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).

thisArg

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. This argument is required.

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.

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

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. In practice, this means it's going to have a length property, and integer ("index") properties in the range (0..length - 1). For example, you could use a NodeList, or a custom object like { 'length': 2, '0': 'eat', '1': 'bananas' }.

You can use push to append an element to an array. And, because push accepts a variable number of arguments, you can also push multiple elements at once.

But, if you pass an array to push, it will actually add that array as a single element, instead of adding the elements individually. So you end up with an array inside an array.

What if that is not what you want? concat does have the desired behavior in this case, but it does not append to the existing array—it instead creates and returns a new array.

But you wanted to append to the existing array... So what now? Write a loop? Surely not?

apply to the rescue!

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

Clever usage of apply allows you to use built-in functions for some tasks that would probably have otherwise been written by looping over the array values.

As an example, here are Math.max/Math.min, used to find out the maximum/minimum value in an array.

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

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

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

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

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

But beware: by 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 (that is, more than tens of thousands of arguments) varies across engines. (The JavaScriptCore engine has hard-coded argument limit of 65536.

This is because the limit (and 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) {
  let min = Infinity;
  let 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;
}

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

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) {
  let oNew = Object.create(this.prototype);
  this.apply(oNew, aArgs);
  return oNew;
};

Example usage:

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

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

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

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