JavaScript Function : Object

constructor

Functions are reusable pieces of code that can be executed with different parameters.

Spec

Function([param1 : String, [param2 : String, [...]]], body : String) : Function

Same as new Function(param1, param2, ..., body).

Spec

Constructors

new Function([param1 : String, [param2 : String, [...]]], body : String) : Function

Creates a new normal Function that has the supplied parameter names and body. If any parameter name contains a ',', it will be split on the ',' and each component will be added as a parameter. Unless the body needs to be modified at run time, Functions are typically created with the function keyword.

See example below on how to construct generator (function*()) and async functions (async function()).

Example:

// The following result in equivalent behavior, but the first
// should be used unless the body changes at run time.
var f1 = function(x, y) { return x + y; };
var f2 = Function('x', 'y','return x + y;');
var f3 = Function('x, y', 'return x + y;');
var f4 = new Function('x', 'y', 'return x + y;');

// The following demonstrate creating generator and async functions
var GeneratorFunction = Object.getPrototypeOf(function*(){}).constructor;

// The following are equivalent:
var g1 = function*(x, y) { yield x; yield y; }
var g2 = GeneratorFunction('x', 'y', 'yield x; yield y;');
var g3 = new GeneratorFunction('x, y', 'yield x; yield y;');

var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;

// The following are equivalent:
var a1 = async function(url) { return (await fetch(url)).status; }
var a2 = AsyncFunction('url', 'return (await fetch(url)).status;');
var a3 = new AsyncFunction('url', 'return (await fetch(url)).status;');

var AsyncGeneratorFunction = Object.getPrototypeOf(async function*(){}).constructor;

// The following are equivalent:
var ag1 = async function*(x, y) { yield await x; yield await y; }
var ag2 = AsyncGeneratorFunction('x', 'y', 'yield await x; yield await y;');
var ag3 = new AsyncGeneratorFunction('x, y', 'yield await x; yield await y;');

Run

Results:

Spec

Instance Properties

length : Number

The number of parameters specified when the function was defined.

Example:

var add = function(x, y) { return x + y; };
console.log(add.length);

Run

Results:

Spec

prototype : Object

The prototype for objects created by calling this function with the new keyword.

Example:

var Button = function(content) { 
  this.content = content;
};
Button.prototype.click = function() {
  console.log(this.content + ' clicked');
}

var myButton = new Button('OK');
myButton.click();

Run

Results:

Spec

Instance Methods

apply([thisArg : Object, [parameters : Array]]) : Object

Call this with the this value inside the function bound to thisArg and the parameters to the function from parameters. Returns the result of the function call. See Reflect.apply().

Example:

var whatsThis = function() { console.log(this); }
whatsThis.apply('hello');

// Call a function that takes a variable number of args
var numbers = [3, 20, 1, 45];
console.log(Math.max.apply(undefined, numbers));

Run

Results:

Spec

bind(thisArg : Object, [param1 : Object, [param2 : Object, [...]]]) : Function

Returns a new function that, when called, will have this equal to thisArg, the first parameter equal to param1, the second parameter equal to param2, etc.

Example:

var Button = function(content) { 
  this.content = content;
};
Button.prototype.click = function() {
  console.log(this.content + ' clicked');
}

var myButton = new Button('OK');
myButton.click();

var looseClick = myButton.click;
looseClick();

var boundClick = myButton.click.bind(myButton);
boundClick();

// Example showing binding some parameters
var sum = function(a, b) {
  return a + b;
};

var add5 = sum.bind(null, 5);
console.log(add5(10));

Run

Results:

Spec

call([thisArg : Object, [param1 : Object, [param2 : Object, [...]]]]) : Object

Call this with the this value inside the function bound to thisArg and parameters (param1, param2, ...). Returns the result of the function call.

Example:

var whatsThis = function() { console.log(this); }
whatsThis.call('hello');

// Call a function that takes a variable number of args
console.log(Math.max.call(undefined, 3, 20, 1, 45));

Run

Results:

Spec

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