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JavaScript Interview Questions and Answers

In this article we have gathered the latest and most asked JavaScript Interview Questions and Answers for both freshers and experienced. The JavaScript Interview Questions cover the topics related to JavaScript include variables and data types, control structures, functions, objects and object-oriented programming, arrays, the Document Object Model (DOM), events, debugging and error handling, browser compatibility, and libraries and frameworks. etc.

JavaScript is a widely-used programming language that is commonly used to add interactivity and other dynamic features to websites. It is a client-side language, which means that the code is executed on the user’s computer rather than on the server. In this blog post, we will cover some Common JavaScript interview questions and answers and its use in web development. Whether you are a beginner who learn JavaScript or an experienced developer looking to brush up on your skills, these questions and answers will provide valuable insight into the language and its capabilities.

JavaScript Interview Questions and Answers

These JavaScript Interview Questions will help job seekers to prepare well for the job interview and cross the panel discussion easily.

So let’s get started :

1. What is JavaScript and why is it used?
2. What are some key features of JavaScript?
3. How is JavaScript different from other programming languages?
4. What are some common uses for JavaScript?
5. Can you give an example of how JavaScript is used on a web page?
6. What is the difference between a var and a let variable in JavaScript?
7. What is the difference between a null and an undefined value in JavaScript?
8. What is the purpose of the Array.prototype.map() method in JavaScript?
9. What is the difference between a == and a === comparison in JavaScript?
10. What is the purpose of the this keyword in JavaScript?
11. What is the purpose of the new keyword in JavaScript?
12. What is the purpose of the try and catch keywords in JavaScript?
13. What is the purpose of the Promise object in JavaScript?
14. What is the difference between a class and an interface in TypeScript?
15. What is the purpose of the async and await keywords in JavaScript?
16. What is the purpose of the Array.prototype.reduce() method in JavaScript?
17. What is the purpose of the Array.prototype.filter() method in JavaScript?
18. What is the purpose of the Array.prototype.sort() method in JavaScript?
19. What is the purpose of the let keyword in JavaScript?
20. What is the purpose of the const keyword in JavaScript?
21. What is the purpose of the Array.prototype.map() method in JavaScript?
22. What is the purpose of the Array.prototype.forEach() method in JavaScript?

1. What is JavaScript and why is it used?

JavaScript is a programming language that is commonly used to add interactivity and other dynamic features to websites. It is a client-side language, which means that the code is executed on the user’s computer rather than on the server. This allows for faster and more responsive websites.


2. What are some key features of JavaScript?

  • JavaScript is an object-oriented language, which means that it allows developers to create and manipulate objects in their code.
  • It is a dynamically-typed language, which means that the data types of variables are determined at runtime rather than being explicitly defined by the programmer.
  • JavaScript is a loosely-typed language, which means that it allows for a certain degree of flexibility in the way that code is written.
  • It has a large and active community of developers, which means that there is a wealth of resources available for learning and using the language.

3. How is JavaScript different from other programming languages?

JavaScript is different from other programming languages in a number of ways. For example:

  • It is primarily a client-side language, whereas many other languages are server-side. This means that JavaScript code is executed on the user’s computer rather than on the server.
  • It is an object-oriented language, whereas some other languages are procedural. This means that JavaScript allows for the creation and manipulation of objects in the code.
  • It is a dynamically-typed language, whereas some other languages are statically-typed. This means that the data types of variables in JavaScript are determined at runtime rather than being explicitly defined by the programmer.

4. What are some common uses for JavaScript?

Some common uses for JavaScript include:

  • Adding interactivity to websites, such as pop-up windows, drop-down menus, and image galleries.
  • Validating form input to ensure that it is accurate and complete before it is submitted to the server.
  • Creating interactive maps and other graphical elements on websites.
  • Creating games and other applications that run in the user’s web browser.
  • Manipulating the content of a web page in real-time, such as changing the text or images on the page.

5. Can you give an example of how JavaScript is used on a web page?

Here is a simple example of how JavaScript can be used on a web page to create a pop-up window:

<script>
function showPopup() {
    alert("Hello! This is a pop-up window.");
}
</script>

<button onclick="showPopup()">Click here to show the pop-up</button>

In this example, the showPopup() function is defined using JavaScript. This function simply displays an alert box with the message “Hello! This is a pop-up window.” The onclick attribute of the <button> element is used to call the showPopup() function when the button is clicked by the user. When the button is clicked, the pop-up window will be displayed.


6. What is the difference between a var and a let variable in JavaScript?

In JavaScript, var and let are both keywords that are used to declare variables. The key difference between the two is that var variables have function-level scope, while let variables have block-level scope. This means that a var variable can be accessed from anywhere within the function in which it is declared, while a let variable can only be accessed from within the block in which it is declared. For example:

function example() {
    // this `var` variable can be accessed from anywhere within the function
    var x = 10;

    if (x > 5) {
        // this `let` variable can only be accessed from within the if block
        let y = 20;
    }
}

7. What is the difference between a null and an undefined value in JavaScript?

In JavaScript, null and undefined are both special values that represent the absence of a value. The key difference between the two is that null is a value that is explicitly assigned to a variable, while undefined is the default value of a variable that has not been assigned a value. For example:

// this variable is assigned the `null` value
let x = null;

// this variable has the `undefined` value because it has not been assigned a value
let y;

Additionally, null is considered to be an object, while undefined is not. This means that, for example, you can call methods on a null value, but not on an undefined value.

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8. What is the purpose of the Array.prototype.map() method in JavaScript?

The Array.prototype.map() method is a built-in method in JavaScript that is used to transform the elements of an array. It takes a callback function as an argument, which is applied to each element of the array, and returns a new array with the results of the callback function applied to each element. This can be useful for performing a variety of operations on arrays, such as converting the elements of an array to a different data type or applying a mathematical operation to each element. For example:

// this array contains a list of numbers
let numbers = [1, 2, 3, 4, 5];

// this uses the `map()` method to convert each element to its square
let squares = numbers.map(x => x * x);

// the resulting array is [1, 4, 9, 16, 25]

9. What is the difference between a == and a === comparison in JavaScript?

In JavaScript, == and === are both operators that are used to compare two values. The key difference between the two is that == performs a type conversion before comparing the values, while === does not. This means that == will consider two values to be equal if they have the same value after being converted to a common type, while === will only consider two values to be equal if they have the same value and are of the same type. For example:

// this comparison will return true because the values are equal after being converted to a common type (string)
"1" == 1

// this comparison will return false because the values are not equal and are not of the same type
"1" === 1

In general, it is considered best practice to use the === operator for comparisons in JavaScript, as it provides more consistent and reliable results. However, there may be cases where using the == operator with type conversion is useful or necessary.


10. What is the purpose of the this keyword in JavaScript?

The this keyword is a special keyword in JavaScript that refers to the object that is currently being referred to by the code. It is commonly used in object-oriented programming to refer to the current instance of an object. The value of this can change depending on the context in which it is used, but it is typically set to the object that is calling the current function. For example:

// this is an object with a method that uses the `this` keyword
let example = {
    x: 10,
    y: 20,
    sum: function() {
        // this refers to the example object, so this.x + this.y is equal to 10 + 20
        return this.x + this.y;
    }
};

In this example, the sum() method uses the this keyword to refer to the x and y properties of the example object. When the method is called, the this keyword will be set to the example object, so the result of the method will be 30.


11. What is the purpose of the new keyword in JavaScript?

The new keyword is a special keyword in JavaScript that is used to create a new instance of an object. It is commonly used in conjunction with constructors, which are special functions that are used to create objects. When the new keyword is used with a constructor, it creates a new object and sets the this keyword to refer to the new object. The constructor can then be used to initialize the properties of the new object. For example:

// this is a constructor function
function Example(x, y) {
    // the `this` keyword refers to the new object that is being created
    this.x = x;
    this.y = y;
}

// this creates a new instance of the Example object
let example = new Example(10, 20);

// the `example` object now has the properties x and y with the values 10 and 20, respectively

In this example, the Example constructor function is used with the new keyword to create a new instance of the Example object. The x and y arguments are passed to the constructor, which assigns their values to the x and y properties of the new object. The example variable now holds a reference to the new object, which can be used to access its properties and methods.


12. What is the purpose of the try and catch keywords in JavaScript?

The try and catch keywords are used in JavaScript to handle errors that may occur in a program. The try keyword is used to define a block of code that may throw an error, while the catch keyword is used to define a block of code that will be executed if an error is thrown. This allows for the graceful handling of errors in a program, without the need for the program to crash or terminate. For example:

try {
    // this code may throw an error
    let x = y + 10;
} catch (error) {
    // this code will be executed if an error is thrown
    console.log("An error occurred: " + error.message);
}

In this example, the code inside the try block attempts to perform a calculation using the y variable, which is not defined. This will cause an error to be thrown. The catch block will catch the error and log a message to the console with the error’s message. This allows for the program to continue executing, rather than crashing or terminating.


13. What is the purpose of the Promise object in JavaScript?

The Promise object is a built-in object in JavaScript that is used to handle asynchronous operations. It represents the eventual completion or failure of an asynchronous operation, and provides a way to specify what will happen when the operation is complete or has failed. This allows for more efficient and elegant handling of asynchronous operations in JavaScript, such as making network requests or working with user input. For example:

// this is a function that returns a promise
function makeRequest() {
    return new Promise((resolve, reject) => {
        // this is an asynchronous operation that may succeed or fail
        setTimeout(() => {
            let success = Math.random() >= 0.5;
            if (success) {
                // this will cause the promise to be resolved with the value "Success!"
                resolve("Success!");
            } else {
                // this will cause the promise to be rejected with the value "Error!"
                reject("Error!");
            }
        }, 1000);
    });
}

// this is how the promise can be used to handle the asynchronous operation
makeRequest()
    .then(result => {
        // this will be executed if the operation is successful
        console.log("The operation was successful: " + result);
    })
    .catch(error => {
        // this will be executed if the operation fails console.log("The operation failed: " + error);
});

In this example, the makeRequest() function returns a Promise object that represents the eventual completion or failure of an asynchronous operation. The then() and catch() methods are used to specify what will happen when the operation is successful or has failed, respectively. The then() method receives a callback function that will be executed with the result of the operation if it is successful, while the catch() method receives a callback function that will be executed with the error if the operation fails. This allows for the asynchronous operation to be handled in a clean and organized way.

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14. What is the difference between a class and an interface in TypeScript?

In TypeScript, a class is a blueprint for creating objects, while an interface defines a contract for classes or objects to implement. A class can have properties and methods, and can extend other classes or implement interfaces. An interface, on the other hand, cannot have any implementation code, but it can define the shape of an object, including its properties and methods. For example:

// this is a class that has a property and a method
class ExampleClass {
    x: number;
    getX() {
        return this.x;
    }
}

// this is an interface that defines the shape of an object
interface ExampleInterface {
    x: number;
    getX(): number;
}

In this example, the ExampleClass class has a property x and a method getX(), which can be used to access the value of x. The ExampleInterface interface, on the other hand, defines the shape of an object that has a property x and a method getX() that returns a number. A class can implement this interface by declaring that it implements the interface and providing the required properties and methods.


15. What is the purpose of the async and await keywords in JavaScript?

The async and await keywords are used in JavaScript to handle asynchronous operations in a more synchronous-like manner. The async keyword is used to declare an asynchronous function, which allows the use of the await keyword inside the function. The await keyword is used to pause the execution of the asynchronous function until a specified asynchronous operation has completed. This allows for cleaner and more intuitive handling of asynchronous operations in JavaScript. For example:

// this is an asynchronous function that uses the `await` keyword
async function example() {
    // this line will pause the execution of the function until the asynchronous operation is complete
    let result = await makeRequest();

    // this line will not be executed until the asynchronous operation has completed
    console.log("The result is: " + result);
}

// this is an example of how the asynchronous function can be called
example();

In this example, the example() function is declared as asynchronous using the async keyword. Inside the function, the await keyword is used to pause the execution of the function until the makeRequest() asynchronous operation has completed. Once the operation has completed, the result of the operation is logged to the console. This allows for the asynchronous operation to be handled in a more synchronous-like manner, without the need for callback functions or other complex constructs.


16. What is the purpose of the Array.prototype.reduce() method in JavaScript?

The Array.prototype.reduce() method is a built-in method in JavaScript that is used to reduce an array to a single value. It takes a callback function as an argument, which is applied to each element of the array and combines them into a single value. This can be useful for performing a variety of operations on arrays, such as calculating the sum or product of all the elements, or creating a new object from the elements of the array. For example:

// this array contains a list of numbers
let numbers = [1, 2, 3, 4, 5];

// this uses the `reduce()` method to calculate the sum of the elements
let sum = numbers.reduce((total, current) => total + current);

// the result is 15

In this example, the reduce() method is used to calculate the sum of the elements of the numbers array. The callback function that is passed to reduce() takes two arguments, total and current, which represent the accumulated value and the current element of the array, respectively. The callback function adds the current element to the total, and the result of the callback function is used as the new value of the total argument in the next iteration. This continues until all the elements of the array have been processed, and the final value of total is the sum of the elements.


17. What is the purpose of the Array.prototype.filter() method in JavaScript?

The Array.prototype.filter() method is a built-in method in JavaScript that is used to filter the elements of an array based on a specified condition. It takes a callback function as an argument, which is applied to each element of the array and returns a new array with only the elements that satisfy the condition. This can be useful for extracting a subset of elements from an array based on certain criteria, or for removing unwanted elements from an array. For example:

// this array contains a list of numbers
let numbers = [1, 2, 3, 4, 5];

// this uses the `filter()` method to return only the even numbers
let evenNumbers = numbers.filter(x => x % 2 === 0);

// the resulting array is [2, 4]

In this example, the filter() method is used to return only the even numbers from the numbers array. The callback function that is passed to filter() checks if the current element is evenly divisible by 2, and returns true if it is and false if it is not. The filter() method then creates a new array with only the elements for which the callback function returned true, which in this case are the even numbers.


18. What is the purpose of the Array.prototype.sort() method in JavaScript?

The Array.prototype.sort() method is a built-in method in JavaScript that is used to sort the elements of an array. It takes an optional callback function as an argument, which is used to specify the sorting criteria. If no callback function is provided, the elements of the array are sorted in ascending order according to their default sort order. This can be useful for organizing the elements of an array in a specific order, such as alphabetical or numerical order. For example:

// this array contains a list of strings
let words = ["apple", "banana", "cherry"];

// this uses the `sort()` method to sort the strings alphabetically
let sortedWords = words.sort();

// the resulting array is ["apple", "banana", "cherry"]

In this example, the sort() method is used to sort the elements of the words array alphabetically. Because no callback function is provided, the elements are sorted in ascending order according to their default sort order, which for strings is alphabetical. The sort() method then returns a new array with the sorted elements, which in this case is the words array in alphabetical order.

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19. What is the purpose of the let keyword in JavaScript?

The let keyword is a new keyword in JavaScript that is used to declare variables. It is similar to the var keyword, but it has a different scope. Variables declared with let are scoped to the block in which they are declared, whereas variables declared with var are scoped to the entire function in which they are declared. This allows for more fine-grained control over the visibility and lifetime of variables in JavaScript. For example:

// this code uses the `let` keyword to declare a variable
let x = 10;

// this code uses the `var` keyword to declare a variable
var y = 20;

// the `x` and `y` variables are accessible in this scope
console.log(x + y);

// this code uses the `let` keyword to declare a variable inside a block
if (x > 0) {
    let z = 30;
    // the `z` variable is only accessible inside this block
    console.log(x + y + z);
}

// this code uses the `var` keyword to declare a variable inside a block
if (y > 0) {
    var w = 40;
    // the `w` variable is accessible in the entire function, not just the block
    console.log(x + y + w);
}

In this example, the x and y variables are declared using the let and var keywords, respectively. The x variable is only accessible in the scope in which it is declared, while the y variable is accessible in the entire function in which it is declared. The z and w variables are also declared using the let and var keywords, respectively, and their scope is determined by the keyword that is used to declare them. The z variable is only accessible inside the block in which it is declared, while the w variable is accessible in the entire function.


20. What is the purpose of the const keyword in JavaScript?

The const keyword is a new keyword in JavaScript that is used to declare constants. A constant is a variable that cannot be reassigned or redeclared. This allows for the creation of variables that have a fixed value and cannot be changed, which can be useful for ensuring that certain values are not accidentally modified. Constants are declared using the const keyword, and their value must be assigned at the time of declaration. For example:

// this code uses the `const` keyword to declare a constant
const PI = 3.14159;

// this code attempts to reassign the value of the constant, but it will fail
PI = 3.14;

// this code uses the `const` keyword to declare a constant and assign its value
const MAX_HEIGHT = 100;

// the constant is now fixed at the value 100 and cannot be changed

In this example, the PI and MAX_HEIGHT constants are declared using the const keyword. The PI constant is given a value of 3.14159 at the time of declaration, but an attempt to reassign its value later on will fail because it is a constant. The MAX_HEIGHT constant is also declared using the const keyword, and its value is assigned at the time of declaration. The value of a constant cannot be changed once it is assigned, so the MAX_HEIGHT constant will always have a value of 100.


21. What is the purpose of the Array.prototype.map() method in JavaScript?

The Array.prototype.map() method is a built-in method in JavaScript that is used to create a new array by applying a callback function to each element of an array. It takes a callback function as an argument, which is applied to each element of the array and returns a new array with the results of the callback function. This can be useful for transforming the elements of an array, or for creating a new array from the elements of an array. For example:

// this array contains a list of numbers
let numbers = [1, 2, 3, 4, 5];

// this uses the `map()` method to square each element of the array
let squaredNumbers = numbers.map(x => x * x);

// the resulting array is [1, 4, 9, 16, 25]

In this example, the map() method is used to create a new array by applying a callback function to each element of the numbers array. The callback function takes the current element as an argument, squares it, and returns the result. The map() method then creates a new array with the results of the callback function, which in this case is a new array with the squared elements of the numbers array.


22. What is the purpose of the Array.prototype.forEach() method in JavaScript?

The Array.prototype.forEach() method is a built-in method in JavaScript that is used to iterate over the elements of an array. It takes a callback function as an argument, which is applied to each element of the array. Unlike a for loop, the forEach() method does not return a new array, but it allows for the execution of a specific operation on each element of the array. This can be useful for performing a specific action on each element of an array, such as logging the elements to the console. For example:

// this array contains a list of numbers
let numbers = [1, 2, 3, 4, 5];

// this uses the `forEach()` method to log each element of the array to the console
numbers.forEach(x => console.log(x));

// this will log the numbers 1, 2, 3, 4, and 5 to the console

In this example, the forEach() method is used to iterate over the elements of the numbers array. The callback function that is passed to forEach() logs the current element to the console. The forEach() method then applies the callback function to each element of the array, which causes the numbers to be logged to the console. Unlike a for loop, the forEach() method does not return a new array, but it allows for the execution of a specific operation on each element of the array.


Thank you for reading this JavaScript interview questions and answers. We hope that it has provided you with some useful information and helped you prepare for your next JavaScript interview. We hope these Java Interview Questions help you in cracking your next programmer job Interview.

All The Best!

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