What is Function.apply in JavaScript?

The apply method calls the function with a given this value and an array-like object as an argument.

Syntax

apply(thisArg, argsArray)

• thisArg: The this value on executing the function. It is an optional argument. However, if thisArg is skipped, the this value is:

  • non-strict mode - global object
  • strict mode - undefined

• argsArray: The arguments to be passed to a function. Arguments are passed as an array.

• The apply method returns the value returned by the function.

Example

var number = 10;

function test(...arguments) {
  let max = arguments.length ? Math.max.apply(null, arguments) : 0;
  return this.number + max;
}

console.log("Calling without thisArg");
console.log( test.apply() ); // 10

let obj = {number :  20};
console.log("Calling with thisArg");
console.log( test.apply(obj));  // 20

console.log("Calling with thisArg and arguments");
console.log( test.apply(obj, [100, 200, 300, 400]) );  // 420

In the code above, we have created a test function that will return the sum of the number property in the this object, and the largest value in the arguments passed.

• First, we called the test function using the apply method without the thisArg argument. In this case, the global object is considered as this.

• Then, we called the test method by passing obj as a thisArg value. Inside the test function, obj is considered as thisArg.

• Lastly, we passed thisArg and arguments to the test function.

Example with strict mode

"use strict" // strict mode
var number = 10;

function test() {
  return this.number;
}

console.log("Calling without thisArg");
console.log( test.apply() ); // error

In the code above, we have called the test function without thisArg. This will result in an error because in strict mode, if we don’t pass the thisArg to the apply function, the this value inside the function is undefined. When trying to access the this value(undefined), we will get an error.

call() and apply() are identical in functionality. The only difference is that call() accepts a list of arguments; whereas, apply() accepts a single array of arguments.