JavaScript this Operator Interview Questions
Last Updated : 04 Jan, 2025
In JavaScript, this is a special keyword that refers to the current object or context. It behaves differently depending on where it's used. If you use this in a regular function, it refers to the global object (in browsers, this is the window). In an object's method, this refers to the object itself.
We are going to discuss some Interview Questions related to This keyword.
1. Why this is Necessary in JavaScript?
- To provide access to an object's properties & methods from methods within that object.
- Efficiency via code reuse.
- Enables reusability and flexibility of functions across different objects.
- Supports object-oriented design principles by linking data (properties) with behavior (methods)
- Enables objects to manage their own state and behavior effectively.
2. What is the default value of this in the global context?
JavaScript console.log(this); // Window (browser) or global object (Node.js)
3. How does this behave inside a simple function?
JavaScript function show() { console.log(this); // Global object or undefined in strict mode } show(); Output<ref *1> Object [global] { global: [Circular *1], clearInterval: [Function: clearInterval], clearTimeout: [Function: clearTimeout], setInterval: [Function: setInterval], setTimeout: [Functio... 4. What is the value of this in strict mode?
JavaScript "use strict"; function test() { console.log(this); } test(); 5. How does 'this' work in an object method?
JavaScript const obj = { name: "GeeksforGeeks", greet() { console.log(this.name); } }; obj.greet(); 6. How does 'this' work with nested functions?
JavaScript const obj = { name: "GeeksforGeeks", greet() { function inner() { console.log(this); } inner(); } }; obj.greet(); Output<ref *1> Object [global] { global: [Circular *1], clearInterval: [Function: clearInterval], clearTimeout: [Function: clearTimeout], setInterval: [Function: setInterval], setTimeout: [Functio... - In strict mode, this inside a regular function is undefined because regular functions do not automatically bind this to their enclosing object.
- To fix this, use an arrow function inside greet() as arrow functions inherit this from their outer context, ensuring this refers to obj.
7. How does this behave in an arrow function?
JavaScript const obj = { name: "Geeks", greet: () => { console.log(this.name); // undefined, as `this` refers to the enclosing scope } }; obj.greet(); 8. What is the value of this inside a class?
JavaScript class Person { constructor(name) { this.name = name; } greet() { console.log(this.name); // Refers to the instance of the class } } const p = new Person("GeeksforGeeks"); p.greet(); // "GeeksforGeeks" 9. What happens to 'this' in an IIFE?
JavaScript (function () { console.log(this); // Global object or undefined in strict mode })(); Output<ref *1> Object [global] { global: [Circular *1], clearInterval: [Function: clearInterval], clearTimeout: [Function: clearTimeout], setInterval: [Function: setInterval], setTimeout: [Functio... The output of the code depends on whether you are running it in strict mode or non-strict mode.
Non-strict mode (default in regular JavaScript): In non-strict mode, when a function is called without an object context (like in your case), this refers to the global object (in the browser, it’s window).
JavaScript // In a browser: Window { ... } // Represents the global window object in a browser Strict mode: In strict mode ('use strict';), this inside a function call refers to undefined instead of the global object.
10. How does 'this' behave in setTimeout?
JavaScript const obj = { name: "GeeksforGeeks", greet() { setTimeout(function () { console.log(this.name); // undefined in strict mode }, 1000); } }; obj.greet(); 11. How to fix 'this' in setTimeout using arrow functions?
JavaScript const obj = { name: "GeeksforGeeks", greet() { setTimeout(() => { console.log(this.name); // "GeeksforGeeks" }, 1000); } }; obj.greet(); In this code:
- When setTimeout is used with an arrow function, this inside the arrow function refers to the outer scope's this, which is the object (obj).
- As a result, the arrow function correctly accesses the name property of obj, and the output will be "GeeksforGeeks".
12. What is 'this' when a function is bound?
JavaScript const obj = { name: "GeeksforGeeks" }; function greet() { console.log(this.name); } const boundGreet = greet.bind(obj); boundGreet(); In this code:
- greet() is a regular function where this would normally depend on how it's called.
- greet.bind(obj) creates a new function (boundGreet) where the value of this is permanently set to the object obj.
- When boundGreet() is called, this inside the function refers to obj, so this.name outputs "GeeksforGeeks".
13. How does 'this' behave in call and apply?
JavaScript const obj = { name: "GeeksforGeeks" }; function greet() { console.log(this.name); } greet.call(obj); greet.apply(obj); OutputGeeksforGeeks GeeksforGeeks
In this code:
- This method calls the function immediately and sets this to obj. It then prints "GeeksforGeeks" because this.name refers to o obj.name.
- Like call(), apply() also calls the function immediately and sets this to obj. The difference is that apply() expects arguments to be passed as an array, but in this case, no arguments are needed, so the result is the same: "GeeksforGeeks".
OutputGeeksforGeeks GeeksforGeeks
14. What happens when "this" is used in a constructor function?
JavaScript function Person(name) { this.name = name; } const p = new Person("GeeksforGeeks"); console.log(p.name); In this code
- In a constructor function, this refers to the new object being created during the invocation of the function with the new keyword.
- When new Person("GeeksforGeeks") is called, this.name = name sets the name property of the new object to "GeeksforGeeks". The new object is returned and assigned to p, so p.name outputs "GeeksforGeeks".
15. How does "this" behave in chaining methods?
JavaScript const obj = { count: 0, increment() { this.count++; return this; }, show() { console.log(this.count); return this; } }; obj.increment().increment().show(); In this code
- In this example, obj has two methods: increment and show. The increment method increases the count by 1 and returns the object itself (this), allowing method chaining.
- The show method prints the current count and also returns the object. Calling obj.increment().increment().show() increases count to 2 and prints 2 because the methods are chained together.
16. What is "this" in an ES module?
In an ES module, this is undefined at the top level because ES modules run in strict mode by default
JavaScript console.log(this); // Undefined in Es modules
17. What is " this " inside the an object returned by the function?
JavaScript function createObject() { return { name: "GeeksforGeeks", greet() { console.log(this.name); } }; } const obj = createObject(); obj.greet(); In this example, the createObject function returns an object with a name property and a greet method. When obj.greet() is called, this refers to the obj itself, so it accesses obj.name and prints "GeeksforGeeks".
18 . How does "this" behave in a factory function when used inside a method of the returned object?
JavaScript function create(name) { return { name, greet() { console.log(this.name); } }; } const person = create("GeeksforGeeks"); person.greet(); In this code
- The createPerson function creates an object with a name property and a greet method.
- When person.greet() is called, this inside the greet method refers to the person object, allowing it to access person.name and print "GeeksforGeeks".
19 . How does "this" behave in a forEach callback?
JavaScript [1, 2, 3].forEach(function () { console.log(this); }, { name: "GeeksforGeeks" }); Output{ name: 'GeeksforGeeks' } { name: 'GeeksforGeeks' } { name: 'GeeksforGeeks' } In this code
- The forEach method iterates over the array [1, 2, 3], with the second argument { name: "GeeksforGeeks" } explicitly set as this inside the loop.
- During each iteration, this refers to { name: "GeeksforGeeks" }, so the object is printed for every element in the array.
20. How can you pass "this" explicitly in a Promise chain?
JavaScript const obj = { name: "GeeksforGeeks", greet() { return Promise.resolve(this.name).then(name => { console.log(name); }); } }; obj.greet(); - In this example, obj.greet() returns a promise that resolves with this.name (which is "GeeksforGeeks").
- Once the promise is resolved, the .then() method prints the value of name, which is "". So, the output is Alice.
20. How does "this" behave with getters and setters?
JavaScript const obj = { name: "Geeks", get greet() { return `Hello, ${this.name}`; }, set greet(newName) { this.name = newName; } }; console.log(obj.greet); // Output: Hello, Geeks obj.greet = "forGeeks"; console.log(obj.greet); // Output: Hello, forGeeks OutputHello, Geeks Hello, forGeeks
In this example, obj has a getter and setter for the greet property. The getter returns a greeting message with this.name, and the setter updates this.name. Initially, obj.greet prints "Hello, Geeks". After setting obj.greet = "forGeeks", it updates name and prints "Hello, forGeeks".
21. How does " this " behave with async iteration?
JavaScript const obj = { name: "GeeksforGeeks", async *greet() { yield `Hello, ${this.name}`; } }; (async () => { for await (const message of obj.greet()) { console.log(message); } })(); OutputHello, GeeksforGeeks
In this code
- The obj.greet() method is an asynchronous generator that yields a greeting message using this.name.
- The for await...of loop iterates over the yielded values and prints "Hello, GeeksforGeeks" because this.name refers to "GeeksforGeeks".
22. How does "this" behave with Map key iteration in JavaScript?
JavaScript const map = new Map(); map.set("key1", { name: "Geeks" }); map.set("key2", { name: "forGeeks" }); map.forEach(function (value, key) { console.log(this.name); }, { name: "GeeksforGeeks" }); OutputGeeksforGeeks GeeksforGeeks
- The map.forEach method iterates through the map, with the second argument { name: "GeeksforGeeks" } being explicitly set as this inside the loop.
- For each item in the map, it prints "GeeksforGeeks" because this.name consistently refers to "GeeksforGeeks".
OutputGeeksforGeeks GeeksforGeeks
23 .How does "this" behave with async/await in JavaScript?
JavaScript const obj = { name: "GeeksforGeeks", async greet() { console.log(this.name); } }; obj.greet(); In this example, obj.greet() is an asynchronous function that logs this.name. Since this refers to the obj object, it prints "GeeksforGeeks", which is the value of obj.name.
24. What is the value of " this " in an event handler in a DOM element?
JavaScript const obj = { name : "GeeksforGeeks", greet() { document.getElementById("btn").addEventListener( "click", function() { console.log(this.name); // Output: undefined }); } }; obj.greet(); 25. How does " this " work with new and constructor functions in JavaScript?
JavaScript function Animal(name) { this.name = name; this.speak = function() { console.log(`${this.name} makes a sound`); }; } const dog = new Animal("Dog"); dog.speak(); In this code
- Animal(name): This is a constructor function. It creates a new object with a name property and a speak method.
- dog.speak(): After creating the dog object, we call the speak() method. It prints "Dog makes a sound", because the name property of dog is "Dog".
Similar Reads
JavaScript Tutorial JavaScript is a programming language used to create dynamic content for websites. It is a lightweight, cross-platform, and single-threaded programming language. It's an interpreted language that executes code line by line, providing more flexibility.Client Side: On the client side, JavaScript works
8 min read
JavaScript Basics
Introduction to JavaScriptJavaScript is a versatile, dynamically typed programming language that brings life to web pages by making them interactive. It is used for building interactive web applications, supports both client-side and server-side development, and integrates seamlessly with HTML, CSS, and a rich standard libra
4 min read
JavaScript VersionsJavaScript is a popular programming language used by developers all over the world. It’s a lightweight and easy-to-learn language that can run on both the client-side (in your browser) and the server-side (on the server). JavaScript was created in 1995 by Brendan Eich.In 1997, JavaScript became a st
2 min read
How to Add JavaScript in HTML Document?To add JavaScript in HTML document, several methods can be used. These methods include embedding JavaScript directly within the HTML file or linking an external JavaScript file.Inline JavaScriptYou can write JavaScript code directly inside the HTML element using the onclick, onmouseover, or other ev
3 min read
JavaScript SyntaxJavaScript syntax refers to the rules and conventions dictating how code is structured and arranged within the JavaScript programming language. This includes statements, expressions, variables, functions, operators, and control flow constructs.Syntaxconsole.log("Basic Print method in JavaScript");Ja
6 min read
JavaScript OutputJavaScript provides different methods to display output, such as console.log(), alert(), document.write(), and manipulating HTML elements directly. Each method has its specific use cases, whether for debugging, user notifications, or dynamically updating web content. Here we will explore various Jav
4 min read
JavaScript CommentsComments help explain code (they are not executed and hence do not have any logic implementation). We can also use them to temporarily disable parts of your code.1. Single Line CommentsA single-line comment in JavaScript is denoted by two forward slashes (//), JavaScript// A single line comment cons
2 min read
JS Variables & Datatypes
Variables and Datatypes in JavaScriptVariables and data types are foundational concepts in programming, serving as the building blocks for storing and manipulating information within a program. In JavaScript, getting a good grasp of these concepts is important for writing code that works well and is easy to understand.Data TypesVariabl
6 min read
Global and Local variables in JavaScriptIn JavaScript, understanding the difference between global and local variables is important for writing clean, maintainable, and error-free code. Variables can be declared with different scopes, affecting where and how they can be accessed. Global VariablesGlobal variables in JavaScript are those de
4 min read
JavaScript LetThe let keyword is a modern way to declare variables in JavaScript and was introduced in ECMAScript 6 (ES6). Unlike var, let provides block-level scoping. This behaviour helps developers avoid unintended issues caused by variable hoisting and scope leakage that are common with var.Syntaxlet variable
6 min read
JavaScript constThe const keyword in JavaScript is a modern way to declare variables, introduced in (ES6). It is used to declare variables whose values need to remain constant throughout the lifetime of the application.const is block-scoped, similar to let, and is useful for ensuring immutability in your code. Unli
5 min read
JavaScript Var StatementThe var keyword is used to declare variables in JavaScript. It has been part of the language since its inception. When a variable is declared using var, it is function-scoped or globally-scoped, depending on where it is declared.Syntaxvar variable = value;It declares a variable using var, assigns it
7 min read
JS Operators
JavaScript OperatorsJavaScript operators are symbols or keywords used to perform operations on values and variables. They are the building blocks of JavaScript expressions and can manipulate data in various ways.There are various operators supported by JavaScript:1. JavaScript Arithmetic OperatorsArithmetic Operators p
5 min read
Operator precedence in JavaScriptOperator precedence refers to the priority given to operators while parsing a statement that has more than one operator performing operations in it. Operators with higher priorities are resolved first. But as one goes down the list, the priority decreases and hence their resolution. ( * ) and ( / )
2 min read
JavaScript Arithmetic OperatorsJavaScript Arithmetic Operators are the operator that operate upon the numerical values and return a numerical value. Addition (+) OperatorThe addition operator takes two numerical operands and gives their numerical sum. It also concatenates two strings or numbers.JavaScript// Number + Number =>
5 min read
JavaScript Assignment OperatorsAssignment operators are used to assign values to variables in JavaScript.JavaScript// Lets take some variables x = 10 y = 20 x = y ; console.log(x); console.log(y); Output20 20 More Assignment OperatorsThere are so many assignment operators as shown in the table with the description.OPERATOR NAMESH
5 min read
JavaScript Comparison OperatorsJavaScript comparison operators are essential tools for checking conditions and making decisions in your code. 1. Equality Operator (==) The Equality operator is used to compare the equality of two operands. JavaScript// Illustration of (==) operator let x = 5; let y = '5'; // Checking of operands c
5 min read
JavaScript Logical OperatorsLogical operators in JavaScript are used to perform logical operations on values and return either true or false. These operators are commonly used in decision-making statements like if or while loops to control the flow of execution based on conditions.In JavaScript, there are basically three types
5 min read
JavaScript Bitwise OperatorsIn JavaScript, a number is stored as a 64-bit floating-point number but bitwise operations are performed on a 32-bit binary number. To perform a bit-operation, JavaScript converts the number into a 32-bit binary number (signed) and performs the operation and converts back the result to a 64-bit numb
5 min read
JavaScript Ternary OperatorThe Ternary Operator in JavaScript is a conditional operator that evaluates a condition and returns one of two values based on whether the condition is true or false. It simplifies decision-making in code, making it more concise and readable. Syntaxcondition ? trueExpression : falseExpressionConditi
4 min read
JavaScript Comma OperatorJavaScript Comma Operator mainly evaluates its operands from left to right sequentially and returns the value of the rightmost operand. JavaScriptlet x = (1, 2, 3); console.log(x); Output3 Here is another example to show that all expressions are actually executed.JavaScriptlet a = 1, b = 2, c = 3; l
2 min read
JavaScript Unary OperatorsJavaScript Unary Operators work on a single operand and perform various operations, like incrementing/decrementing, evaluating data type, negation of a value, etc.Unary Plus (+) OperatorThe unary plus (+) converts an operand into a number, if possible. It is commonly used to ensure numerical operati
4 min read
JavaScript in and instanceof operatorsJavaScript Relational Operators are used to compare their operands and determine the relationship between them. They return a Boolean value (true or false) based on the comparison result.JavaScript in OperatorThe in-operator in JavaScript checks if a specified property exists in an object or if an e
3 min read
JavaScript String OperatorsJavaScript String Operators are used to manipulate and perform operations on strings. There are two operators which are used to modify strings in JavaScript. These operators help us to join one string to another string.1. Concatenate OperatorConcatenate Operator in JavaScript combines strings using
3 min read
JS Statements
JS Loops
JavaScript LoopsLoops in JavaScript are used to reduce repetitive tasks by repeatedly executing a block of code as long as a specified condition is true. This makes code more concise and efficient.Suppose we want to print 'Hello World' five times. Instead of manually writing the print statement repeatedly, we can u
3 min read
JavaScript For LoopJavaScript for loop is a control flow statement that allows code to be executed repeatedly based on a condition. It consists of three parts: initialization, condition, and increment/decrement. Syntaxfor (statement 1 ; statement 2 ; statement 3){ code here...}Statement 1: It is the initialization of
4 min read
JavaScript While LoopThe while loop executes a block of code as long as a specified condition is true. In JavaScript, this loop evaluates the condition before each iteration and continues running as long as the condition remains true.Syntaxwhile (condition) { Code block to be executed}Here's an example that prints from
3 min read
JavaScript For In LoopThe JavaScript for...in loop iterates over the properties of an object. It allows you to access each key or property name of an object.JavaScriptconst car = { make: "Toyota", model: "Corolla", year: 2020 }; for (let key in car) { console.log(`${key}: ${car[key]}`); }Outputmake: Toyota model: Corolla
3 min read
JavaScript for...of LoopThe JavaScript for...of loop is a modern, iteration statement introduced in ECMAScript 2015 (ES6). Works for iterable objects such as arrays, strings, maps, sets, and more. It is better choice for traversing items of iterables compared to traditional for and for in loops, especially when we have bre
3 min read
JavaScript do...while LoopA do...while loop in JavaScript is a control structure where the code executes repeatedly based on a given boolean condition. It's similar to a repeating if statement. One key difference is that a do...while loop guarantees that the code block will execute at least once, regardless of whether the co
4 min read
JS Perfomance & Debugging
JS Object
Objects in JavascriptAn object in JavaScript is a data structure used to store related data collections. It stores data as key-value pairs, where each key is a unique identifier for the associated value. Objects are dynamic, which means the properties can be added, modified, or deleted at runtime.There are two primary w
4 min read
Object Oriented Programming in JavaScriptObject Oriented Programming (OOP) is a style of programming that uses classes and objects to model real-world things like data and behavior. A class is a blueprint that defines the properties and methods an object can have, while an object is a specific instance created from that class. Why OOP is N
3 min read
JavaScript ObjectsIn our previous article on Introduction to Object Oriented Programming in JavaScript we have seen all the common OOP terminology and got to know how they do or don't exist in JavaScript. In this article, objects are discussed in detail.Creating Objects:In JavaScript, Objects can be created using two
6 min read
Creating objects in JavaScriptAn object in JavaScript is a collection of key-value pairs, where keys are strings (properties) and values can be any data type. Objects can be created using object literals, constructors, or classes. Properties are defined with key-value pairs, and methods are functions defined within the object, e
5 min read
JavaScript JSON ObjectsJSON (JavaScript Object Notation) is a handy way to share data. It's easy for both people and computers to understand. In JavaScript, JSON helps organize data into simple objects. Let's explore how JSON works and why it's so useful for exchanging information.const jsonData = { "key1" : "value1", ...
3 min read
JavaScript Object ReferenceJavaScript Objects are the most important data type and form the building blocks for modern JavaScript. The "Object" class represents the JavaScript data types. Objects are quite different from JavaScript’s primitive data types (Number, String, Boolean, null, undefined, and symbol). It is used to st
4 min read
JS Function
Functions in JavaScriptFunctions in JavaScript are reusable blocks of code designed to perform specific tasks. They allow you to organize, reuse, and modularize code. It can take inputs, perform actions, and return outputs.JavaScriptfunction sum(x, y) { return x + y; } console.log(sum(6, 9)); // output: 15Function Syntax
4 min read
How to write a function in JavaScript ?JavaScript functions serve as reusable blocks of code that can be called from anywhere within your application. They eliminate the need to repeat the same code, promoting code reusability and modularity. By breaking down a large program into smaller, manageable functions, programmers can enhance cod
4 min read
JavaScript Function CallThe call() method is a predefined JavaScript method. It can be used to invoke (call) a method with an owner object as an argument (parameter). This allows borrowing methods from other objects, executing them within a different context, overriding the default value, and passing arguments. Syntax: cal
2 min read
Different ways of writing functions in JavaScriptA JavaScript function is a block of code designed to perform a specific task. Functions are only executed when they are called (or "invoked"). JavaScript provides different ways to define functions, each with its own syntax and use case.Below are the ways of writing functions in JavaScript:Table of
3 min read
Difference between Methods and Functions in JavaScriptGrasping the difference between methods and functions in JavaScript is essential for developers at all levels. While both are fundamental to writing effective code, they serve different purposes and are used in various contexts. This article breaks down the key distinctions between methods and funct
3 min read
Explain the Different Function States in JavaScriptIn JavaScript, we can create functions in many different ways according to the need for the specific operation. For example, sometimes we need asynchronous functions or synchronous functions. Â In this article, we will discuss the difference between the function Person( ) { }, let person = Person ( )
3 min read
JavaScript Function Complete ReferenceA JavaScript function is a set of statements that takes inputs, performs specific computations, and produces outputs. Essentially, a function performs tasks or computations and then returns the result to the user.Syntax:function functionName(Parameter1, Parameter2, ..) { // Function body}Example: Be
3 min read
JS Array
JavaScript ArraysIn JavaScript, an array is an ordered list of values. Each value, known as an element, is assigned a numeric position in the array called its index. The indexing starts at 0, so the first element is at position 0, the second at position 1, and so on. Arrays can hold any type of data—such as numbers,
7 min read
JavaScript Array MethodsTo help you perform common tasks efficiently, JavaScript provides a wide variety of array methods. These methods allow you to add, remove, find, and transform array elements with ease.Javascript Arrays Methods1. JavaScript Array length The length property of an array returns the number of elements i
7 min read
Best-Known JavaScript Array MethodsAn array is a special variable in all programming languages used to store multiple elements. JavaScript array come with built-in methods that every developer should know how to use. These methods help in adding, removing, iterating, or manipulating data as per requirements.There are some Basic JavaS
6 min read
Important Array Methods of JavaScriptJavaScript arrays are powerful tools for managing collections of data. They come with a wide range of built-in methods that allow developers to manipulate, transform, and interact with array elements.Some of the most important array methods in JavaScript areTable of Content1. JavaScript push() Metho
7 min read
JavaScript Array ReferenceJavaScript Array is used to store multiple elements in a single variable. It can hold various data types, including numbers, strings, objects, and even other arrays. It is often used when we want to store a list of elements and access them by a single variable.Syntax:const arr = ["Item1", "Item2", "
4 min read