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.
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.
The source for this interactive example is stored in a GitHub repository. If you’d like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
let boundFunc = func.bind(thisArg[, arg1[, arg2[, …argN]]])
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.
Because older browsers are generally also slower browsers, it is far more critical than most people recognize to create performance polyfills to make the browsing experience in outdated browsers slightly less horrible.
Thus, presented below are two options for Function.prototype.bind() polyfills:
new operator.new operator on bound functions.Generally, in most code it’s very rare to see new used on a bound function, so it is generally best to go with the first option.
if (!Function.prototype.bind) (function(){
var slice = Array.prototype.slice;
Function.prototype.bind = function() {
var thatFunc = this, thatArg = arguments[0];
var args = slice.call(arguments, 1);
if (typeof thatFunc !== 'function') {
throw new TypeError('Function.prototype.bind - ' +
'what is trying to be bound is not callable');
}
return function(){
var funcArgs = args.concat(slice.call(arguments))
return thatFunc.apply(thatArg, funcArgs);
};
};
})();
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(){
var ArrayPrototypeSlice = Array.prototype.slice;
Function.prototype.bind = function(otherThis) {
if (typeof this !== 'function') {
throw new TypeError('Function.prototype.bind - what is trying to be bound is not callable');
}
var baseArgs= ArrayPrototypeSlice.call(arguments, 1),
baseArgsLength = baseArgs.length,
fToBind = this,
fNOP = function() {},
fBound = function() {
baseArgs.length = baseArgsLength;
baseArgs.push.apply(baseArgs, arguments);
return fToBind.apply(
fNOP.prototype.isPrototypeOf(this) ? this : otherThis, baseArgs
);
};
if (this.prototype) {
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:
Array.prototype.slice(),
Array.prototype.concat(),
Function.prototype.call()
and Function.prototype.apply(),
built-in methods to have their original values.
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.)prototype property. (Proper bound functions have none.)length
property does not agree with that mandated by ECMA-262: it creates functions with length of
0. A full implementation—depending on the length of the target function and the number of pre-specified
arguments—may return a non-zero length.
name
property is not derived from the original function name. According to ECMA-262, name of the returned bound function
should be “bound” + name of target function (note the space character).If you choose to use this partial implementation, you must not rely on those cases where behavior deviates from ECMA-262, 5th edition! Thankfully, these deviations from the specification rarely (if ever) come up in most coding situations. If you do not understand any of the deviations from the specification above, then it is safe in this particular case to not worry about these noncompliant deviation details.
If it’s absolutely necessary and performance is not a concern, a far slower (but more specification-compliant solution) can be found at https://github.com/Raynos/function-bind.
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:
pre data-role="codeBlock" data-info="js" class="language-javascript">this.x = 9;
const module = {
x: 81,
getX: function() { return this.x; }
};
module.getX();
const retrieveX = module.getX;
retrieveX();
const boundGetX = retrieveX.bind(module);
boundGetX();
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 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);
const result1 = addArguments(1, 2);
const leadingThirtysevenList = list.bind(null, 37);
const addThirtySeven = addArguments.bind(null, 37);
const list2 = leadingThirtysevenList();
const list3 = leadingThirtysevenList(1, 2, 3);
const result2 = addThirtySeven(5);
const result3 = addThirtySeven(5, 10);
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.floor(Math.random() * 12) + 1;
}
LateBloomer.prototype.bloom = function() {
window.setTimeout(this.declare.bind(this), 1000);
};
LateBloomer.prototype.declare = function() {
console.log(`I am a beautiful flower with ${this.petalCount} petals!`);
};
const flower = new LateBloomer();
flower.bloom();
Warning: This section demonstrates JavaScript capabilities and documents some edge cases of the bind() method.
The methods shown below are not the best way to do things, and probably should not be used in any production environment.
Bound functions are automatically suitable for use with the new
operator to construct new instances created by the target function. When a bound function is used to construct a
value, the provided this is ignored.
However, provided arguments are still prepended to the constructor call:
pre data-role="codeBlock" data-info="js" class="language-javascript">function Point(x, y) {
this.x = x;
this.y = y;
}
Point.prototype.toString = function() {
return `${this.x},${this.y}`;
};
const p = new Point(1, 2);
p.toString();
const YAxisPoint = Point.bind(null, 0);
const emptyObj = {};
const YAxisPoint = Point.bind(emptyObj, 0);
const axisPoint = new YAxisPoint(5);
axisPoint.toString();
axisPoint instanceof Point;
axisPoint instanceof YAxisPoint;
new YAxisPoint(17, 42) instanceof Point;
Note that you need not do anything special to create a bound function for use with new.
The corollary is that you need not do anything special to create a bound function to be called plainly, even if you
would rather require the bound function to only be called using new.
YAxisPoint(13);
`${emptyObj.x},${emptyObj.y}`;
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.prototype,
with the this value set to the slice()
function of Array.prototype. This means that additional apply() calls can be eliminated:
const unboundSlice = Array.prototype.slice;
const slice = Function.prototype.apply.bind(unboundSlice);
slice(arguments);
| Specification |
|---|
| [ECMAScript (ECMA-262) |
| The definition of ‘Function.prototype.bind’ in that specification.](https://tc39.es/ecma262/#sec-function.prototype.bind) |
The compatibility table on this page is generated from structured data. If you’d like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Update compatibility data on GitHub
| Desktop | Mobile | Server | |
|---|---|---|---|
| Chrome | Edge | Firefox | |
| — | — | — | — |
bind |
Chrome Full support 7 | Edge Full support 12 | Firefox Full support 4 |
Our team will review your report. Once we verify the information you have supplied we will update this browser compatability table accordingly.
You can join the GitHub repository to see updates and commits for this table data:
https://github.com/mdn/browser-compat-data
Our goal is to provide accurate, real values for all our compatibility data tables. Notifying MDN of inaccurate data or supplying new data pushes us further towards our goal of providing 100% real values to the developer community.
Thank you for helping.
Please select the browser or browsers which are affected.
Briefly outline the issue you are highlighting. Minimum 10 and maximum 1,000 characters.
Browser documentation and release notes are good supporting items to accompany your message. A demo hosted on services like Codepen or JSBin are perfect for providing real examples of your findings.
Connection error:Sorry, we can’t seem to reach the server. We are working to fix the problem. Please try again later.
Full support
Full support