Top 50 Javascript Interview Questions
Expertly Ace the Top 50 JavaScript Interview Questions
As a widely used scripting language, JavaScript is an essential skill for any web developer or programmer. During job interviews, candidates are often asked a variety of questions to test their knowledge and understanding of this language. Some of the top 50 JavaScript interview questions cover topics such as the basic syntax, DOM manipulation, event handling, jQuery, AJAX, and more. It is important to not only have a good grasp of the language itself but also to understand its use cases and best practices. Many questions also assess problem-solving skills and the ability to write efficient and optimized code. Overall, being proficient in JavaScript and being able to answer these top 50 interview questions can greatly increase a candidate's chances of landing a job as a JavaScript developer.
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1 What is JavaScript?
JavaScript is a high level, interpreted programming language used to create dynamic and interactive web pages, as well as server side applications. It runs on the client side, meaning it is executed in the user's browser, providing a more engaging and responsive experience for users.
2) What is the difference between var, let, and const in JavaScript?
Var declares a variable globally or locally to an entire function, let declares a block scoped variable, and const declares an unchangeable value. Variables declared with let and const have block level scope, meaning they can only be accessed within the block of code they were declared in.
3) What are the different data types in JavaScript?
JavaScript has six primitive data types: string, number, boolean, undefined, null, and symbol. It also has a seventh data type, object, which includes functions, arrays, and objects.
4) How do you detect the data type of a variable in JavaScript?
The typeof operator is used to determine the data type of a variable or expression in JavaScript. It returns a string indicating the type of data, such as “string” or “number.”
5) Explain event bubbling and event capturing in JavaScript.
Event bubbling is when a child element's event triggers the same event on its parent elements, propagating up the DOM tree. Event capturing is the opposite, where the event is handled starting at the highest level parent element and propagating down to the child element where it originated.
6) What is the difference between == and === in JavaScript?
The double equals (==) compares the value of two operands without considering their types, while the triple equals (===) not only compares the values but also checks for their types to be equal.
7) What is the “this” keyword in JavaScript and how does it work?
The “this” keyword refers to the current execution context of a function. It can be used to access properties and methods within an object. The value of “this” is determined by how a function is called, and can change depending on the context in which it is used.
8) How do you handle errors in JavaScript?
There are a few ways to handle errors in JavaScript, such as try/catch blocks, error handling functions, and using the throw keyword to manually throw an error. Proper error handling can help prevent a program from crashing and provide helpful feedback to the user.
9) What is a closure in JavaScript?
A closure is a function that has access to variables defined in its outer function scope, even after the outer function has returned. This concept is frequently used with event handlers and in implementing private variables and methods in objects.
10) What is hoisting in JavaScript?
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their scope before code execution. This allows variables to be used before they are declared, although it is best practice to declare them at the top of their scope.
11) Explain the concept of callback functions in JavaScript.
A callback function is a function that is passed as an argument to another function and is executed later in the code. This allows for asynchronous programming, where a function continues to execute while waiting for a task to complete.
12) What is the difference between null and undefined in JavaScript?
Null and undefined both represent the absence of a value. However, null is explicitly assigned to a variable, while undefined means a variable has been declared but has not been assigned a value.
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13) What is a higher order function in JavaScript?
A higher order function is a function that takes another function as an argument, or returns a function as its result. This allows for advanced functional programming techniques in JavaScript.
14) How do you handle asynchronous code in JavaScript?
Asynchronous code is handled using callbacks, promises, or the newer async/await syntax. These techniques allow code to continue executing while waiting for the result of an asynchronous operation, improving performance and responsiveness.
15) What is the difference between synchronous and asynchronous code in JavaScript?
Synchronous code is executed line by line and blocks further execution until the current operation is completed. Asynchronous code is non blocking and allows other code to be executed while waiting for a particular operation to complete.
16) What is the difference between a deep copy and a shallow copy in JavaScript?
A deep copy creates a new instance of an object and copies all of its properties and values recursively. A shallow copy creates a new object that references the original object's properties, meaning changes to the original object will also affect the shallow copy.
17) How do you implement inheritance in JavaScript?
JavaScript uses prototype based inheritance, where objects can inherit properties and methods from a parent or prototype object. This can be achieved using the prototype chain or by using the newer class syntax introduced in ES6.
18) What is the difference between anonymous and named functions in JavaScript?
Anonymous functions do not have a name and can only be called through a reference to the variable they are assigned to. Named functions have a name and can be called directly by their name.
19) How do you handle cross site scripting (XSS) attacks in JavaScript?
XSS attacks can be prevented by properly sanitizing user input, encoding output, and implementing strict content security policies.
20) What are the different ways to define a JavaScript object?
Objects can be defined using object literal notation, object constructor functions, and the newer ES6 class syntax.
21) Explain event delegation in JavaScript.
Event delegation is a technique where a parent element is used to handle events on behalf of its children. This allows for more efficient event handling, especially in situations where new elements are added dynamically.
22) How do you convert a string to an integer in JavaScript?
The parseInt() and Number() methods can be used to convert a string to an integer in JavaScript. They will both return NaN (Not a Number) if the string cannot be converted to a number.
23) What is the difference between a global variable and a local variable in JavaScript?
Global variables are declared outside of a function and can be accessed from any part of the code. Local variables are declared within a function and can only be accessed within that function.
24) How do you clone an object in JavaScript?
Objects can be cloned using the Object.assign() method, the spread (…) operator, or using the JSON.parse() and JSON.stringify() methods.
25) What is the difference between == and === in JavaScript?
The double equals (==) compares the value of two operands without considering their types, while the triple equals (===) not only compares the values but also checks for their types to be equal.
26) How does prototypal inheritance differ from classical inheritance?
In classical inheritance, objects inherit directly from classes, while in prototypal inheritance, objects are created from a prototype object, which they inherit properties and methods from.
27) What is the DOM and how do you manipulate it using JavaScript?
The Document Object Model (DOM) is a representation of an HTML document as a tree structure. It can be manipulated using JavaScript methods such as getElementById(), getElementsByClassName(), and querySelector().
28) What is the difference between == and === in JavaScript?
The double equals (==) compares the value of two operands without considering their types, while the triple equals (===) not only compares the values but also checks for their types to be equal.
29) How do you implement error handling in JavaScript?
Error handling in JavaScript can be implemented using try/catch blocks, error handling functions, or the throw keyword to manually throw an error.
30) What is the difference between isNaN() and Number.isNaN() in JavaScript?
isNaN() attempts to convert the tested value to a number before checking if it is NaN, while Number.isNaN() only returns true if the tested value is actually NaN.
31) How do you implement hoisting in JavaScript?
Hoisting occurs automatically in JavaScript, allowing variables and function declarations to be moved to the top of their scope.
32) What is a promise in JavaScript?
A promise is an object that represents the eventual completion or failure of an asynchronous operation. It allows for more predictable handling of asynchronous code and supports error handling and chaining multiple operations.
33) How do you handle errors in promises in JavaScript?
Errors in promises can be handled using the catch() method, which will be called if any of the operations in the promise chain throw an error. Additionally, the reject handler in the promise constructor can be used to handle errors within the promise itself.
34) What is a generator function in JavaScript?
A generator function allows for pausing and resuming the execution of a function using the yield keyword. This can be useful for writing asynchronous code in a synchronous manner.
33) How do you implement destructuring in JavaScript?
Destructuring allows for extracting specific values from objects or arrays into their own variables, making code more concise and readable. It can be used with object and array literals, as well as function parameters.
36) What are the different ways to define a variable in JavaScript?
Variables can be defined using the var, let, or const keywords. Var is function scoped, let is block scoped, and const is block scoped and values cannot be re assigned.
37) What is the difference between a method and a function in JavaScript?
A method is a function that is a property of an object, while a regular function is not. Methods are usually called with an object context, while functions are called directly.
38) What is the difference between a cookie and a local storage in JavaScript?
Cookies are small pieces of data stored in a browser for websites, while local storage is designed to store larger amounts of data locally and persistently in the browser.
39) How do you implement sorting in JavaScript?
The sort() method can be used to sort arrays in JavaScript, either alphabetically or numerically. A custom sorting function can also be passed as a parameter to the sort() method.
40) What is a constructor function in JavaScript?
A constructor function is used to create objects based on a template or blueprint. It is similar to a class in other programming languages and is used to maintain consistency in the creation of objects.
41) How do you implement authentication using JavaScript?
Authentication can be implemented using various techniques such as cookies, token based authentication, or using third party authentication providers such as OAuth.
42) What is a single page application in JavaScript?
A single page application (SPA) is a web application that loads a single HTML page and dynamically updates the page as the user interacts with it, providing a more seamless and responsive user experience.
43) How do you implement routing in a single page application?
Routing in a single page application is typically implemented using a library or framework, such as Vue Router or React Router, which maps specific routes to specific components in the application.
44) What is garbage collection in JavaScript?
Garbage collection is a process in which the JavaScript engine automatically frees up memory by removing objects that are no longer referenced or in use.
45) How do you handle memory leaks in JavaScript?
Memory leaks occur when an object is kept in memory unnecessarily. They can be avoided by properly managing references, using let and const for variable declarations, and avoiding global variables.
46) What is currying in JavaScript?
Currying is a technique used to transform a function with multiple arguments into a series of functions, each with a single argument. This can be useful for creating reusable utility functions.
47) How do you convert an object to an array in JavaScript?
The Object.entries() method can be used to convert an object to an array of key value pairs, while the Object.keys() and Object.values() methods can be used to convert an object's keys and values, respectively, to an array.
48) What is a data attribute in JavaScript?
Data attributes are custom HTML attributes that can be used to store additional information about elements. They can be accessed using the dataset property in JavaScript.
49) How do you implement lazy loading in JavaScript?
Lazy loading is a technique used to defer loading of non critical resources until they are needed. This can improve performance by reducing the initial load time of a page. It can be implemented using libraries or using native browser functionality such as the “loading” attribute for images.
50) What is a higher order component in JavaScript?
A higher order component (HOC) is a function that takes in a component and returns a new, enhanced component. This is useful for adding additional functionality to components without modifying their original code.
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