30+ JavaScript Interview Questions And Answers
JavaScript is a versatile scripting language used for creating engaging websites. It powers interactive features such as animations, forms, and dynamic loading of content. Here, we offer a collection of interview questions to assess candidates' understanding and proficiency in JavaScript at various levels.
Most asked JavaScript interview questions
Beginners
1.
What are variables in JavaScript?
1.
What are variables in JavaScript?
Variables are containers for storing data values. In JavaScript, we use the var, let, or const keywords to declare variables.
2.
Explain the difference between '=='' and '===' in JavaScript.
2.
Explain the difference between '=='' and '===' in JavaScript.
The '==' operator checks for equality in value only, while '===' checks for both value and type.
3.
Describe what a function is in JavaScript.
3.
Describe what a function is in JavaScript.
A function is a block of code designed to perform a particular task. It's executed when called.
4.
What is a callback function?
4.
What is a callback function?
A callback function is a function passed into another function as an argument to be executed later.
5.
What are arrays in JavaScript?
5.
What are arrays in JavaScript?
Arrays are special variables that can hold more than one value at a time.
6.
Can you show me a simple example of a loop in JavaScript?
for (let i = 0; i < 5; i++) {
console.log(i);
}
6.
Can you show me a simple example of a loop in JavaScript?
for (let i = 0; i < 5; i++) {
console.log(i);
}
The provided code will print the numbers from 0 to 4 to the console.
7.
What is a JavaScript object?
7.
What is a JavaScript object?
A JavaScript object is a collection of properties, where each property is defined as a key-value pair.
8.
Explain 'null' in JavaScript.
8.
Explain 'null' in JavaScript.
Null is a special value representing 'no value' or 'nothing'.
9.
What is the use of 'parseInt' in JavaScript?
9.
What is the use of 'parseInt' in JavaScript?
'parseInt' converts a string to an integer.
10.
Let's guess, what does the following code return: '7' + 1 + 5?
'7' + 1 + 5
10.
Let's guess, what does the following code return: '7' + 1 + 5?
'7' + 1 + 5
The code will return the string '715' due to type coercion.
11.
What is the event loop?
11.
What is the event loop?
The event loop handles asynchronous callbacks in JavaScript. It's a fundamental part of the runtime model.
12.
What do you understand by 'scope' in JavaScript?
12.
What do you understand by 'scope' in JavaScript?
Scope refers to the current context of code, which determines the accessibility of variables.
13.
Can JavaScript code generate random numbers?
13.
Can JavaScript code generate random numbers?
Yes, using Math.random() you can generate a random number between 0 and 1.
14.
What does the 'this' keyword refer to in JavaScript?
14.
What does the 'this' keyword refer to in JavaScript?
The 'this' keyword refers to the object the function is a property of.
15.
Can you explain what a closure is in JavaScript with an example?
function outerFunction() {
let count = 0;
function innerFunction() {
count++;
console.log(count);
}
return innerFunction;
}
15.
Can you explain what a closure is in JavaScript with an example?
function outerFunction() {
let count = 0;
function innerFunction() {
count++;
console.log(count);
}
return innerFunction;
}
A closure is a function having access to the parent scope, even after the parent function has closed.
Advanced
1.
Explain the concept of 'hoisting' in JavaScript.
1.
Explain the concept of 'hoisting' in JavaScript.
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.
2.
What is the 'Prototype' chain?
2.
What is the 'Prototype' chain?
The 'Prototype' chain is a feature in JavaScript where an object can inherit properties from its prototype ancestor.
3.
Can you define what IIFE (Immediately Invoked Function Expression) is?
3.
Can you define what IIFE (Immediately Invoked Function Expression) is?
IIFE is a function that runs as soon as it is defined. It's used to create a private scope.
4.
What is 'event delegation' in JavaScript?
4.
What is 'event delegation' in JavaScript?
Event delegation is a technique to handle events efficiently by using a single handler on a parent element.
5.
Describe 'promises' in JavaScript.
5.
Describe 'promises' in JavaScript.
Promises represent the eventual completion (or failure) of an asynchronous operation and its resulting value.
6.
How does 'async/await' improve working with asynchronous code in JavaScript?
6.
How does 'async/await' improve working with asynchronous code in JavaScript?
'async/await' makes asynchronous code easier to write and read, avoiding the callback hell.
7.
Can you explain how 'this' keyword works in arrow functions?
7.
Can you explain how 'this' keyword works in arrow functions?
Arrow functions don't have their own 'this' context and they inherit it from the enclosing execution context.
8.
What is the significance of 'use strict' in JavaScript?
8.
What is the significance of 'use strict' in JavaScript?
'use strict' enforces stricter parsing and error handling of JavaScript codes.
9.
How does 'webpack' help with JavaScript development?
9.
How does 'webpack' help with JavaScript development?
Webpack bundles JavaScript files for usage in a browser, which can improve performance and manage dependencies.
10.
Explain the concept of destructuring in JavaScript.
10.
Explain the concept of destructuring in JavaScript.
Destructuring is a way to unpack values from arrays or properties from objects into distinct variables.
11.
What does the following code return:
[1, 2, 3].map(num => "Number: " + num)
11.
What does the following code return:
[1, 2, 3].map(num => "Number: " + num)
The code returns a new array: ['Number: 1', 'Number: 2', 'Number: 3'].
12.
What is the difference between 'null' and 'undefined'?
12.
What is the difference between 'null' and 'undefined'?
'null' is an assigned value representing no value, while 'undefined' means a variable has been declared but not assigned a value.
13.
Can you explain the difference between an 'attribute' and a 'property' in JavaScript?
13.
Can you explain the difference between an 'attribute' and a 'property' in JavaScript?
Attributes are defined in the HTML document while properties are on DOM nodes.
14.
Explain what 'memoization' is in JavaScript.
14.
Explain what 'memoization' is in JavaScript.
Memoization is a programming technique used to speed up function calls by caching the results.
15.
Demonstrate a 'spread operator' and explain its uses.
const a = [1, 2];
const b = [...a, 3, 4];
15.
Demonstrate a 'spread operator' and explain its uses.
const a = [1, 2];
const b = [...a, 3, 4];
The spread operator allows an iterable to expand in places where arguments are expected. It's used for array construction and concatenation.
JavaScript Interview Tips
Understanding the Question
When faced with difficult interview questions, the first step is to ensure you understand the question completely. Listen carefully and don't hesitate to ask for clarification if needed. It's important to grasp the core of the question before jumping to an answer. Break down the question into parts if it's complex. Restating the question in your own words can also help confirm your understanding with the interviewer. Always take a moment to compose your thoughts before answering, rushing can lead to misunderstandings or incomplete responses.
Structured Approach
Addressing challenging questions often requires a structured approach. One common strategy is the STAR method (Situation, Task, Action, Result), which is particularly effective for behavioral interview questions. It helps organize your thoughts and provides a clear narrative of your experience. For technical questions, a structured approach might include outlining the steps you would take to solve a problem, discussing potential challenges, and how you'd overcome them. Clear and logical thinking is beneficial in conveying complex subjects.
Reflecting on Experience
When answering tough interview questions, relate your responses to your past experiences. Share specific examples that demonstrate how you've handled similar situations successfully. Employ your anecdotes to showcase your problem-solving abilities, expertise, and professional growth. It’s influential to reflect on what you learned from the experience and how it applies to the potential job. Personal stories are memorable and can distinguish you from other candidates.
Honesty and Transparency
It is essential to stay truthful when answering difficult questions. If you encounter a question that targets an area you're less familiar with, it's acceptable to admit it. You can then discuss how you would take steps to learn or how you plan to bridge that knowledge gap. Employers value honesty and the willingness to learn. This approach can also apply when discussing past mistakes or challenges; being open about them and emphasizing what you learned can be seen as a strength.
Keeping Calm Under Pressure
During interviews, you're being evaluated not only on your answers but also on how you handle stress. If you're presented with a tough question, maintain your composure. Take a deep breath, consider the question thoughtfully, and keep a positive attitude. It's acceptable to take a brief pause to collect your thoughts. Interviewers appreciate candidates who can stay calm under pressure. Even if you're unsure of the answer, demonstrating resilience and composure is important.
FAQs
How much does a JavaScript developer cost?
The cost of a JavaScript developer may vary based on their experience and location. On average, you can expect to invest starting from $45/hour for vetted talent. Read more: How much does a JavaScript developer cost?
Where can I hire a JavaScript developer?
FireHire is your go-to destination to connect with elite JavaScript developers. We take care of the search and vetting to match you with the right expertise. Read more: Where can I hire a JavaScript developer?
What is the hourly rate for JavaScript developer?
Hourly rates for JavaScript developers can range widely, but at FireHire, competitive rates starting from $45/hour get you access to top-tier Front-End developers. Read more: What is the hourly rate for JavaScript developer?
Why using FireHire for hiring JavaScript developers is the best choice?
Choose FireHire for a unique blend of swift, risk-free hiring, access to a broad range of tech expertise, and the assurance of a 30-day replacement guarantee.
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