Review of Week 2 Day 4 Learning Objectives

1. Identify the difference between const, let, and var declarations

2. Explain the difference between const, let, and var declarations

var a = "a";

• var is the historical keyword used for variable declaration.
• var declares variables in function scope, or global scope if not inside a function.
• We consider var to be deprecated and it is never used in this course.

let b = "b";

• let is the keyword we use most often for variable declaration.
• let declares variables in block scope.
• variables declared with let are re-assignable.

const c = "c";

• const is a specialized form of let that can only be used to initialize a variable.
• Except when it is declared, you cannot assign to a const variable.
• const scopes variables the same way that let does.

3. Predict the evaluation of code that utilizes function scope, block scope, lexical scope, and scope chaining

Consider this run function, inside which foo and bar have function scope. i and baz are scoped to the block expression.

// function and block scope in this example
function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar);

  {
    console.log(foo);
    let baz = "Bazz";
    console.log(baz);
  }

  console.log(baz); // ReferenceError
}

run();

Notice that referencing baz from outside it’s block results in JavaScript throwing a ReferenceError.

Consider this run function, inside of which foo has function scope.

function run() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo
}

run();

Consider this func1 function and it’s nested scopes.

// global scope
function func1(arg1) {
  // func1 scope

  return function func2(arg2) {
    // func2 scope

    return function func3(arg3) {
      // func3 scope

      console.log(arg1, arg2, arg3);
    }
  }
}

6. Implement a closure and explain how the closure effects scope

const adder = (arg1) => {
  return (arg2) => {
    return arg1 + arg2;
  }
};

const func2 = adder(2);
const result = func2(2);
console.log(result); // => 4;

```js function