How to Use apply() Method in JavaScript: Simple Guide

In JavaScript, apply() is a method used to call a function with a given this value and arguments provided as an array or array-like object. It allows you to control the context of this inside the function and pass arguments dynamically.

Syntax

The apply() method is called on a function and takes two arguments:

• thisArg: The value to use as this inside the function.
• argsArray: An array or array-like object containing arguments to pass to the function.

It looks like this:

javascript

functionName.apply(thisArg, argsArray);

Example

This example shows how to use apply() to call a function with a specific this value and arguments passed as an array.

javascript

const person = {
  fullName: function(city, country) {
    return this.firstName + ' ' + this.lastName + ', ' + city + ', ' + country;
  }
};

const person1 = {
  firstName: 'John',
  lastName: 'Doe'
};

const result = person.fullName.apply(person1, ['New York', 'USA']);
console.log(result);

Output

John Doe, New York, USA

Common Pitfalls

Common mistakes when using apply() include:

• Passing arguments as separate values instead of an array.
• Using null or undefined as thisArg unintentionally, which defaults this to the global object or undefined in strict mode.
• Confusing apply() with call(), which takes arguments separately, not as an array.

Here is an example showing the wrong and right way:

javascript

// Wrong: arguments not in an array
function greet(greeting, punctuation) {
  console.log(greeting + ', ' + this.name + punctuation);
}

const obj = { name: 'Alice' };
// greet.apply(obj, 'Hello', '!'); // This causes an error

// Right: arguments passed as an array

greet.apply(obj, ['Hello', '!']); // Output: Hello, Alice!

Output

Hello, Alice!

Quick Reference

Part	Description
functionName.apply(thisArg, argsArray)	Calls function with specified this and arguments as array
thisArg	Object to use as this inside the function
argsArray	Array or array-like object of arguments to pass
Returns	Result of the function call

Key Takeaways

Use apply() to call a function with a specific this value and arguments as an array.

The first argument sets the this context inside the function.

The second argument must be an array or array-like object of parameters.

apply() differs from call() by accepting arguments as an array.

Avoid passing arguments separately or using null/undefined unintentionally as thisArg.