Function.prototype.apply()

The apply() method calls a function with a given this value and arguments provided as an array (or an array-like object).

Syntax

fun.apply(thisArg, [argsArray])

Parameters

thisArg

The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.

argsArray

An array-like object, specifying the arguments with which fun should be called, or null or undefined if no arguments should be provided to the function. Starting with ECMAScript 5 these arguments can be a generic array-like object instead of an array. See below for browser compatibility information.

Return value

The result of calling the function with the specified this value and arguments.

Description

You can assign a different this object when calling an existing function. this refers to the current object, the calling object. With apply, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.

apply is very similar to call(), except for the type of arguments it supports. You can use an arguments array instead of a named set of parameters. With apply, you can use an array literal, for example, fun.apply(this, ['eat', 'bananas']), or an Array object, for example, fun.apply(this, new Array('eat', 'bananas')).

You can also use arguments for the argsArray parameter. arguments is a local variable of a function. It can be used for all unspecified arguments of the called object. Thus, you do not have to know the arguments of the called object when you use the apply method. You can use arguments to pass all the arguments to the called object. The called object is then responsible for handling the arguments.

Since ECMAScript 5th Edition you can also use any kind of object which is array-like, so in practice this means it's going to have a property length and integer properties in the range (0...length). As an example you can now use a NodeList or a custom object like { 'length': 2, '0': 'eat', '1': 'bananas' }.

Examples

Using apply to chain constructors

You can use apply to chain constructors for an object, similar to Java. In the following example we will create a global Function method called construct, which will enable you to use an array-like object with a constructor instead of an arguments list.

Function.prototype.construct = function (aArgs) {
  var oNew = Object.create(this.prototype);
  this.apply(oNew, aArgs);
  return oNew;
};

Function.prototype.construct = function(aArgs) {
  var fConstructor = this, fNewConstr = function() { 
    fConstructor.apply(this, aArgs); 
  };
  fNewConstr.prototype = fConstructor.prototype;
  return new fNewConstr();
};

Example usage:

function MyConstructor() {
  for (var nProp = 0; nProp < arguments.length; nProp++) {
    this['property' + nProp] = arguments[nProp];
  }
}

var myArray = [4, 'Hello world!', false];
var myInstance = MyConstructor.construct(myArray);

console.log(myInstance.property1);                // logs 'Hello world!'
console.log(myInstance instanceof MyConstructor); // logs 'true'
console.log(myInstance.constructor);              // logs 'MyConstructor'

Using apply and built-in functions

Clever usage of apply allows you to use built-ins functions for some tasks that otherwise probably would have been written by looping over the array values. As an example here we are going to use Math.max/Math.min to find out the maximum/minimum value in an array.

// min/max number in an array
var numbers = [5, 6, 2, 3, 7];

// using Math.min/Math.max apply
var max = Math.max.apply(null, numbers); 
// This about equal to Math.max(numbers[0], ...)
// or Math.max(5, 6, ...)

var min = Math.min.apply(null, numbers);

// vs. simple loop based algorithm
max = -Infinity, min = +Infinity;

for (var i = 0; i < numbers.length; i++) {
  if (numbers[i] > max) {
    max = numbers[i];
  }
  if (numbers[i] < min) {
    min = numbers[i];
  }
}

But beware: in using apply this way, you run the risk of exceeding the JavaScript engine's argument length limit. The consequences of applying a function with too many arguments (think more than tens of thousands of arguments) vary across engines (JavaScriptCore has hard-coded argument limit of 65536), because the limit (indeed even the nature of any excessively-large-stack behavior) is unspecified. Some engines will throw an exception. More perniciously, others will arbitrarily limit the number of arguments actually passed to the applied function. (To illustrate this latter case: if such an engine had a limit of four arguments [actual limits are of course significantly higher], it would be as if the arguments 5, 6, 2, 3 had been passed to apply in the examples above, rather than the full array.) If your value array might grow into the tens of thousands, use a hybrid strategy: apply your function to chunks of the array at a time:

function minOfArray(arr) {
  var min = Infinity;
  var QUANTUM = 32768;

  for (var i = 0, len = arr.length; i < len; i += QUANTUM) {
    var submin = Math.min.apply(null, 
                                arr.slice(i, Math.min(i+QUANTUM, len)));
    min = Math.min(submin, min);
  }

  return min;
}

var min = minOfArray([5, 6, 2, 3, 7]);

Using apply in "monkey-patching"

Apply can be the best way to monkey-patch a built-in function of Firefox, or JS libraries. Given someobject.foo function, you can modify the function in a somewhat hacky way, like so:

var originalfoo = someobject.foo;
someobject.foo = function() {
  // Do stuff before calling function
  console.log(arguments);
  // Call the function as it would have been called normally:
  originalfoo.apply(this, arguments);
  // Run stuff after, here.
}

This method is especially handy where you want to debug events, or interface with something that has no API like the various .on([event]... events, such as those usable on the Devtools Inspector).

Specifications

Browser compatibility

See also

• arguments object
• Function.prototype.bind()
• Function.prototype.call()
• Functions and function scope
• Reflect.apply()