Javascript Function apply()

// object definition
const personName = {
  firstName: "Taylor",
  lastName: "Jackson",
};

// function definition
function greet(wish, message) {
  return `${this.firstName}, ${wish}. ${message}`;
}

// calling greet() function by passing two arguments
let result = greet.apply(personName, ["Good morning", "How are you?"]);

console.log(result);

// Output:
// Taylor, Good morning. How are you?

apply() Syntax

The syntax of the apply() method is:

func.apply(thisArg, argsArray)

Here, func is a function.

apply() Parameters

The apply() method can take two parameters:

• thisArg - The value of this which is provided while calling func.
• argsArray (optional) - An array containing the arguments to the function.

apply() Return Value

• Returns the result of the called function with the specified this value and arguments.

Example 1: apply() Method to call a Function

// object definition
const personName = {
  firstName: "Taylor",
  lastName: "Jackson",
};

// function definition
function greet(wish, message) {
  return `${this.firstName}, ${wish}. ${message}`;
}

// calling greet() function by passing two arguments
let result = greet.apply(personName, ["Good morning", "How are you?"]);

console.log(result);

Output

Taylor, Good morning. How are you?

In the above program, we have used the apply() method to invoke the greet() function.

Inside the apply() method, the parameter:

• personName - is this value
• ["Good morning", "How are you?"] - are values for "wish" and "message" parameters of the greet() function

The method calls the greet() function so the result variable holds the return value of greet().

Apart from calling a function, we can use the apply() method for:

• Function borrowing
• Append an array

Example 2: apply() for Function Borrowing

// object definition
const car = {
  name: "BMW",
  description() {
    return `The ${this.name} is of ${this.color} color.`;
  },
};

// object definition
const bike = {
  name: "Duke",
  color: "black",
};

// bike is borrowing description() method from car using apply() 
let result = car.description.apply(bike);

console.log(result);

Output

The Duke is of black color.

In the above program, we have borrowed the method of car object in the bike object with the help of the apply() method.

Example 3: apply() to Append two Arrays

let color1 = ["Red", "Green", "Blue"];
let color2 = ["Yellow", "Black"];

// appending two arrays color1 and color2
color1.push.apply(color1, color2);

console.log(color1);

Output

[ 'Red', 'Green', 'Blue', 'Yellow', 'Black' ]

Example 4 : Using apply() with built-in functions

const numbers = [5, 1, 4, 3, 4, 6, 8];

// using apply() with Math object 
let max = Math.max.apply(null, numbers);

console.log(max); 

// without using apply() with Math object 
let max1 = Math.max(5, 1, 4, 3, 4, 6, 8)
;

console.log(max1); 

Output

8
8

Recommended Reading: JavaScript Function call()