entries property, but it will not pick properties from object’s prototype.

for/in

for/in loop works with any object after the in.

for(let p in o) { 
  console.log(o[p]); 
}

for(let i in a) console.log(i);

The for/in loop does not actually enumerate all properties of an object. It does not enumerate properties whose names are symbols. And of the properties whose names are strings, it only loops over the enumerableproperties.

with

The with statement runs a block of code as if the properties of a specified object were variables in scope for that code.

The with statement is forbidden in strict mode  and should be considered deprecated in non-strict mode: avoid using it whenever possible.

document.forms[0].address.value

with(document.forms[0]) {
  name.value = "";
  address.value = "";
  email.value = "";
}

debugger

If a debugger program is available and is running, then an implementation may (but is not required to) perform some kind of debugging action.

In practice, this statement acts like a breakpoint: execution of JavaScript code stops, and you can use the debugger to print variables’ values, examine the call stack, and so on.

Note that it is not enough to have a debugger available: the debugger statement won’t start the debugger for you. If you’re using a web browser and have the developer tools console open, however, this statement will cause a breakpoint.

use strict

Strict mode is a restricted subset of the language that fixes important language deficiencies and provides stronger error checking and increased security.

The differences between strict mode and non-strict mode are the following:

• The with statement is not allowed in strict mode.

• In strict mode, all variables must be declared: a ReferenceError is thrown if you assign a value to an identifier that is not a declared variable, function, function parameter, catch clause parameter, or property of the global object.

• In non-strict mode, this implicitly declares a global variable by adding a new property to the global object.

• In strict mode, functions invoked as functions (rather than as methods) have a this value of undefined. (In non-strict mode, functions invoked as functions are always passed the global object as their this value.)

• A function is invoked with call() or apply() , the this value is exactly the value passed as the first argument to call() or apply(). (In non-strict mode, null and undefined values are replaced with the global object and non-object values are converted to objects.)

• In strict mode, assignments to non-writable properties and attempts to create new properties on non-extensible objects throw a TypeError. (In non-strict mode, these attempts fail silently.)

• In strict mode, code passed to eval() cannot declare variables or define functions in the caller’s scope as it can in non-strict mode. Instead, variable and function definitions live in a new scope created for the eval(). This scope is discarded when the eval() returns.

• In strict mode, the Arguments object  in a function holds a static copy of the values passed to the function. In non-strict mode, the Arguments object has “magical” behavior in which elements of the array and named function parameters both refer to the same value.

• In strict mode, a SyntaxError is thrown if the delete operator is followed by an unqualified identifier such as a variable, function, or function parameter. (In non-strict mode, such a delete expression does nothing and evaluates to false.)

• In strict mode, an attempt to delete a non-configurable property throws a TypeError. (In non-strict mode, the attempt fails and the delete expression evaluates to false.)

• In strict mode, it is a syntax error for an object literal to define two or more properties by the same name. (In non-strict mode, no error occurs.)

• ...

Objects

In addition to its name and value, each property has three property attributes:

• The writable attribute specifies whether the value of the property can be set.

• The enumerable attribute specifies whether the property name is returned by a for/in loop.

• The configurable attribute specifies whether the property can be deleted and whether its attributes can be altered.

Prototypes

All objects created by object literals have the same prototype object, Object.prototype.

Objects created using the new keyword and a constructor invocation use the value of the prototype property of the constructor function as their prototype.

Object created by new Object() inherits from Object.prototype, just as the object created by {} does. Similarly, the object created by new Array() uses Array.prototype as its prototype, and the object created by new Date() uses Date.prototype as its prototype.

Almost all objects have a prototype, but only a relatively small number of objects have a prototype property. It is these objects with prototype properties that define the prototypes for all the other objects.

Most built-in constructors (and most user-defined constructors) have a prototype that inherits from Object.prototype.

Date.prototype inherits properties from Object.prototype, so a Date object created by new Date() inherits properties from both Date.prototype and Object.prototype. This linked series of prototype objects is known as a prototype chain.

Creating Objects

Objects can be created with object literals, with the new keyword, and with the Object.create() function.

let book = {
  "main title": "JavaScript",
  "sub-title": "The Definitive Guide", 
  for: "all audiences", 
  author: {
    firstname: "David", .
    surname: "Flanagan"
  }
};

Use Object.create to guard against accidental modifications:

let o = { x: "don't change this value" };
library.function(Object.create(o));

Note: the library function can modify the passed in object, but not the original o object

Access Object Properties with an array ([]) notation

let addr = "";
for(let i = 0; i < 4; i++) {
  addr += customer[`address${i}`] + "\n";
}

Inheritance

How to query for property which may be undefined

Deleting properties

The delete operator only deletes own properties, not inherited ones. (To delete an inherited property, you must delete it from the prototype object in which it is defined. Doing this affects every object that inherits from that prototype.)

delete does not remove properties that have a configurable attribute of false.

Certain properties of built-in objects are non-configurable, as are properties of the global object created by variable declaration and function declaration.

Testing properties

To check whether an object has a property with a given name. You can do this with the in operator, with the hasOwnProperty() and propertyIsEnumerable() methods, or simply by querying the property

( != undefined).

in & query

Advantage of using in: in can distinguish between properties that do not exist and properties that exist but have been set to undefined.

hasOwnProperty

The propertyIsEnumerable() returns true only if the named property is an own property and its enumerable attribute is true.

Enumerating properties

To guard against enumerating inherited properties with for/in, you can add an explicit check inside the loop body:

for(let p in o) {
  if (!o.hasOwnProperty(p)) continue; 
}

for(let p in o) {
  if (typeof o[p] === "function") continue;
}

Functions you can use to get an array of property names

• Object.keys() returns an array of the names of the enumerable own properties of an object. It does not include non-enumerable properties, inherited properties, or properties whose name is a Symbol.

• Object.getOwnPropertyNames() works like Object.keys() but returns an array of the names of nonenumerable own properties as well.

• Object.getOwnPropertySymbols() returns own properties whose names are Symbols, whether or not they are enumerable.

• Reflect.ownKeys() returns all own property names, both enumerable and non-enumerable, and both string and Symbol.

Extending Objects

To copy the properties of one object to another object

let target = {x: 1}, source = {y: 2, z: 3};
for(let key of Object.keys(source)) {
  target[key] = source[key];
}

One reason to assign properties from one object into another is when you have an object that defines default values for many properties and you want to copy those default properties into another object if a property by that name does not already exist in that object. Using Object.assign() naively will not do what you want:

Object.assign(o, defaults);

Instead, use one of the following:,

Serializing Objects

The functions JSON.stringify() and JSON.parse() serialize and restore JavaScript objects.

Object methods

toString(), valueOf(), loLocaleString(), toJSON()

Extended Object Literal Syntax

Shorthand Properties

let x = 1, y = 2;
let o = {
  x: x,
  y: y
};

Computer Property Names

let o = {};
o[PROPERTY_NAME] = 1;
o[computePropertyName()] = 2;

let p = {
  [PROPERTY_NAME]: 1,
  [computePropertyName()]: 2
};

Symbols as Property Names

const extension = Symbol("my extension symbol");
let o = {
  [extension]: {}
};
o[extension].x = 0;

Two Symbols created with the same string argument are still different from one another.

The point of Symbols is not security, but to define a safe extension mechanism for JavaScript objects. If you get an object from third-party code that you do not control and need to add some of your own properties to that object but want to be sure that your properties will not conflict with any properties that may already exist on the object, you can safely use Symbols as your property names.

Spread Operator

You can copy the properties of an existing object into a new object using the “spread operator” ... inside an object literal:

Shorthand Methods

let square = {
  area: function() {
    return this.side * this.side; },
  side: 10
};

let square = {
  area() { 
    return this.side * this.side; },
  side: 10
};

When you write a method using this shorthand syntax, the property name can take any of the forms that are legal in an object literal: in addition to a regular JavaScript identifier like the name area above, you can also use string literals and computed property names, which can include Symbol property names:

const METHOD_NAME = "m";
const symbol = Symbol();
let weirdMethods = {
  "method With Spaces"(x) { return x + 1; },
  [METHOD_NAME](x) { return x + 2; },
  [symbol](x) { return x + 3; }
};

Property Getters and Setters

let o = {
  dataProp: value,
  get accessorProp() { return this.dataProp; },
  set accessorProp(value) { this.dataProp = value; }
};

Arrays

Creating Arrays

• Array literals

• The ... spread operator on an iterable object

• The Array() constructor

• The Array.of() and Array.from() factory methods

Array literals

If an array literal contains multiple commas in a row, with no value between, the array is sparse

Array literal syntax allows an optional trailing comma, so [,,] has a length of 2, not 3.

The Spread Operator

create a copy of an array - modifying the copy does not change the original

Array.of()

When the Array() constructor function is invoked with one numeric argument, it uses that argument as an array length. But when invoked with more than one numeric argument, it treats those arguments as elements for the array to be created. This means that the Array() constructor cannot be used to create an array with a single numeric element.

Array.from()

It is also a simple way to make a copy of an array:

Array.from() is also important because it defines a way to make a true-array copy of an array-like object. Array-like objects are non-array objects that have a numeric length property and have values stored with properties whose names happen to be integers.

Array.from() also accepts an optional second argument. If you pass a function as the second argument, then as the new array is being built, each element from the source object will be passed to the function you specify, and the return value of the function will be stored in the array instead of the original value.

Reading and Writing Array Elements

What is special about arrays is that when you use property names that are non-negative integers , the array automatically maintains the value of the length property for you.

JavaScript converts the numeric array index you specify to a string—the index 1 becomes the string "1", then uses that string as a property name.

It is helpful to clearly distinguish an array index from an object property name. All indexes are property names, but only property names that are integers between 0 and 2³¹ are indexes. All arrays are objects, and you can create properties of any name on them. If you use properties that are array indexes, however, arrays have the special behavior of updating their length property as needed.

Note that you can index an array using numbers that are negative or that are not integers. When you do this, the number is converted to a string, and that string is used as the property name. Since the name is not a non-negative integer, it is treated as a regular object property, not an array index.

The fact that array indexes are simply a special type of object property name means that JavaScript arrays have no notion of an “out of bounds” error. When you try to query a nonexistent property of any object, you don’t get an error; you simply get undefined.

Sparse Arrays

Sparse arrays can be created with the Array() constructor or simply by assigning to an array index larger than the current array length.

you can also make an array sparse with the delete operator. 

Note that when you omit a value in an array literal (using repeated commas as in [1,,3]), the resulting array is sparse, and the omitted elements simply do not exist

Array Length

if you set the length property to a nonnegative integer n smaller than its current value, any array elements whose index is greater than or equal to n are deleted from the array.

You can also set the length property of an array to a value larger than its current value. Doing this does not actually add any new elements to the array; it simply creates a sparse area at the end of the array.

Adding and Deleting Array Elements

You can also use the push() method to add one or more values to the end of an array.

You can use the unshift() method to insert a value at the beginning of an array, shifting the existing array elements to higher indexes.

The pop() method is the opposite of push(): it removes the last element of the array and returns it, reducing the length of an array by 1.

Similarly, the shift() method removes and returns the first element of the array, reducing the length by 1 and shifting all elements down to an index one lower than their current index.

You can delete array elements with the delete operator

Iterating Arrays

The easiest way to loop through each of the elements of an array (or any iterable object) is with the for/ofloop

let letters = [..."Hello world"];
let string = "";
for(let letter of letters) {
  string += letter;
}

It has no special behavior for sparse arrays and simply returns undefined for any array elements that do not exist.

If you want to use a for/of loop for an array and need to know the index of each array element, use the entries() method of the array

let letters = [..."Hello world"];
let everyother = "";
for(let [index, letter] of letters.entries()) {
  if (index % 2 === 0) everyother += letter; 
}

Another good way to iterate arrays is with forEach(). This is not a new form of the for loop, but an array method that offers a functional approach to array iteration.

let letters = [..."Hello world"];
let uppercase = "";
letters.forEach(letter => { 
  uppercase += letter.toUpperCase();
});

You can also loop through the elements of an array with a for loop.

for(let i = 0, len = letters.length; i < len; i++) {
  // loop body 
}

Multidimensional Arrays

Create a multidimensional array

for(let i = 0; i < table.length; i++) {
  table[i] = new Array(10); 
}

for(let row = 0; row < table.length; row++) {
  for(let col = 0; col < table[row].length; col++) {
    table[row][col] = row * col;
  }
}

Array Methods

Array Iterator Methods

First, all of these methods accept a function as their first argument and invoke that function once for each element (or some elements) of the array. If the array is sparse, the function you pass is not invoked for nonexistent elements. In most cases, the function you supply is invoked with three arguments: the value of the array element, the index of the array element, and the array itself.

data.forEach(value => { sum += value; });

data.forEach(function(v, i, a) {
  a[i] = v + 1; 
});

Note that map() returns a new array: it does not modify the array it is invoked on. If that array is sparse, your function will not be called for the missing elements, but the returned array will be sparse in the same way as the original array: it will have the same length and the same missing elements.

To close the gaps in a sparse array, you can do this:

And to close gaps and remove undefined and null elements, you can use filter, like this:

Unlike filter(), however, find() and findIndex() stop iterating the first time the predicate finds an element. When that happens, find() returns the matching element, and findIndex() returns the index of the matching element. If no matching element is found, find() returns undefined and findIndex()returns -1.

When you invoke reduce() with no initial value, it uses the first element of the array as the initial value.

reduceRight() works just like reduce(), except that it processes the array from highest index to lowest (right-to-left), rather than from lowest to highest. You might want to do this if the reduction operation has right-to-left associativity

Flattening arrays with flat() and flatMap()

Calling a.flatMap(f) is the same as (but more efficient than) a.map(f).flat():

Adding arrays with concat()

Stacks and Queues with push(), pop(), shift(), and unshift()

The push() and pop() methods allow you to work with arrays as if they were stacks. The push() method appends one or more new elements to the end of an array and returns the new length of the array.

The unshift() and shift() methods behave much like push() and pop(), except that they insert and remove elements from the beginning of an array rather than from the end.

You can implement a queue data structure by using push() to add elements at the end of an array and shift() to remove them from the start of the array. Note differences in unshift with single and multiple values.

Subarrays with slice(), splice(), fill(), and copyWithin()

splice() is a general-purpose method for inserting or removing elements from an array. splice() can delete elements from an array, insert new elements into an array, or perform both operations at the same time. 

The first argument to splice() specifies the array position at which the insertion and/or deletion is to begin. The second argument specifies the number of elements that should be deleted from (spliced out of) the array.

Unlike concat(), splice() inserts arrays themselves, not the elements of those arrays.

copyWithin() copies a slice of an array to a new position within the array. It modifies the array in place and returns the modified array, but it will not change the length of the array.

Array Searching and Sorting Methods

let a = ["ant", "Bug", "cat", "Dog"];
a.sort(); // a == ["Bug","Dog","ant","cat"]; 
a.sort(function(s,t) {
  let a = s.toLowerCase();
  let b = t.toLowerCase();
  if (a < b) return -1;
  if (a > b) return 1;
  return 0;
});

indexOf() and lastIndexOf() compare their argument to the array elements using the equivalent of the === operator. If your array contains objects instead of primitive values, these methods check to see if two references both refer to exactly the same object. If you want to actually look at the content of an object, try using the find() method with your own custom predicate function instead.

indexOf() and lastIndexOf() take an optional second argument that specifies the array index at which to begin the search. Negative values are allowed for the second argument and are treated as an offset from the end of the array.

indexOf() will not detect the NaN value in an array, but includes() will

When sort() is called with no arguments, it sorts the array elements in alphabetical order. To sort an array into some order other than alphabetical, you must pass a comparison function as an argument to sort().

Array to String Conversions

The join() method converts all the elements of an array to strings and concatenates them, returning the resulting string.

Arrays, like all JavaScript objects, have a toString() method. For an array, this method works just like the join() method with no arguments:

Static Array Functions

Array-Like Objects

It is often perfectly reasonable to treat any object with a numeric length property and corresponding non-negative integer properties as a kind of array.

let a = {}; 
let i = 0;
while(i < 10) {
  a[i] = i * i;
  i++;
}
a.length = i;
// Now iterate through it as if it were a real array
let total = 0;
for(let j = 0; j < a.length; j++) {
  total += a[j];
}

Since array-like objects do not inherit from Array.prototype, you cannot invoke array methods on them directly. You can invoke them indirectly using the Function.call method.

Strings as Arrays

Functions

In addition to the arguments, each invocation has another value - the invocation context - that is the value of the this keyword.

Function Declarations

function printprops(o) {
  for(let p in o) {
    console.log(`${p}: ${o[p]}\n`);
  }
}

Function declaration statements are “hoisted” to the top of the enclosing script, function, or block so that functions defined in this way may be invoked from code that appears before the definition.

Function Expressions

const f = function fact(x) {
  if (x <= 1) return 1;
  return x * fact(x-1);
}

Arrow Functions

If the body of your arrow function is a single return statement but the expression to be returned is an object literal, then you have to put the object literal inside parentheses to avoid syntactic ambiguity between the curly braces of a function body and the curly braces of an object literal

Arrow functions differ from functions defined in other ways in one critical way: they inherit the value of the this keyword from the environment in which they are defined rather than defining their own invocation context as functions defined in other ways do.

Nested Functions

function hypotenuse(a, b) {
  function square(x) { return x*x; }
  return Math.sqrt(square(a) + square(b));
}

Invoking Functions

For function invocation in non-strict mode, the invocation context (the this value) is the global object. In strict mode, however, the invocation context is undefined.

Constructor Invocation

A constructor invocation creates a new, empty object that inherits from the object specified by the prototypeproperty of the constructor.

Indirect invocation

JavaScript functions are objects, and like all JavaScript objects, they have methods. Two of these methods, call() and apply(), invoke the function indirectly. Both methods allow you to explicitly specify the this value for the invocation, which means you can invoke any function as a method of any object, even if it is not actually a method of that object.

Function Arguments and Parameters

Optional Parameters and Defaults

When a function is invoked with fewer arguments than declared parameters, the additional parameters are set to their default value, which is normally undefined.

function getPropertyNames(o, a) {
  a = a || [];
  for(let property in o) a.push(property);
  return a;
}

function getPropertyNames(o, a = []) {
  for(let property in o) a.push(property);
  return a;
}

One interesting case is that, for functions with multiple parameters, you can use the value of a previous parameter to define the default value of the parameters that follow it

const rectangle = (width, height = width*2) => ({width, height});

Rest Parameters and Variable-Length Argument Lists

Rest parameters enable us to write functions that can be invoked with arbitrarily more arguments than parameters.

function max(first=-Infinity, ...rest) {
  let maxValue = first; 

  for(let n of rest) {
    if (n > maxValue) {
      maxValue = n;
    }
  }

  return maxValue;
}

max(1, 10, 100, 2, 3, 1000, 4, 5, 6)

within the body of a function, the value of a rest parameter will always be an array. The array may be empty, but a rest parameter will never be undefined.

This type of function is called variadic functions, variable arity functions, or vararg functions.

The Arguments Object

Within the body of any function, the identifier arguments refers to the Arguments object for that invocation.

function max(x) {
  let maxValue = -Infinity;

  for(let i = 0; i < arguments.length; i++) {
    if (arguments[i] > maxValue)
      maxValue = arguments[i];
    }
  return maxValue;
}

max(1, 10, 100, 2, 3, 1000, 4, 5, 6)

you should avoid using it in any new code you write.

The Spread Operator for Function Calls

let numbers = [5, 2, 10, -1, 9, 100, 1];

Math.min(...numbers)

function timed(f) {
  return function(...args) {
    console.log(`Entering function ${f.name}`);
    let startTime = Date.now();
    try {
      return f(...args); 
    }
    finally {
      console.log(`Exiting ${f.name} after ${Date.now() - startTime}ms`);
    }
  };
}

// Compute the sum of the numbers between 1 and n by brute force
function benchmark(n) {
  let sum = 0;
  for(let i = 1; i <= n; i++) sum += i;
  return sum;
}

// Now invoke the timed version of that test function
timed(benchmark)(1000000)

Destructuring Function Arguments into Parameters

function vectorAdd(v1, v2) {
  return [v1[0] + v2[0], v1[1] + v2[1]];
}

vectorAdd([1,2], [3,4])

function vectorAdd([x1,y1], [x2,y2]) {
  return [x1 + x2, y1 + y2];
}

vectorAdd([1,2], [3,4])

function vectorMultiply({x, y}, scalar) {
  return { x: x*scalar, y: y*scalar };
}

vectorMultiply({x: 1, y: 2}, 2)

function vectorMultiply({x,y}, scalar) {
  return { x: x*scalar, y: y*scalar};
}
vectorMultiply({x: 1, y: 2}, 2)

Argument Types

Adding code to check the types of arguments

function sum(a) {
  let total = 0;
  for(let element of a) { 
    if (typeof element !== "number") {
      throw new TypeError("sum(): elements must be numbers");
    }
    total += element;
  }
  return total;
}

Functions as Values

function square(x) { return x * x; }

Functions can also be assigned to object properties rather than variables.

Functions don’t even require names at all, as when they’re assigned to array elements:

a[0] accesses first element of the array, which is "x => x*x", (a[1]) passes parameter, which is 20.

Examples of using functions as data

function add(x,y) { return x + y; }
function subtract(x,y) { return x - y; }
function multiply(x,y) { return x * y; }
function divide(x,y) { return x / y; }

function operate(operator, operand1, operand2) {
  return operator(operand1, operand2);
}

or:

const operators = {
  add: (x,y) => x+y,
  subtract: (x,y) => x-y,
  multiply: (x,y) => x*y,
  divide: (x,y) => x/y,
  pow: Math.pow 
};

function operate2(operation, operand1, operand2) {
  if (typeof operators[operation] === "function") {
    return operators[operation](operand1, operand2);
  }
  else throw "unknown operator";
}

Defining Your Own Function Properties

When a function needs a “static” variable whose value persists across invocations, it is often convenient to use a property of the function itself.

For example, suppose you want to write a function that returns a unique integer whenever it is invoked. The function must never return the same value twice. In order to manage this, the function needs to keep track of the values it has already returned, and this information must persist across function invocations.

uniqueInteger.counter = 0;
function uniqueInteger() {
  return uniqueInteger.counter++;
}

uniqueInteger()

uniqueInteger()

Compute factorials and cache results as properties of the function itself.

function factorial(n) {
  if (Number.isInteger(n) && n > 0) { 
    if (!(n in factorial)) { 
      factorial[n] = n * factorial(n-1);
    }
    return factorial[n]; 
  } 
  else {
    return NaN; 
  }
}

factorial[1] = 1;

factorial(6)

factorial[5]

Functions as Namespaces

Variables declared within a function are not visible outside of the function. For this reason, it is sometimes useful to define a function simply to act as a temporary namespace in which you can define variables without cluttering the global namespace.

Variables that would have been global become local to the function. Following code defines only a single global variable: the function name chunkNamespace.

function chunkNamespace() {
  // Chunk of code goes here
  // Any variables defined in the chunk are local to this function 
  // instead of cluttering up the global namespace.
}

chunkNamespace();

If defining even a single property is too much, you can define and invoke an anonymous function in a single expression - IIEF (immediately invoked function expression)

(function() { 
// chunkNamespace() function rewritten as an unnamed expression.
// Chunk of code goes here
}());

Closures

JavaScript uses lexical scoping. This means that functions are executed using the variable scope that was in effect when they were defined, not the variable scope that is in effect when they are invoked.

In order to implement lexical scoping, the internal state of a JavaScript function object must include not only the code of the function but also a reference to the scope in which the function definition appears.

This combination of a function object and a scope (a set of variable bindings) in which the function’s variables are resolved is called a closure.

Closures become interesting when they are invoked from a different scope than the one they were defined in. This happens most commonly when a nested function object is returned from the function within which it was defined.

let scope = "global scope";

function checkscope() {
  let scope = "local scope"; 
  function f() { return scope; } 
  return f();
}

let scope = "global scope";

function checkscope() {
  let scope = "local scope"; 
  function f() { return scope; } 
  return f;
}

Closures capture the local variables of a single function invocation and can use those variables as private state.

let uniqueInteger = (function() { 
  let counter = 0; 
  return function() { return counter++; };
}());

it is the return value of the function that is being assigned to uniqueInteger.

Private variables like counter need not be exclusive to a single closure: it is perfectly possible for two or more nested functions to be defined within the same outer function and share the same scope.

function counter() {
  let n = 0;
  return {
    count: function() { return n++; },
    reset: function() { n = 0; }
  };
}

You can combine this closure technique with property getters and setters

function counter(n) { 
  return {
    get count() { return n++; },
    set count(m) {
      if (m > n) n = m;
      else throw Error("count can only be set to a larger value")
    }
  };
}

Define a private variable and two nested functions to get and set the value of that variable.

function addPrivateProperty(o, name, predicate) {
  let value; 
  o[`get${name}`] = function() { return value; };
  o[`set${name}`] = function(v) {
    if (predicate && !predicate(v)) {
      throw new TypeError(`set${name}: invalid value ${v}`);
    }
    else {
      value = v;
    }
  };
}

Function Properties, Methods, and Constructor

Since functions are objects, they can have properties and methods, just like any other object.

The length Property

The read-only length property of a function specifies the arity of the function—the number of parameters it declares in its parameter list, which is usually the number of arguments that the function expects.

The name Property

This property is primarily useful when writing debugging or error messages.

The prototype Property

When a function is used as a constructor, the newly created object inherits properties from the prototype object.

The call() and apply() Methods

call() and apply() allow you to indirectly invoke a function as if it were a method of some other object. The first argument to both call() and apply() is the object on which the function is to be invoked; this argument is the invocation context and becomes the value of the this keyword within the body of the function.

To invoke the function f() as a method of the object o (passing no arguments),

To pass two numbers to the function f() and invoke it as if it were a method of the object o,

f.call(o, 1, 2);

The apply() method is like the call() method, except that the arguments to be passed to the function are specified as an array:

f.apply(o, [1,2]);

The trace() function defined uses the apply() method instead of a spread operator, and by doing that, it is able to invoke the wrapped method with the same arguments and the same this value as the wrapper method

function trace(o, m) {
  let original = o[m]; 
  o[m] = function(...args) {
    console.log(new Date(), "Entering:", m); 
    let result = original.apply(this, args);
    console.log(new Date(), "Exiting:", m); 
    return result; 
  };
}

The bind() Method

The primary purpose of bind() is to bind a function to an object.

The most common use case for calling bind() is to make non-arrow functions behave like arrow functions.

Partial application is a common technique in functional programming and is sometimes called currying.

The toString() Method

Most (but not all) implementations of this toString() method return the complete source code for the function

The Function() Constructor

The Function() constructor is best thought of as a globally scoped version of eval()  that defines new variables and functions in its own private scope. You will probably never need to use this constructor in your code.

Functional Programming

Processing Arrays with Functions

Skipping

Higher-Order Functions

A higher-order function is a function that operates on functions, taking one or more functions as arguments and returning a new function.

function not(f) {
  return function(...args) { 
    let result = f.apply(this, args);
    return !result; 
  };
}

Returns a new function that maps one array to another

const map = function(a, ...args) { return a.map(...args); };

function mapper(f) {
  return a => map(a, f);
}

const increment = x => x + 1;

const incrementAll = mapper(increment);

incrementAll([1,2,3]

Example that takes two functions, f and g, and returns a new function that computes f(g()):

function compose(f, g) {
  return function(...args) {
    return f.call(this, g.apply(this, args));
  };
}

Memoization

We defined a factorial function that cached its previously computed results. In functional programming, this kind of caching is called memoization.

Classes

JavaScript’s classes and prototype-based inheritance mechanism are substantially different from the classes and class-based inheritance mechanism of Java.

Classes and Prototypes

If we define a prototype object and then use Object.create() to create objects that inherit from it, we have defined a JavaScript class.

Factory function that returns a new range object:

function range(from, to) {
  let r = Object.create(range.methods);
  r.from = from;
  r.to = to;
  return r;
}

range.methods = {
  includes(x) { return this.from <= x && x <= this.to; },
  *[Symbol.iterator]() {
    for(let x = Math.ceil(this.from); x <= this.to; x++)
      yield x;
  },
  toString() { return "(" + this.from + "..." + this.to +")"; }
};

Classes and Constructors

A constructor is a function designed for the initialization of newly created objects.

The critical feature of constructor invocations is that the prototype property of the constructor is used as the prototype of the new object.

While almost all objects have a prototype, only a few objects have a prototype property. It is function objects that have a prototype property.

This means that all objects created with the same constructor function inherit from the same object and are therefore members of the same class.

A Range class using a constructor

function Range(from, to) {
  this.from = from;
  this.to = to;
}

Range.prototype = {
  includes: function(x) { return this.from <= x && x <= this.to; },
  [Symbol.iterator]: function*() {
    for(let x = Math.ceil(this.from); x <= this.to; x++)
      yield x;
  },
  toString: function() { return "(" + this.from + "..." + this.to + ")"; }
};

Because the Range() constructor is invoked with new, it does not have to call Object.create() or take any action to create a new object.

In the first example, the prototype was range.methods. This was a convenient and descriptive name, but arbitrary. In the second example, the prototype is Range.prototype, and this name is mandatory. 

An invocation of the Range() constructor automatically uses Range.prototype as the prototype of the new Range object.

Constructors, Class Identity, and instanceof

Two objects are instances of the same class if and only if they inherit from the same prototype object.

The instanceof operator is not checking whether r was actually initialized by the Range constructor. Instead, it is checking whether r inherits from Range.prototype.

function Strange() {}
Strange.prototype = Range.prototype;

new Strange() instanceof Range

If you want to test the prototype chain of an object for a specific prototype and do not want to use the constructor function as an intermediary, you can use the isPrototypeOf() method

range.methods.isPrototypeOf(r);

The constructor Property

Every regular JavaScript function automatically has a prototype property. The value of this property is an object that has a single, non-enumerable constructor property. 

The value of the constructor property is the function object

let F = function() {};

let p = F.prototype;

let c = p.constructor;

c === F

Instances of the Range class, as defined, do not have a constructor property. We can remedy this problem by explicitly adding a constructor to the prototype:

Range.prototype = {
  constructor: Range
};

Another common technique that you are likely to see in older JavaScript code is to use the predefined prototype object with its constructor property and add methods to it one at a time with code like this:

Range.prototype.includes = function(x) {
  return this.from <= x && x <= this.to;
};
Range.prototype.toString = function() {
  return "(" + this.from + "..." + this.to + ")";
};

Classes with the class Keyword

class Range {
  constructor(from, to) {
    this.from = from;
    this.to = to;
  }
  includes(x) { return this.from <= x && x <= this.to; }
  *[Symbol.iterator]() {
    for(let x = Math.ceil(this.from); x <= this.to; x++)
      yield x;
  }
  toString() { return `(${this.from}...${this.to})`; }
}

Although class bodies are superficially similar to object literals, they are not the same thing. In particular, they do not support the definition of properties with name/value pairs.

If your class does not need to do any initialization, you can omit the constructor keyword and its body, and an empty constructor function will be implicitly created for you.

If you want to define a class that subclasses - or inherits from - another class, you can use the extends keyword with the class keyword:

class Span extends Range {
  constructor(start, length) {
    if (length >= 0) {
      super(start, start + length);
    }
    else {
      super(start + length, start);
    }
  }
}

class declarations have both statement and expression forms

Static methods

You can define a static method within a class body by prefixing the method declaration with the static keyword. Static methods are defined as properties of the constructor function rather than properties of the prototype object.

static parse(s) {
  let matches = s.match(/^\((\d+)\.\.\.(\d+)\)$/);
  if (!matches) {
    throw new TypeError(`Cannot parse Range from "${s}".`)
  }
  return new Range(parseInt(matches[1]),
  parseInt(matches[2]));
}

The method defined by this code is Range.parse(), not Range.prototype.parse(), and you must invoke it through the constructor, not through an instance:

let r = Range.parse('(1...10)');

Getters, Setters, and other Method Forms

Within a class body, you can define getter and setter methods  just as you can in object literals. The only difference is that in class bodies, you don’t put a comma after the getter or setter.

Public, Private, and Static Fields

The ES6 standard only allows the creation of methods (including getters, setters, and generators) and static methods; it does not include syntax for defining fields.

If you want to define a field on a class instance, you must do that in the constructor function or in one of the methods. And if you want to define a static field for a class, you must do that outside the class body, after the class has been defined.

Standardization is underway, however, for extended class syntax that allows the definition of instance and static fields, in both public and private forms.

class Buffer {
  constructor() {
    this.size = 0;
    this.capacity = 4096;
    this.buffer = new Uint8Array(this.capacity);
  }
}

class Buffer {
  size = 0;
  capacity = 4096;
  buffer = new Uint8Array(this.capacity);
}

The same proposal that seeks to standardize these instance fields also defines private (with the # prefix) instance fields.

class Buffer {
  #size = 0;
  get size() { return this.#size; }
}

A related proposal seeks to standardize the use of the static keyword for fields.

static integerRangePattern = /^\((\d+)\.\.\.(\d+)\)$/;
static parse(s) {
  let matches = s.match(Range.integerRangePattern);
  if (!matches) {
    throw new TypeError(`Cannot parse Range from "${s}".`)
  }
  return new Range(parseInt(matches[1]), matches[2]);
}

Adding Methods to Existing Classes

We can augment JavaScript classes simply by adding new methods to their prototype objects.

if (!String.prototype.startsWith) {
  String.prototype.startsWith = function(s) {
    return this.indexOf(s) === 0;
  };
}

Number.prototype.times = function(f, context) {
  let n = this.valueOf();
  for(let i = 0; i < n; i++) f.call(context, i);
};

Subclasses

Subclasses and Prototypes

Span subclass of the Range class.  This subclass will work just like a Range, but instead of initializing it with a start and an end, we’ll instead specify a start and a distance, or span.

function Span(start, span) {
  if (span >= 0) {
    this.from = start;
    this.to = start + span;
  }
  else {
    this.to = start;
    this.from = start + span;
  }
}

Ensure that the Span prototype inherits from the Range

Span.prototype = Object.create(Range.prototype);

We don't want to inherit Range.prototype.constructor, so we define our own constructor property:

Span.prototype.constructor = Span;

Span overrides the toString() method

A robust subclassing mechanism needs to allow classes to invoke the methods and constructor of their superclass, but prior to ES6, JavaScript did not have a simple way to do these things.

Subclasses with extends and super

class EZArray extends Array {
  get first() { return this[0]; }
  get last() { return this[this.length-1]; }
}

Example demonstrates the use of the super keyword to invoke the constructor and methods of the superclass

class TypedMap extends Map {
  constructor(keyType, valueType, entries) {
    if (entries) {
      for(let [k, v] of entries) {
        if (typeof k !== keyType || typeof v !== valueType) {
          throw new TypeError(`Wrong type for entry [${k}, ${v}]`);
        }
     }
   }
 
    super(entries);
    this.keyType = keyType;
    this.valueType = valueType;
  }
  set(key, value) {
    if (this.keyType && typeof key !== this.keyType) {
      throw new TypeError(`${key} is not of type${this.keyType}`);
    }
    if (this.valueType && typeof value !== this.valueType)
    {
      throw new TypeError(`${value} is not of type ${this.valueType}`);
    }
    return super.set(key, value);
  }
}

You may not use the this keyword in your constructor until after you have invoked the superclass constructor with super(). This enforces a rule that superclasses get to initialize themselves before subclasses do.

Once private fields are supported, we could change these properties to #keyType and #valueType so that they could not be altered from the outside.

Class Hierarchies and Abstract Classes

Define abstract classes—classes that do not include a complete implementation—to serve as a common superclass for a group of related subclasses.

Modules

Automating Closure-Based Modularity

Imagine a tool that takes a set of files, wraps the content of each of those files within an immediately invoked function expression, keeps track of the return value of each function, and concatenates everything into one big file.

const modules = {};
function require(moduleName) { return modules[moduleName]; }


modules["sets.js"] = (function() {
  const exports = {};
  exports.BitSet = class BitSet { ... };
  return exports;
}());


modules["stats.js"] = (function() {
  const exports = {};
  const sum = (x, y) => x + y;
  const square = x = > x * x;
  exports.mean = function(data) { ... };
  exports.stddev = function(data) { ... };
  return exports;
}());

writing code like the following to make use of those modules

const stats = require("stats.js");
const BitSet = require("sets.js").BitSet;
// Now write code using those modules
let s = new BitSet(100);
s.insert(10);
s.insert(20);
s.insert(30);
let average = stats.mean([...s]);

Modules in ES6

ES6 adds import and export keywords to JavaScript and finally supports real modularity as a core language feature.

ES6 modularity is conceptually the same as Node modularity: each file is its own module, and constants, variables, functions, and classes defined within a file are private to that module unless they are explicitly exported.

ES6 Exports

To export a constant, variable, function, or class from an ES6 module, simply add the keyword export before the declaration

export const PI = Math.PI;

export function degreesToRadians(d) { return d * PI / 180; }

export class Circle {
  constructor(r) { this.r = r; }
  area() { return PI * this.r * this.r; }
}

or:

export { Circle, degreesToRadians, PI };

It is common to write modules that export only one value (typically a function or class), and in this case, we usually use export default instead of export

export default class BitSet {
  // implementation omitted
}

ES6 Imports

import BitSet from './bitset.js';

import { mean, stddev } from "./stats.js";

When importing from a module that defines many exports, however, you can easily import everything with an import statement like this:

import * as stats from "./stats.js";

With the wildcard import shown in the previous example, the importing module would use the imported mean() and stddev() functions through the stats object, invoking them as stats.mean() and stats.stddev().

Note: not finished.

The JavaScript Standard Library

The Set Class

Sets are not ordered or indexed, and they do not allow duplicates.

let s = new Set();
let t = new Set([1, s]);

let t = new Set(s);

let unique = new Set("Mississippi");

The argument to the Set() constructor need not be an array: any iterable object (including other Set objects) is allowed.

The add() method takes a single argument; if you pass an array, it adds the array itself to the set, not the individual array elements. add() always returns the set it is invoked on, however, so if you want to add multiple values to a set, you can used chained method calls like.

it is very important to understand that set membership is based on strict equality checks, like the === operator performs.

The most important thing we do with sets is not to add and remove elements from them, but to check to see whether a specified value is a member of the set:

The Set class is iterable, which means that you can use a for/of loop to enumerate all of the elements of a set:

let sum = 0;
for(let p of oneDigitPrimes) { 
  sum += p; // and add them up
}

Because Set objects are iterable, you can convert them to arrays and argument lists with the ... spread operator

JavaScript Set class always remembers the order that elements were inserted in, and it always uses this order when you iterate a set: the first element inserted will be the first one iterated (assuming you haven’t deleted it first), and the most recently inserted element will be the last one iterated.

Set class also implements a forEach() method

let product = 1;
oneDigitPrimes.forEach(n => { product *= n; });

The Map Class

let m = new Map();
let n = new Map([["one", 1],["two", 2]]);

let copy = new Map(n);

let o = { x: 1, y: 2};
let p = new Map(Object.entries(o));

map is a set of keys, each of which has an associated value. This is not quite the same as a set of key/value pairs.

use has() to check whether a map includes the specified key; use delete() to remove a key (and its associated value) from the map; use clear() to remove all key/value pairs from the map; and use the size property to find out how many keys a map contains.

set() method of Map can be chained.

Any JavaScript value can be used as a key or a value in a Map. This includes null, undefined, and NaN, as well as reference types like objects and arrays.

Map compares keys by identity, not by equality.

Iterate over map:

Map class iterates in insertion order

If you want to iterate just the keys or just the associated values of a map, use the keys() and values() methods: these return iterable objects that iterate keys and values, in insertion order. (The
entries() method returns an iterable object that iterates key/value pairs, but this is exactly the same as iterating the map directly.)

[...m.keys()] 
[...m.values()]
[...m.entries()]

Map objects can also be iterated using the forEach()

m.forEach((value, key) => {...}

Note that the value parameter comes before the key parameter.

WeakMap and WeakSet

The WeakMap class is a variant (but not an actual subclass) of the Map class that does not prevent its key values from being garbage collected.

WeakMap keys must be objects or arrays; primitive values are not subject to garbage collection and cannot be used as keys.

WeakMap implements only the get(), set(), has(), and delete() methods. In particular, WeakMap is not iterable and does not define keys(), values(), or forEach(). If WeakMap was iterable, then its keys would be reachable and it wouldn’t be weak.

Similarly, WeakMap does not implement the size property because the size of a WeakMap could change at any time as objects are garbage collected

Typed Arrays and Binary Data

They differ from regular arrays in some very important ways

• The elements of a typed array are all numbers. Unlike regular JavaScript numbers, however, typed arrays allow you to specify the type (signed and unsigned integers and IEEE-754 floating point) and size (8 bits to 64 bits) of the numbers to be stored in the array.

• You must specify the length of a typed array when you create it, and that length can never change.

• The elements of a typed array are always initialized to 0 when the array is created.

Int8Array()

Uint8Array()

Uint8ClampedArray()

Int16Array()

Uint32Array()

Uint16Array()

Int32Array()

BigInt64Array()

BigUint64Array()

Float32Array()

let bytes = new Uint8Array(1024);

let matrix = new Float64Array(9);

let sudoku = new Int8Array(81);

Initialize with values

let white = Uint8ClampedArray.of(255, 255, 255, 0);

let ints = Uint32Array.from(white);

one more way to create typed arrays that involves the ArrayBuffer type

let buffer = new ArrayBuffer(1024*1024);

buffer.byteLength

Typed arrays are not true arrays, but they re-implement most array methods, so you can use them pretty much just like you’d use regular arrays:

Remember that typed arrays have fixed lengths, so the length property is read-only, and methods that change the length of the array (such as push(), pop(), unshift(), shift(), and splice()) are not implemented for typed arrays. Methods that alter the contents of an array without changing the length (such as sort(), reverse(), and fill()) are implemented.

Determine Endianess and DataView

let littleEndian = new Int8Array(new Int32Array([1]).buffer)
[0] === 1;

You can use the DataView class, which defines methods for reading and writing values from an ArrayBuffer with explicitly specified byte ordering. Refer to book for more examples.

Pattern Matching with Regular Expressions

RegExp objects may be created with the RegExp() constructor, of course, but they are more often created using a special literal syntax.

let pattern = /s$/;

let pattern = new RegExp("s$");

Regular expressions can also have one or more flag characters that affect how they work

let pattern = /s$/i;

Punctuation characters have special meanings in regular expressions: ^ $ . * + ? = ! : | \ / ( ) [ ] { }. Other punctuation characters, such as quotation marks and @, do not have special meaning and simply match themselves literally in a regular expression.

If you use the RegExp() constructor, keep in mind that any backslashes in your regular expression need to be doubled, since strings also use backslashes as an escape character.

REPETITIONS

let r = /\d{2,4}/;

r = /\w{3}\d?/;

r = /\s+java\s+/;

r = /[^(]*/;

If you want to match repetitions of more complicated expressions, you’ll need to define a group with parentheses

Be careful when using the * and ? repetition characters. Since these characters may match zero instances of whatever precedes them, they are allowed to match nothing.

NON-GREEDY REPETITION

It is also possible to specify that repetition should be done in a non-greedy way. Simply follow the repetition character or characters with a question mark: ??, +?, *?, or even {1,5}?.

"aaa"

/a+/

"aaa"

"aaa"

/a+?/

"a"

Note that using non-greedy repetition may not always produce the results you expect. This is because regular expression pattern matching is done by findingthe first position in the string at which a match is possible. Since a match is possible starting at the first character of the string, shorter matches starting at subsequent characters are never even considered.

ALTERNATION, GROUPING, AND REFERENCES

If the left alternative matches, the right alternative is ignored, even if it would have produced a “better” match

Another purpose of parentheses in regular expressions is to define subpatterns within the complete pattern. When a regular expression is successfully matched against a target string, it is possible to extract the portions of the target string that matched any particular parenthesized subpattern. For example, suppose you are looking for one or more lowercase letters followed by one or more digits. You might use the pattern /[a-z]+\d+/. But suppose you only really care about the digits at the end of each match. If you put that part of the pattern in parentheses (/[a-z]+(\d+)/), you can extract the digits from any matches you find,

A related use of parenthesized subexpressions is to allow you to refer back to a subexpression later in the same regular expression. This is done by following a \ character by a digit or digits. The digits refer to the position of the parenthesized subexpression within the regular expression. For example, \1 refers back to the first subexpression, and \3 refers to the third.

/['"][^'"]*['"]/

/(['"])[^'"]*\1/

Note (?:...) syntax:

In pattern "/([Jj]ava(?:[Ss]cript)?)\sis\s(fun\w*)/" \2 refers to the text matched by (fun\w*) since (?:[Ss]cript)?) in not remembered.

SPECIFYING MATCH POSITION

regular expression anchors because they anchor the pattern to a specific position in the search string. The most commonly used anchor elements are ^, which ties the pattern to the beginning of the string, and $, which anchors the pattern to the end of the string.

To search for “Java” as a word by itself you can try the pattern /\sJava\s/, which requires a space before and after the word. But there are two problems with this solution. First, it does not match “Java” at the beginning or the end of a string, but only if it appears with space on either side. Second, when this pattern does find a match, the matched string it returns has leading and trailing spaces, which is not quite what’s needed. So instead of matching actual space characters with \s, match (or anchor to) word boundaries with \b. The resulting expression is /\bJava\b/.

The element \B anchors the match to a location that is not a word boundary. Thus, the pattern /\B[Ss]cript/ matches “JavaScript” and “postscript”, but not “script” or “Scripting”.

You can also use arbitrary regular expressions as anchor conditions.

If you include an expression within (?= and ) characters, it is a lookahead assertion, and it specifies that the enclosed characters must match, without actually matching them.

If you instead introduce an assertion with (?!, it is a negative lookahead assertion.

FLAGS

Flags are specified after the second / character of a regular expression literal or as a string passed as the second argument to the RegExp() constructor.

String Methods for Pattern Matching

SEARCH()

Strings support four methods that use regular expressions.

search() does not support global searches; it ignores the g flag of its regular expression argument.

REPLACE()

parenthesized subexpressions of a regular expression are numbered from left to right and that the regular expression remembers the text that each subexpression matches.

to replace quotation marks in a string with other characters:

If your RegExp uses named capture groups, then you can refer to the matching text by name rather than by number:

Instead of passing a replacement string as the second argument to replace(), you can also pass a function that will be invoked to compute the replacement value.

Example to convert decimal integers in a string to hexadecimal:

MATCH()

If the regular expression does not have the g flag set, match() does not do a global search; it simply searches for the first match. In this nonglobal case, match() still returns an array, but the array elements are completely different.

Thus, if match() returns an array a, a[0] contains the complete match, a[1] contains the substring that matched the first parenthesized expression, and so on.

let url = /(\w+):\/\/([\w.]+)\/(\S*)/;
let text = "Visit my blog at http://www.example.com/~david";
let match = text.match(url);
let fullurl, protocol, host, path;
if (match !== null) {

  fullurl = match[0];

  protocol = match[1];

  host = match[2];

  path = match[3];

In this non-global case, the array returned by match() also has some object properties in addition to the numbered array elements.

input property refers to the string on which match() was called

The index property is the position within that string at which the match starts.

if the regular expression contains named capture groups, then the returned array also has a groups property whose value is an object.

There are also important but less dramatic differences in behavior when the y flag is set. Refer to book for examples.

MATCHALL()

Instead of returning an array of matching substrings like match() does, however, it returns an iterator that yields the kind of match objects that match() returns when used with a non-global RegExp.

SPLIT()

Surprisingly, if you call split() with a RegExp delimiter and the regular expression includes capturing groups, then the text that matches the capturing groups will be included in the returned array.

The RegExp Class

The RegExp() constructor is useful when a regular expression is being dynamically created and thus cannot be represented with the regular expression literal syntax.

let zipcode = new RegExp("\\d{5}", "g");

TEST()

Returns true or false by calling exec().

EXEC()

let pattern = /Java/g;
let text = "JavaScript > Java";
let match;
while((match = pattern.exec(text)) !== null) {
  console.log(`Matched ${match[0]} at ${match.index}`);
  console.log(`Next search begins at ${pattern.lastIndex}`);
}

THE LASTINDEX PROPERTY AND REGEXP REUSE

The use of the lastIndex property with the g and y flags is a particularly awkward part of this API.  When you use these flags, you need to be particularly careful when calling the match(), exec(), or test() methods because the behavior of these methods depends on lastIndex, and the value of lastIndex depends on what you have previously done with the RegExp object.

To find the index of all <p> tags within a string of HTML text:

let match, positions = [];
while((match = /<p>/g.exec(html)) !== null) { 
  positions.push(match.index);
}

If the html string contains at least one <p> tag, then it will loop forever. For each iteration of the loop, we’re creating a new RegExp object with lastIndex set to 0, so exec() always begins at the start of the string, and if there is a match, it will keep matching over and over. The  solution, of course, is to define the RegExp once, and save it to a variable so that we’re using the same RegExp object for each iteration of the loop.

On the other hand, sometimes reusing a RegExp object is the wrong thing to do. Suppose, for example, that we want to loop through all of the words in a dictionary to find words that contain pairs of double letters.

let dictionary = [ "apple", "book", "coffee" ];
let doubleLetterWords = [];
let doubleLetter = /(\w)\1/g;
for(let word of dictionary) {
  if (doubleLetter.test(word)) {
    doubleLetterWords.push(word);
  }
}

Because we set the g flag on the RegExp, the lastIndex property is changed after successful matches, and the test() method (which is based on exec()) starts searching for a match at the position specified by lastIndex. After matching the “pp” in “apple”, lastIndex is 3, and so we start searching the word “book” at position 3 and do not see the “oo” that it contains.

Dates and Times

let now = new Date();

let epoch = new Date(0);

let century = new Date(2100,
0, 
1,
2, 3, 4, 5);

let century = new Date(Date.UTC(2100, 0, 1));

If you print a date (with console.log(century), for example), it will, by default, be printed in your local time zone. If you want to display a date in UTC, you should explicitly convert it to a string with toUTCString() or toISOString().

if you pass a string to the Date() constructor, it will attempt to parse that string as a date and time specification

Once you have a Date object, various get and set methods allow you to query and modify the year, month, day-of-month, hour, minute, second, and millisecond fields of the Date. Each of these methods hastwo forms: one that gets or sets using local time and one that gets or sets using UTC time.

Note that the methods for querying the day-of-month are getDate() and getUTCDate(). The more natural-sounding functions getDay() and getUTCDay() return the day-of-week (0 for Sunday through 6 for Saturday). The day-of-week is read-only, so there is not a corresponding setDay() method.

Timestamps

JavaScript represents dates internally as integers that specify the number of milliseconds since (or before) midnight on January 1, 1970, UTC time.

For any Date object, the getTime() method returns this internal value, and the setTime() method sets it.

d.setTime(d.getTime() + 30000);

The static Date.now() method returns the current time as a timestamp and is helpful when you want to measure how long your code takes to run

let startTime = Date.now();
reticulateSplines(); // Do some time-consuming operation
let endTime = Date.now();
console.log(`Spline reticulation took ${endTime -startTime}ms.`);

adds three months and two weeks to the current date:

Formatting and Parsing Date Strings

let d = new Date(2020, 0, 1, 17, 10, 30);

d.toString()

d.toUTCString()

d.toLocaleDateString()

d.toLocaleTimeString()

d.toISOString()

there is also a static Date.parse() method that takes a string as its argument, attempts to parse it as a date and time, and returns a timestamp representing that date. Date.parse() is able to parse the same strings that the Date() constructor can and is guaranteed to be able to parse the output of toISOString(), toUTCString(), and toString().

Error Classes

One good reason to use an Error object is that, when you create an Error, it captures the state of the JavaScript stack, and if the exception is uncaught, the stack trace will be displayed with the error message, which will help you debug the issue.

Error objects have two properties: message and name, and a toString() method. Node and all modern browsers also define a stack property on Error objects.

Subclasses are EvalError, RangeError, ReferenceError, SyntaxError, TypeError, and URIError.

You should feel free to define your own Error subclasses that best encapsulate the error conditions of your own program.

class HTTPError extends Error {
  constructor(status, statusText, url) {
    super(`${status} ${statusText}: ${url}`);
    this.status = status;
    this.statusText = statusText;
    this.url = url;
  }
  get name() { return "HTTPError"; }
}

let error = new HTTPError(404, "Not Found", "http://example.com/");

error.status

error.message

error.name

JSON Serialization and Parsing

JavaScript supports JSON serialization and deserialization with the two functions JSON.stringify() and JSON.parse().

let o = {s: "", n: 0, a: [true, false, null]};

let s = JSON.stringify(o);

let copy = JSON.parse(s);

Inefficient way of creating a deep copy of an object

function deepcopy(o) {
  return JSON.parse(JSON.stringify(o));
}

Typically, you pass only a single argument to JSON.stringify() and JSON.parse(). Both functions accept an optional second argument that allows us to extend the JSON format.

JSON.stringify() also takes an optional third argument. If you would like your JSONformatted string to be human-readable (if it is being used as a configuration file, for example), then you should pass null as the second argument and pass a number or string as the third argument. If the third argument is a number, then it will use that number of spaces for each indentation level. If the third argument is a string of whitespace (such as '\t'), it will use that string for each level of indent.

JSON Customizations

If JSON.stringify() is asked to serialize a value that is not natively supported by the JSON format, it looks to see if that value has a toJSON() method, and if so, it calls that method and then stringifies the return value in place of the original value. Date objects implement toJSON(): it returns the same string that toISOString() method does.

If you need to re-create Date objects (or modify the parsed object inany other way), you can pass a “reviver” function as the second argument to JSON.parse().

let data = JSON.parse(text, function(key, value) {
  if (key[0] === "_") return undefined;
  if (typeof value === "string" && /^\d\d\d\d-\d\d-\d\dT\d\d:\d\d:\d\d.\d\d\dZ$/.test(value)) {
    return new Date(value);
  }
  return value;
});

The Console API

Console functions that print their arguments like console.log() have a little-known feature: if the first argument is a string that includes %s, %i, %d, %f, %o, %O, or %c, then this first argument is treated as format string, and the values of subsequent arguments are substituted into the string in place of the two-character % sequences.

URL API

let url = new URL("https://example.com:8000/path/name?q=term#fragment");

url.href

url.origin

url.protocol

url.host

url.hostname

url.port

url.pathname

url.search

url.hash

let url = new URL("https://example.com");

url.pathname = "api/search";

url.search = "q=test";

url.toString()

One of the important features of the URL class is that it correctly adds punctuation and escapes special characters in URLs when that is needed

let url = new URL("https://example.com");

url.pathname = "path with spaces";

url.pathname

url.search = "q=foo#bar";

url.search

url.href

Often, however, HTTP requests encode the values of multiple form fields or multiple API parameters into the query portion of a URL. In this format, the query portion of the URL is a question mark followed by one or more name/value pairs, which are separated from one another by ampersands.

If you want to encode these kinds of name/value pairs into the query portion of a URL, then the searchParams property will be more useful than the search property.

The value of the searchParams property is a URLSearchParams object.

Timers

setTimeout() and setInterval()—that allow programs to ask the browser to invoke a function after a specified amount of time has elapsed or to invoke the function repeatedly at a specified interval.

setTimeout(() => { console.log("Ready..."); }, 1000);
setTimeout(() => { console.log("set..."); }, 2000);
setTimeout(() => { console.log("go!"); }, 3000);

If you want to invoke a function repeatedly, use setInterval()

Both setTimeout() and setInterval() return a value. If you save this value in a variable, you can then use it later to cancel the execution of the function by passing it to clearTimeout() or clearInterval().

let clock = setInterval(() => {
  console.clear();
  console.log(new Date().toLocaleTimeString());
}, 1000);

setTimeout(() => { clearInterval(clock); }, 10000);

Iterators and Generators

The iterator method of an iterable object does not have a conventional name but uses the Symbol, Symbol.iterator as its name. So a simple for/of loop over an iterable object iterable could also be written the hard way, like this:

let iterable = [99];
let iterator = iterable[Symbol.iterator]();
for(let result = iterator.next(); !result.done; result =iterator.next()) {
  console.log(result.value) // result.value == 99
}

When you want to iterate though a “partially used” iterator:

Implementing Iterable Objects

we will implement the Range class one more time, making it iterable without relying on a generator. 

In order to make a class iterable, you must implement a method whose name is the Symbol Symbol.iterator

class Range {
  constructor (from, to) {
    this.from = from;
    this.to = to;
  }
  has(x) { return typeof x === "number" && this.from <= x && x <= this.to; }
  toString() { return `{ x | ${this.from} ≤ x ≤ ${this.to}}`; }

  [Symbol.iterator]() {
    let next = Math.ceil(this.from); 
    let last = this.to; 
    return { 
      next() {
        return (next <= last) ? { value: next++ } : { done: true }; 
      },
      [Symbol.iterator]() { return this; }
    };
  }
}

for(let x of new Range(1,10)) console.log(x);

[...new Range(-2,2)]

In addition to making your classes iterable, it can be quite useful to define functions that return iterable values.

Return an iterable object that iterates the result of applying f()  to each value from the source iterable

function map(iterable, f) {
  let iterator = iterable[Symbol.iterator]();
  return { 
    [Symbol.iterator]() { return this; },
    next() {
      let v = iterator.next();
      if (v.done) {
        return v;
      }
      else {
        return { value: f(v.value) };
      }
    }
  };
}

Return an iterable object that filters the specified iterable, iterating only those elements for which the predicate returns true

function filter(iterable, predicate) {
  let iterator = iterable[Symbol.iterator]();
  return { 
    [Symbol.iterator]() { return this; },
    next() {
      for(;;) {
       let v = iterator.next();
       if (v.done || predicate(v.value)) {
         return v;
       }
     }
   }
 };
}

Generators

Particularly useful when the values to be iterated are not the elements of a data structure, but the result of a computation.

To create a generator, you must first define a generator function - defined with the keyword function* rather than function

When you invoke a generator function, it does not actually execute the function body, but instead returns a generator object. This generator object is an iterator.

Calling its next() method causes the body of the generator function to run from the start (or whatever its current position is) until it reaches a yield statement.

The value of the yield statement becomes the value returned by the next() call on the iterator.

function* oneDigitPrimes() { 
  yield 2; 
  yield 3;
  yield 5; 
  yield 7;
}

let primes = oneDigitPrimes();

primes.next().value

primes.next().value

primes.next().value

primes.next().value

primes.next().done

Generators have a Symbol.iterator method to make them iterable

let sum = 0;
for(let prime of oneDigitPrimes()) sum += prime;

sum

Like regular functions, however, we can also define generators in expression form.

const seq = function*(from,to) {
  for(let i = from; i <= to; i++) yield i;
};

[...seq(3,5)]

In classes and object literals, we can use shorthand notation to omit the function keyword entirely when we define methods.

let o = {
  x: 1, y: 2, z: 3,
  *g() {
    for(let key of Object.keys(this)) {
      yield key;
    }
  }
};

Generators often make it particularly easy to define iterable classes.

*[Symbol.iterator]() {
  for(let x = Math.ceil(this.from); x <= this.to; x++)
    yield x;
}

Generator Examples

Generators are more interesting if they actually generate the values they yield by doing some kind of computation.

generator function that yields Fibonacci numbers

function* fibonacciSequence() {
  let x = 0, y = 1;
  for(;;) {
    yield y;
    [x, y] = [y, x+y];
  }
}

If this generator is used with the ... spread operator, it will loop until memory is exhausted and the program crashes.

Use it in a for/of loop, however

function fibonacci(n) {
  for(let f of fibonacciSequence()) {
    if (n-- <= 0) return f;
  }
}

fibonacci(20)

This kind of infinite generator becomes more useful with a take() generator like this

function* take(n, iterable) {
  let it = iterable[Symbol.iterator]();
  while(n-- > 0) {
    let next = it.next(); 
    if (next.done) return; 
    else yield next.value; 
  }
}

[...take(5, fibonacciSequence())]

Asynchronous Javascript

Promises, new in ES6, are objects that represent the not-yet-available result of an asynchronous operation.

The keywords async and await were introduced in ES2017 and provide new syntax that simplifies asynchronous programming by allowing you to structure your Promise based code as if it was synchronous.

Asynchronous iterators and the for/await loop were introduced in ES2018 and allow you to work with streams of asynchronous events using simple loops that appear synchronous.

Asynchronous Programming with Callbacks

Timers

let updateIntervalId = setInterval(checkForUpdates, 60000);

function stopCheckingForUpdates() {
  clearInterval(updateIntervalId);
} 

Events

Event-driven JavaScript programs register callback functions for specified types of events in specified contexts, and the web browser invokes those functions whenever the specified events occur. 

These callback functions are called event handlers or event listeners, and they are registered with addEventListener()

Ask the web browser to return an object representing the HTML <button> element that matches this CSS selector:

Now register a callback function to be invoked when the user clicks on that button

Network Events

JavaScript running in the browser can fetch data from a web server with code like this:

function getCurrentVersionNumber(versionCallback) {
  let request = new XMLHttpRequest();
  request.open("GET", "http://www.example.com/api/version");
  request.send();

  request.onload = function() {
    if (request.status === 200) {
      let currentVersion = parseFloat(request.responseText);
      versionCallback(null, currentVersion);
    } 
    else {
      versionCallback(response.statusText, null);
    }
  };

  request.onerror = request.ontimeout = function(e) {
    versionCallback(e.type, null);
  };
}

Promises

Promises, a core language feature designed to simplify asynchronous programming.

A Promise is an object that represents the result of an asynchronous computation. That result may or may not be ready yet, and the Promise API is intentionally vague about this: there is no way to synchronously get the value of a Promise; you can only ask the Promise to call a callback function when the value is ready.

One real problem with callback-based asynchronous programming is that it is common to end up with callbacks inside callbacks inside callbacks, with lines of code so highly indented that it is difficult to read. 

Promises allow this kind of nested callback to be re-expressed as a more linear Promise chain that tends to be easier to read and easier to reason about.

Another problem with callbacks is that they can make handling errors difficult. If an asynchronous function (or an asynchronously invoked callback) throws an exception, there is no way for that exception to propagate back to the initiator of the asynchronous operation. This is a fundamental fact about asynchronous programming: it breaks exception handling. Promises help here by standardizing a way to handle errors and providing a way for errors to propagate correctly through a chain of promises.

Note that Promises represent the future results of single asynchronous computations. They cannot be used to represent repeated asynchronous computations, however. 

We can’t use Promises to replace setInterval() because that function invokes a callback function repeatedly, which is something that Promises are just not designed to do.

Using Promises

How we would use this Promise returning utility function

getJSON(url).then(jsonData => {
  // callback function that will be asynchronously invoked with the parsed JSON value when it becomes available.
});

The Promise object defines a then() instance method. Instead of passing our callback function directly to getJSON(), we instead pass it to the then() method. When the HTTP response arrives, the body of that response is parsed as JSON, and the resulting parsed value is passed to the function that we passed to then().

If you call the then() method of a Promise object multiple times, each of the functions you specify will be called when the promised computation is complete. 

Unlike many event listeners, though, a Promise represents a single computation, and each function registered with then() will be invoked only once.

function displayUserProfile(profile) { ...}

getJSON("/api/user/profile").then(displayUserProfile);

HANDLING ERRORS WITH PROMISES

Asynchronous operations, particularly those that involve networking, can typically fail in a number of ways, and robust code has to be written to handle the errors that will inevitably occur.

if getJSON() runs normally, it passes its result to displayUserProfile(). If there is an error (the user is not logged in, the server is down, the user’s internet connection dropped, the request timed out, etc.), then getJSON() passes an Error object to handleProfileError().

In practice, it is rare to see two functions passed to then(). There is a better and more idiomatic way of handling errors when working with Promises. 

To understand it, first consider what happens if getJSON() completes normally but an error occurs in displayUserProfile(). That callback function is invoked asynchronously when getJSON() returns, so it is also asynchronous and cannot meaningfully throw an exception (because there is no code on the call stack to handle it).

getJSON("/api/user/profile").then(displayUserProfile).catch(handleProfileError);

With this code, a normal result from getJSON() is still passed to displayUserProfile(), but any error in getJSON() or in displayUserProfile() (including any exceptions thrown by displayUserProfile) get passed to handleProfileError().

Chaining Promises

One of the most important benefits of Promises is that they provide a natural way to express a sequence of asynchronous operations as a linear chain of then() method invocations, without having to nest each operation within the callback of the previous one.

fetch(documentURL)
.then(response => response.json()) 
.then(document => {return render(document); })
.then(rendered => {cacheInDatabase(rendered); })
.catch(error => handle(error));

has largely been replaced by the newer, Promise-based Fetch API. In its simplest form, this new HTTP API is just the function fetch(). That promise is fulfilled when the HTTP response begins to arrive and the HTTP status and headers are available.

fetch("/api/user/profile")
  .then(response => {
    if (response.ok &&  response.headers.get("Content-Type") === "application/json") {
      // What can we do here? We don't actually have the response body yet.
    }
});

But although the initial Promise is fulfilled, the body of the response may not yet have arrived. So these text() and json() methods for accessing the body of the response themselves return Promises.

fetch("/api/user/profile")
  .then(response => {
    return response.json();
  })
  .then(profile => {
    displayUserProfile(profile);
  });

There is a second then() in the chain, which means that the first invocation of the then() method must itself return a Promise. That is not how Promises work, however.

When we write a chain of .then() invocations, we are not registering multiple callbacks on a single Promise object. Instead, each invocation of the then() method returns a new Promise object. That new Promise object is not fulfilled until the function passed to then() is complete.

fetch(theURL)       // task 1; returns promise 1
  .then(callback1)  // task 2; returns promise 2
  .then(callback2); // task 3; returns promise 3

Resolving Promises

There is actually a fourth Promise object involved as which brings up the point of what it means for a Promise to be “resolved.” 

fetch() returns a Promise object which, when fulfilled, passes a Response object to the callback function we register. This Response object has .text(), .json(), and other methods to request the body of the HTTP response in various forms. But since the body may not yet have arrived, these methods must return Promise objects.

“task 2” calls the .json() method and returns its value. This is the fourth Promise object, and it is the return value of the callback1() function.

Let's consider:

function c1(response) {
  let p4 = response.json();
  return p4;
}

 // callback 1
 // returns promise 4

function c2(profile) { 
  displayUserProfile(profile);
}

// callback 2

let p1 = fetch("/api/user/profile");

promise 1, task 1

let p2 = p1.then(c1);

promise 2, task 2

let p3 = p2.then(c2);

promise 3, task 3

In order for Promise chains to work usefully, the output of task 2 must become the input to task 3. The input to task 3 is the body of the URL that was fetched, parsed as a JSON object. But the return value of callback c1 is not a JSON object, but Promise p4 for that JSON object.

When p1 is fulfilled, c1 is invoked, and task 2 begins. And when p2 is fulfilled, c2 is invoked, and task 3 begins. 

And when p2 is fulfilled, c2 is invoked, and task 3 begins. But just because task 2 begins when c1 is invoked,it does not mean that task 2 must end when c1 returns. 

Promises are about managing asynchronous tasks, and if task 2 is asynchronous, then that task will not be complete by the time the callback returns.

When you pass a callback c to the then() method, then() returns a Promise p and arranges to asynchronously invoke c at some later time. The callback performs some computation and returns a value v. When the callback returns, p is resolved with the value v. When a Promise is resolved with a value that is not itself a Promise, it is immediately fulfilled with that value. 

So if c returns a non-Promise, that return value becomes the value of p, p is fulfilled and we are done. But if the return value v is itself a Promise, then p is resolved but not yet fulfilled.

At this stage, p cannot settle until the Promise v settles. If v is fulfilled, then p will be fulfilled to the same value. If v is rejected, then p will be rejected for the same reason. This is what the “resolved” state of a Promise means

the Promise has become associated with, or “locked onto,” another Promise. We don’t know yet whether p will be fulfilled or rejected, but our callback c no longer has any control over that. p is “resolved” in the sense that its fate now depends entirely on what happens to Promise v.

Let’s bring this back to our URL-fetching example. When c1 returns p4, p2 is resolved. But being resolved is not the same as being fulfilled, so task 3 does not begin yet. When the full body of the HTTP response becomes available, then the .json() method can parse it and use that parsed value to fulfill p4. When p4 is fulfilled, p2 is automatically fulfilled as well, with the same parsed JSON value. At this point, the parsed JSON object is passed to c2, and task 3 begins.

More on Promises and Errors

With synchronous code, if you leave out error-handling code, you’ll at least get an exception and a stack trace that you can use to figure out what is going wrong. With asynchronous code, unhandled exceptions will often go unreported, and errors can occur silently, making them much harder to debug. The good news is that the .catch() method makes it easy to handle errors when working with Promises.

THE CATCH AND FINALLY METHODS

The .catch() method of a Promise is simply a shorthand way to call .then() with null as the first argument and an error-handling callback as the second argument.

Normal exceptions don’t work with asynchronous code. The .catch() method of Promises is an alternative that does work for asynchronous code.

fetch("/api/user/profile")
  .then(response => {
    if (!response.ok) {
      return null;
    }
    let type = response.headers.get("content-type");
    if (type !== "application/json") {
      throw new TypeError(`Expected JSON, got ${type}`);
    }
    return response.json();
  })
  .then(profile => {
    if (profile) {
      displayUserProfile(profile);
    }
    else { 
      displayLoggedOutProfilePage();
    }
  })
  .catch(e => {
  if (e instanceof NetworkError) {
    displayErrorMessage("Check your internet connection.");
  }
  else if (e instanceof TypeError) {
    displayErrorMessage("Something is wrong with our server!");
  }
  else {
    console.error(e);
  }
});

The first thing that could fail is the fetch() request itself. Let's say p1 was rejected with a NetworkError object.

We didn’t pass an error-handling callback function as the second argument to the .then() call, so p2 rejects as well with the same NetworkError object.

Without a handler, though, p2 is rejected, and then p3 is rejected for the same reason.

At this point, the c3 error-handling callback is called, and the NetworkError-specific code within it runs.

There are a couple of things worth noting about this code. First, notice that the error object thrown with a regular, synchronous throw statement ends up being handled asynchronously with a .catch() method invocation in a Promise chain. This should make it clear why this shorthand method is preferred over passing a second argument to .then(), and also why it is so idiomatic to end Promise chains with a .catch() call.

it is also perfectly valid to use .catch() elsewhere in a Promise chain.  If one of the stages in your Promise chain can fail with an error, and if the error is some kind of recoverable error that should not stop the rest of the chain from running, then you can insert a .catch() call in the chain, resulting in code that might look like this:

startAsyncOperation()
  .then(doStageTwo)
  .catch(recoverFromStageTwoError)
  .then(doStageThree)
  .then(doStageFour)
  .catch(logStageThreeAndFourErrors);

If the callback returns normally, then the .catch() callback will be skipped, and the return value of the previous callback will become the input to the next .then() callback. 

Once an error has been passed to a .catch() callback, it stops propagating down the Promise chain. A .catch() callback can throw a new error, but if it returns normally, than that return value is used to resolve and/or fulfill the associated Promise, and
the error stops propagating.

Sometimes, in complex network environments, errors can occur more or less at random, and it can be appropriate to handle those errors by simply retrying the asynchronous request.

queryDatabase()
  .catch(e => wait(500).then(queryDatabase)) 
  .then(displayTable)
  .catch(displayDatabaseError);

Promises in Parallel

Sometimes,we want to execute a number of asynchronous operations in parallel. The function Promise.all() can do this. Promise.all() takes an array of Promise objects as its input and returns a Promise. 

The returned Promise will be rejected if any of the input Promises are rejected. Otherwise, it will be fulfilled with an array of the fulfillment values of each of the input Promises.

const urls = [ /* zero or more URLs here */ ];
promises = urls.map(url => fetch(url).then(r => r.text()));
Promise.all(promises)
  .then(bodies => { /* do something with the array of strings */ })
  .catch(e => console.error(e));

The Promise returned by Promise.all() rejects when any of the input Promises is rejected. This happens immediately upon the first rejection and can happen while other input Promises are still pending. In ES2020, Promise.allSettled() takes an array of input
Promises and returns a Promise, just like Promise.all() does. But Promise.allSettled() never rejects the returned Promise, and it does not fulfill that Promise until all of the input Promises have settled. The Promise resolves to an array of objects, with one object for each input Promise. Each of these returned objects has a status property set to “fulfilled” or “rejected.” If the status is “fulfilled”, then the object will also have a value property that gives the fulfillment value. And if the status is “rejected”, then the object will also have a reason property that gives the error or rejection value of the corresponding Promise.

Promise.allSettled([Promise.resolve(1), Promise.reject(2),3]).then(results => {
  results[0] // => { status: "fulfilled", value: 1 }
  results[1] // => { status: "rejected", reason: 2 }
  results[2] // => { status: "fulfilled", value: 3 }
});

Occasionally, you may want to run a number of Promises at once but may only care about the value of the first one to fulfill. In that case, you can use Promise.race() instead of Promise.all(). It returns a Promise that is fulfilled or rejected when the first of the Promises in the input array is fulfilled or rejected.

Making Promises

Promises in Sequence

async and await

These new keywords dramatically simplify the use of Promises and allow us to write Promise-based, asynchronous code that looks like synchronous code that blocks while waiting for network responses or other asynchronous events.

Asynchronous code can’t return a value or throw an exception the way that regular synchronous code can. And this is why Promises are designed the way the are. The value of a fulfilled Promise is like the return value of a synchronous function. And the value of a rejected Promise is like a value thrown by a synchronous function.

async and await take efficient, Promise-based code and hide the Promises so that your asynchronous code can be as easy to read and as easy to reason about as inefficient, blocking, synchronous code.

Given a Promise object p, the expression await p waits until p settles. If p fulfills, then the value of await p is the fulfillment value of p. On the other hand, if p is rejected, then the await p expression throws the rejection value of p.

let response = await fetch("/api/user/profile");
let profile = await response.json();

It is critical to understand right away that the await keyword does not cause your program to block and literally do nothing until the specified Promise settles. The code remains asynchronous, and the await simply disguises this fact. This means that any code that uses await is itself asynchronous.

async Functions

Because any code that uses await is asynchronous, there is one critical rule: you can only use the await keyword within functions that have been declared with the async keyword.

async function getHighScore() {
  let response = await fetch("/api/user/profile");
  let profile = await response.json();
  return profile.highScore;
}

Declaring a function async means that the return value of the function will be a Promise even if no Promise-related code appears in the body of the function.

The getHighScore() function is declared async, so it returns a Promise. And because it returns a Promise, we can use the await keyword with it:

displayHighScore(await getHighScore());

Awaiting Multiple Promises

Suppose that we’ve written our getJSON() function using async:

async function getJSON(url) {
  let response = await fetch(url);
  let body = await response.json();
  return body;
}

And now suppose that we want to fetch two JSON values with this function

let value1 = await getJSON(url1);

let value2 = await getJSON(url2);

The problem with this code is that it is unnecessarily sequential: the fetch of the second URL will not begin until the first fetch is complete. If the second URL does not depend on the value obtained from the firstURL, then we should probably try to fetch the two values at the same time.

let [value1, value2] = await Promise.all([getJSON(url1), getJSON(url2)]);

The for/await Loop

Suppose you have an array of URLs:

const urls = [url1, url2, url3];

You can call fetch() on each URL to get an array of Promises:

const promises = urls.map(url => fetch(url));

We could now use Promise.all() to wait for all the Promises in the array to be fulfilled. But suppose we want the results of the first fetch as soon as they become available and don’t want to wait for all the URLs to be fetched.

for(const promise of promises) {
  response = await promise;
  handle(response);
}

for await (const response of promises) {
  handle(response);
}

both examples will only work if they are within functions declared async; a for/await loop is no different than a regular await expression in that way

Implementing Asynchronous Iterators