Function.prototype.bind() - JavaScript | MDN

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Function.prototype.bind()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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

Try it

const module = {
  x: 42,
  getX() {
    return this.x;
  },
};

const unboundGetX = module.getX;
console.log(unboundGetX()); // The function gets invoked at the global scope
// Expected output: undefined

const boundGetX = unboundGetX.bind(module);
console.log(boundGetX());
// Expected output: 42

Syntax

js
bind(thisArg)
bind(thisArg, arg1)
bind(thisArg, arg1, arg2)
bind(thisArg, arg1, arg2, /* …, */ argN)

Parameters

thisArg

The value to be passed as the this parameter to the target function func when the bound function is called. If the function is not in strict mode, null and undefined will be replaced with the global object, and primitive values will be converted to objects. The value is ignored if the bound function is constructed using the new operator.

arg1, …, argN Optional

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

Return value

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

Description

The bind() function creates a new bound function. Calling the bound function generally results in the execution of the function it wraps, which is also called the target function. The bound function will store the parameters passed — which include the value of this and the first few arguments — as its internal state. These values are stored in advance, instead of being passed at call time. You can generally see const boundFn = fn.bind(thisArg, arg1, arg2) as being equivalent to const boundFn = (...restArgs) => fn.call(thisArg, arg1, arg2, ...restArgs) for the effect when it's called (but not when boundFn is constructed).

A bound function can be further bound by calling boundFn.bind(thisArg, /* more args */), which creates another bound function boundFn2. The newly bound thisArg value is ignored, because the target function of boundFn2, which is boundFn, already has a bound this. When boundFn2 is called, it would call boundFn, which in turn calls fn. The arguments that fn ultimately receives are, in order: the arguments bound by boundFn, arguments bound by boundFn2, and the arguments received by boundFn2.

js
"use strict"; // prevent `this` from being boxed into the wrapper object

function log(...args) {
  console.log(this, ...args);
}
const boundLog = log.bind("this value", 1, 2);
const boundLog2 = boundLog.bind("new this value", 3, 4);
boundLog2(5, 6); // "this value", 1, 2, 3, 4, 5, 6

A bound function may also be constructed using the new operator if its target function is constructable. Doing so acts as though the target function had instead been constructed. The prepended arguments are provided to the target function as usual, while the provided this value is ignored (because construction prepares its own this, as seen by the parameters of Reflect.construct). If the bound function is directly constructed, new.target will be the target function instead. (That is, the bound function is transparent to new.target.)

js
class Base {
  constructor(...args) {
    console.log(new.target === Base);
    console.log(args);
  }
}

const BoundBase = Base.bind(null, 1, 2);

new BoundBase(3, 4); // true, [1, 2, 3, 4]

However, because a bound function does not have the prototype property, it cannot be used as a base class for extends.

js
class Derived extends class {}.bind(null) {}
// TypeError: Class extends value does not have valid prototype property undefined

When using a bound function as the right-hand side of instanceof, instanceof would reach for the target function (which is stored internally in the bound function) and read its prototype instead.

js
class Base {}
const BoundBase = Base.bind(null, 1, 2);
console.log(new Base() instanceof BoundBase); // true

The bound function has the following properties:

length

The length of the target function minus the number of arguments being bound (not counting the thisArg parameter), with 0 being the minimum value.

name

The name of the target function plus a "bound " prefix.

The bound function also inherits the prototype chain of the target function. However, it doesn't have other own properties of the target function (such as static properties if the target function is a class).

Examples

Creating a bound function

The most common 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:

js
// Top-level 'this' is bound to 'globalThis' in scripts.
this.x = 9;
const module = {
  x: 81,
  getX() {
    return this.x;
  },
};

// The 'this' parameter of 'getX' is bound to 'module'.
console.log(module.getX()); // 81

const retrieveX = module.getX;
// The 'this' parameter of 'retrieveX' is bound to 'globalThis' in non-strict mode.
console.log(retrieveX()); // 9

// Create a new function 'boundGetX' with the 'this' parameter bound to 'module'.
const boundGetX = retrieveX.bind(module);
console.log(boundGetX()); // 81

Note: If you run this example in strict mode, the this parameter of retrieveX will be bound to undefined instead of globalThis, causing the retrieveX() call to fail.

If you run this example in an ECMAScript module, top-level this will be bound to undefined instead of globalThis, causing the this.x = 9 assignment to fail.

If you run this example in a Node CommonJS module, top-level this will be bound to module.exports instead of globalThis. However, the this parameter of retrieveX will still be bound to globalThis in non-strict mode and to undefined in strict mode. Therefore, in non-strict mode (the default), the retrieveX() call will return undefined because this.x = 9 is writing to a different object (module.exports) from what getX is reading from (globalThis).

In fact, some built-in "methods" are also getters that return bound functions — one notable example being Intl.NumberFormat.prototype.format(), which, when accessed, returns a bound function that you can directly pass as a callback.

Partially applied functions

Another 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.

js
function list(...args) {
  return args;
}

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

console.log(list(1, 2, 3)); // [1, 2, 3]

console.log(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);

console.log(leadingThirtySevenList()); // [37]
console.log(leadingThirtySevenList(1, 2, 3)); // [37, 1, 2, 3]
console.log(addThirtySeven(5)); // 42
console.log(addThirtySeven(5, 10)); // 42
// (the last argument 10 is ignored)

With setTimeout()

By default, within setTimeout(), the this keyword will be set to globalThis, which is window in browsers. 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.

js
class LateBloomer {
  constructor() {
    this.petalCount = Math.floor(Math.random() * 12) + 1;
  }
  bloom() {
    // Declare bloom after a delay of 1 second
    setTimeout(this.declare.bind(this), 1000);
  }
  declare() {
    console.log(`I am a beautiful flower with ${this.petalCount} petals!`);
  }
}

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

You can also use arrow functions for this purpose.

js
class LateBloomer {
  bloom() {
    // Declare bloom after a delay of 1 second
    setTimeout(() => this.declare(), 1000);
  }
}

Bound functions used as constructors

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.

js
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'

// The thisArg's value doesn't matter because it's ignored
const YAxisPoint = Point.bind(null, 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. new.target, instanceof, this etc. all work as expected, as if the constructor was never bound. The only difference is that it can no longer be used for extends.

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. If you call it without new, the bound this is suddenly not ignored.

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

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

// The modifications to `this` is now observable from the outside
console.log(emptyObj); // { x: 0, y: 13 }

If you wish to restrict a bound function to only be callable with new, or only be callable without new, the target function must enforce that restriction, such as by checking new.target !== undefined or using a class instead.

Binding classes

Using bind() on classes preserves most of the class's semantics, except that all static own properties of the current class are lost. However, because the prototype chain is preserved, you can still access static properties inherited from the parent class.

js
class Base {
  static baseProp = "base";
}

class Derived extends Base {
  static derivedProp = "derived";
}

const BoundDerived = Derived.bind(null);
console.log(BoundDerived.baseProp); // "base"
console.log(BoundDerived.derivedProp); // undefined
console.log(new BoundDerived() instanceof Derived); // true

Transforming methods to utility functions

bind() is also helpful in cases where you want to transform a method which requires a specific this value to a plain utility function that accepts the previous this parameter as a normal parameter. This is similar to how general-purpose utility functions work: instead of calling array.map(callback), you use map(array, callback), which allows you to use map with array-like objects that are not arrays (for example, arguments) without mutating Object.prototype.

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:

js
const slice = Array.prototype.slice;

// …

slice.call(arguments);

Note that you can't save slice.call and call it as a plain function, because the call() method also reads its this value, which is the function it should call. In this case, you can use bind() to bind the value of this for call(). In the following piece of code, slice() is a bound version of Function.prototype.call(), with the this value bound to Array.prototype.slice(). This means that additional call() calls can be eliminated:

js
// Same as "slice" in the previous example
const unboundSlice = Array.prototype.slice;
const slice = Function.prototype.call.bind(unboundSlice);

// …

slice(arguments);