Function.prototype.bind()

The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.

thisArg

The value to be passed as the this parameter to the target function func when the bound function is called. The value is ignored if the bound function is constructed using the new operator. When using bind to create a function (supplied as a callback) inside a setTimeout, any primitive value passed as thisArg is converted to object. If no arguments are provided to bind, or if the thisArg is null or undefined, the this of the executing scope is treated as the thisArg for the new function.

arg1, arg2, ...argN Optional

Arguments to prepend to arguments provided to the bound function when invoking func.

A copy of the given function with the specified this value, and initial arguments (if provided).

The bind() function creates a new bound function, which is an exotic function object (a term from ECMAScript 2015) that wraps the original function object. Calling the bound function generally results in the execution of its wrapped function.

A bound function has the following internal properties:

[[BoundTargetFunction]]

The wrapped function object

[[BoundThis]]

The value that is always passed as this value when calling the wrapped function.

[[BoundArguments]]

A list of values whose elements are used as the first arguments to any call to the wrapped function.

[[Call]]

Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression.

When a bound function is called, it calls internal method [[Call]] on [[BoundTargetFunction]], with following arguments Call(boundThis, ...args). Where boundThis is [[BoundThis]], args is [[BoundArguments]], followed by the arguments passed by the function call.

A bound function may also be constructed using the new operator. Doing so acts as though the target function had instead been constructed. The provided this value is ignored, while prepended arguments are provided to the emulated function.

The simplest use of bind() is to make a function that, no matter how it is called, is called with a particular this value.

A common mistake for new JavaScript programmers is to extract a method from an object, then to later call that function and expect it to use the original object as its this (e.g., by using the method in callback-based code).

Without special care, however, the original object is usually lost. Creating a bound function from the function, using the original object, neatly solves this problem:

this.x = 9;    // 'this' refers to global 'window' object here in a browser
const module = {
  x: 81,
  getX: function() { return this.x; }
};

module.getX();
//  returns 81

const retrieveX = module.getX;
retrieveX();
//  returns 9; the function gets invoked at the global scope

//  Create a new function with 'this' bound to module
//  New programmers might confuse the
//  global variable 'x' with module's property 'x'
const boundGetX = retrieveX.bind(module);
boundGetX();
//  returns 81

The next simplest use of bind() is to make a function with pre-specified initial arguments.

These arguments (if any) follow the provided this value and are then inserted at the start of the arguments passed to the target function, followed by whatever arguments are passed to the bound function at the time it is called.

function list() {
  return Array.prototype.slice.call(arguments);
}

function addArguments(arg1, arg2) {
  return arg1 + arg2;
}

const list1 = list(1, 2, 3);
//  [1, 2, 3]

const result1 = addArguments(1, 2);
//  3

// Create a function with a preset leading argument
const leadingThirtysevenList = list.bind(null, 37);

// Create a function with a preset first argument.
const addThirtySeven = addArguments.bind(null, 37);

const list2 = leadingThirtysevenList();
//  [37]

const list3 = leadingThirtysevenList(1, 2, 3);
//  [37, 1, 2, 3]

const result2 = addThirtySeven(5);
//  37 + 5 = 42

const result3 = addThirtySeven(5, 10);
//  37 + 5 = 42
//  (the second argument is ignored)

By default within setTimeout(), the this keyword will be set to the window (or global) object. When working with class methods that require this to refer to class instances, you may explicitly bind this to the callback function, in order to maintain the instance.

function LateBloomer() {
  this.petalCount = Math.floor(Math.random() * 12) + 1;
}

// Declare bloom after a delay of 1 second
LateBloomer.prototype.bloom = function() {
  window.setTimeout(this.declare.bind(this), 1000);
};

LateBloomer.prototype.declare = function() {
  console.log(`I am a beautiful flower with ${this.petalCount} petals!`);
};

const flower = new LateBloomer();
flower.bloom();
//  after 1 second, calls 'flower.declare()'

Bound functions are automatically suitable for use with the new operator to construct new instances created by the target function. When a bound function is used to construct a value, the provided this is ignored.

However, provided arguments are still prepended to the constructor call:

function Point(x, y) {
  this.x = x;
  this.y = y;
}

Point.prototype.toString = function() {
  return `${this.x},${this.y}`;
};

const p = new Point(1, 2);
p.toString();
// '1,2'

//  not supported in the polyfill below,

//  works fine with native bind:

const YAxisPoint = Point.bind(null, 0/*x*/);

const emptyObj = {};
const YAxisPoint = Point.bind(emptyObj, 0/*x*/);

const axisPoint = new YAxisPoint(5);
axisPoint.toString();                    // '0,5'

axisPoint instanceof Point;              // true
axisPoint instanceof YAxisPoint;         // true
new YAxisPoint(17, 42) instanceof Point; // true

Note that you need not do anything special to create a bound function for use with new.

The corollary is that you need not do anything special to create a bound function to be called plainly, even if you would rather require the bound function to only be called using new.

//  Example can be run directly in your JavaScript console
//  ...continued from above

//  Can still be called as a normal function
//  (although usually this is undesired)
YAxisPoint(13);

`${emptyObj.x},${emptyObj.y}`;
// >  '0,13'

If you wish to support the use of a bound function only using new, or only by calling it, the target function must enforce that restriction.

bind() is also helpful in cases where you want to create a shortcut to a function which requires a specific this value.

Take Array.prototype.slice(), for example, which you want to use for converting an array-like object to a real array. You could create a shortcut like this:

const slice = Array.prototype.slice;

// ...

slice.apply(arguments);

With bind(), this can be simplified.

In the following piece of code, slice() is a bound function to the apply() function of Function, with the this value set to the slice() function of Array.prototype. This means that additional apply() calls can be eliminated:

//  same as "slice" in the previous example
const unboundSlice = Array.prototype.slice;
const slice = Function.prototype.apply.bind(unboundSlice);

// ...

slice(arguments);

• Polyfill of Function.prototype.bind in core-js
• Function.prototype.apply()
• Function.prototype.call()
• Functions