function.apply

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

Syntax

function.apply(thisArg, [argsArray])

Parameters

thisArg

Optional. The value of this provided for the call to func. 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

Optional. An array-like object, specifying the arguments with which func 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 use an arguments array instead of a list of arguments (parameters). With apply, you can also use an array literal, for example, func.apply(this, ['eat', 'bananas']), or an Array object, for example, func.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-1). As an example you can now use a NodeList or a custom object like { 'length': 2, '0': 'eat', '1': 'bananas' }.

Examples

Using apply to append an array to another

We can use push to append an element to an array. And, because push accepts a variable number of arguments, we can also push multiple elements at once. But, if we pass an array to push, it will actually add that array as a single element, instead of adding the elements individually, so we end up with an array inside an array. What if that is not what we want? concat does have the behaviour we want in this case, but it does not actually append to the existing array but creates and returns a new array. But we wanted to append to our existing array... So what now? Write a loop? Surely not?

apply to the rescue!

var array = ['a', 'b'];
var elements = [0, 1, 2];
array.push.apply(array, elements);
console.info(array); // ["a", "b", 0, 1, 2]

Using apply and built-in functions

Clever usage of apply allows you to use built-in 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 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 oNew = {};
  oNew.__proto__ = 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();
};

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

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'

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

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