thisArg

The value of this provided for the call to func. If the function is not in strict mode, null and undefined will be replaced with the global object, and primitive values will be converted to objects.

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.

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.

You can also use any kind of object which is array-like as the second parameter. In practice, this means that it needs 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 also use arguments, for example:

function wrapper() {
  return anotherFn.apply(null, arguments);
}

With the rest parameters and parameter spread syntax, this can be rewritten as:

function wrapper(...args) {
  return anotherFn(...args);
}

In general, fn.apply(null, args) is equivalent to fn(...args) with the parameter spread syntax.

You can use Array.prototype.push() to append an element to an array. 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, ending up with an array inside an array. On the other hand, Array.prototype.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.

In this case, you can use apply to implicitly "spread" an array as a series of arguments.

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

The same effect can be achieved with the spread syntax.

const array = ['a', 'b'];
const elements = [0, 1, 2];
array.push(...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;
  const QUANTUM = 32768;

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

  return min;
}

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

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