JavaScript is one of the most popular programming languages in the world and a core technology for web development. As a result, JavaScript interview questions frequently come up in tech job interviews, testing a candidate’s knowledge of JavaScript fundamentals, APIs, asynchronous programming, and more.
In this complete guide, we have compiled the most common and important JavaScript interview questions to help you prepare for your upcoming interview.
Basic JavaScript Interview Questions
JavaScript is a lightweight, interpreted programming language that runs in any browser. Here are some common basic interview questions on core JavaScript:
What are the different data types in JavaScript?
The latest ECMAScript standard defines seven data types in JavaScript:
- Primitive data types:
- Number – for numbers like 100, 3.14 etc.
- String – for strings of text e.g. “Hello”
- Boolean – for true/false values
- Null – a null value
- Undefined – a variable declared but no value assigned
- Symbol – data type whose instances are unique
- Non-primitive data type:
- Object – for more complex data structures like arrays, dates etc.
What is the difference between null and undefined?
- Null is an assigned value representing no value. It is an empty value variable defined by a developer.
- Undefined means a variable has been declared but not defined yet. It is assigned by JavaScript automatically.
What are JavaScript closures?
JavaScript closures allow a function to access variables from an enclosing scope or outer function even after the outer function has closed. This persistence of the function scope/context is called closure in JavaScript.
What is the difference between == and === operators?
- The equality operator == allows loose equality with type conversion.
- The strict equality operator === enforces strict equality with no type conversion.
For example:
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console.log(1 == ‘1’); // true
console.log(1 === ‘1’); // false
Intermediate JavaScript Interview Questions
Here are some slightly more advanced JavaScript interview questions and answers on concepts like scoping, inheritance, promises etc:
What is hoisting in JavaScript?
Hoisting in JavaScript allows access to function declarations and variables before they are declared. Variables are hoisted to the top and initialized with a default value of undefined. Function declarations are hoisted completely.
How does prototypal inheritance work in JavaScript?
All JavaScript objects have a hidden [[Prototype]] property that makes inheritance possible in JavaScript. The prototype object contains properties and methods that can be accessed by all instances via the __proto__ property.
When a property is accessed on an object:
- If the property exists on the object itself, that value is used
- If not, the JavaScript engine looks down the prototype chain until it finds the property.
What are JavaScript promises?
Promises represent the eventual completion or failure of an asynchronous operation. They provide an alternative approach for asynchronous code handling compared to callbacks.
A promise can be in three states:
- Pending – initial state
- Fulfilled – operation completed successfully
- Rejected – operation failed
Promises avoid callback hell and make asynchronous code look synchronous.
What are classes in JavaScript?
JavaScript classes are templates for creating objects. They provide reuseable code using inheritance, constructors and methods.
Syntax:
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class ClassName {
constructor() { … }
method1() { … }
method2() { … }
}
Advanced JavaScript Interview Questions
Let’s look at some advanced JS interview questions now:
Explain event delegation
Event delegation allows adding event listeners to a parent element instead of adding them directly to child elements.
The listener on the parent element analyzes bubbled events from descendants to find a match on child elements.
Benefits:
- Less memory usage
- Easier to add/remove elements
What is the difference between synchronous and asynchronous code in JavaScript?
Synchronous code is executed in sequence, blocking further execution until completion.
Asynchronous code allows other code to execute while waiting for long-running tasks via callbacks, promises, async/await etc.
Asynchronous programming avoids blocking and improves performance of web apps.
What are common memory leaks in JavaScript?
Some common memory leaks:
- Global variables holding unwanted references to objects
- Event listeners not removed after use
- Outdated references to DOM nodes after removal
- Cached references in closures
These continue to occupy memory though no longer required.
What is memoization in JavaScript?
Memoization is the process of caching the output of expensive function calls to avoid repeated computations on same inputs.
This technique is commonly used for optimizing recursive functions.
JavaScript Coding Interview Questions
Some sample coding problems you may encounter:
FizzBuzz
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// Print numbers from 1 to 100
// For multiples of 3 print “Fizz” instead of number
// For multiples of 5 print “Buzz”
// For multiples of 3 and 5 print “FizzBuzz”
Palindrome Checker
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function isPalindrome(str) {
// Remove spaces, lowercase
str = str.replace(/\s/g, “”).toLowerCase();
// Check if str is equal to reverse of str
return str === str.split(“”).reverse().join(“”);
}
Fibonacci Series Generator
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function fibonacci(n) {
// Base case
if (n <= 1) return n;
return fibonacci(n–1) + fibonacci(n–2);
}
Conclusion
This covers the most important JavaScript interview questions and answers for both freshers and experienced developers.
Key topics include data types, closures, prototypes, promises, async/await, DOM manipulation and more.
Practice these questions thoroughly so you can confidently answer them during interviews. Keep learning more advanced JavaScript concepts to take your skills to the next level.
Best of luck with your JavaScript job interview!