Function.prototype.apply()

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.

Normally, when calling a function, the value of this inside the function is the object that the function was accessed on. With apply(), you can assign an arbitrary value as this when calling an existing function, without first attaching the function to the object as a property. This allows you to use methods of one object as generic utility functions.

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, except args is expected to be an array-like object in the former case with apply(), and an iterable object in the latter case with 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 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 otherwise require manually looping over a collection (or using the spread syntax).

For example, we can use Math.max() and Math.min() to find out the maximum and 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() (or the spread syntax) with an arbitrarily long arguments list, you run the risk of exceeding the JavaScript engine's argument length limit.

The consequences of calling a function with too many arguments (that is, more than tens of thousands of arguments) is unspecified and varies across engines. (The JavaScriptCore engine has a hard-coded argument limit of 65536.) Most engines throw an exception; but there's no normative specification preventing other behaviors, such as arbitrarily limiting 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