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.

Syntax

fun.bind(thisArg[, arg1[, arg2[, ...]]])

Parameters

thisArg

The value to be passed as the this parameter to the target function when the bound function is called. The value is ignored if the bound function is constructed using the new operator.

arg1, arg2, ...

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

Description

The bind() function creates a new function (a bound function) with the same function body (internal call property in ECMAScript 5 terms) as the function it is being called on (the bound function's target function) with the this value bound to the first argument of bind(), which cannot be overridden. bind() also accepts leading default arguments to provide to the target function when the bound function is called. 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.

Examples

Creating a bound 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 that 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; 
var module = {
  x: 81,
  getX: function() { return this.x; }
};

module.getX(); // 81

var retrieveX = module.getX;
retrieveX(); 
// 9, because in this case, "this" refers
// to the global object

// Create a new function with 'this' bound to module
// New programmers (like myself) might confuse the
// global var getX with module's property getX
var boundGetX = retrieveX.bind(module);
boundGetX(); // 81

Partially applied functions (currying)

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 the arguments passed to the bound function, whenever the bound function is called.

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

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

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

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

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

With setTimeout

By default within window.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.ceil(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!');
};

var flower = new LateBloomer();
flower.bloom();  
// after 1 second, triggers the 'declare' method

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:

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

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

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

var emptyObj = {};
var YAxisPoint = Point.bind(emptyObj, 0/*x*/);
// not supported in the polyfill below,
// works fine with native bind:
var YAxisPoint = Point.bind(null, 0/*x*/);

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

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

Note that you need do nothing special to create a bound function for use with new. The corollary is that you need do nothing 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
// ...continuing 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 use of a bound function only using new, or only by calling it, the target function must enforce that restriction.

Creating shortcuts

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:

var 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.prototype, 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
var unboundSlice = Array.prototype.slice;
var slice = Function.prototype.apply.bind(unboundSlice);

// ...

slice(arguments);

Polyfill

The bind function is an addition to ECMA-262, 5th edition; as such it may not be present in all browsers. You can partially work around this by inserting the following code at the beginning of your scripts, allowing use of much of the functionality of bind() in implementations that do not natively support it.

if (!Function.prototype.bind) {
  Function.prototype.bind = function(oThis) {
    if (typeof this !== 'function') {
      // closest thing possible to the ECMAScript 5
      // internal IsCallable function
      throw new TypeError('Function.prototype.bind - what is trying to be bound is not callable');
    }

    var aArgs   = Array.prototype.slice.call(arguments, 1),
        fToBind = this,
        fNOP    = function() {},
        fBound  = function() {
          return fToBind.apply(this instanceof fNOP
                 ? this
                 : oThis,
                 aArgs.concat(Array.prototype.slice.call(arguments)));
        };

    if (this.prototype) {
      // Function.prototype doesn't have a prototype property
      fNOP.prototype = this.prototype; 
    }
    fBound.prototype = new fNOP();

    return fBound;
  };
}

Some of the many differences (there may well be others, as this list does not seriously attempt to be exhaustive) between this algorithm and the specified algorithm are:

• The partial implementation relies on Array.prototype.slice(), Array.prototype.concat(), Function.prototype.call() and Function.prototype.apply(), built-in methods to have their original values.
• The partial implementation creates functions that do not have immutable "poison pill" caller and arguments properties that throw a TypeError upon get, set, or deletion. (This could be added if the implementation supports Object.defineProperty, or partially implemented [without throw-on-delete behavior] if the implementation supports the __defineGetter__ and __defineSetter__ extensions.)
• The partial implementation creates functions that have a prototype property. (Proper bound functions have none.)
• The partial implementation creates bound functions whose length property does not agree with that mandated by ECMA-262: it creates functions with length 0, while a full implementation, depending on the length of the target function and the number of pre-specified arguments, may return a non-zero length.

If you choose to use this partial implementation, you must not rely on those cases where behavior deviates from ECMA-262, 5th edition! With some care, however (and perhaps with additional modification to suit specific needs), this partial implementation may be a reasonable bridge to the time when bind() is widely implemented according to the specification.

Specifications

Browser compatibility

See also

• Function.prototype.apply()
• Function.prototype.call()
• Functions