Frequently Asked Questions for JavaScript Interview

Introduction

JavaScript is a popular language that people use to make websites and server programs. The thing that makes JavaScript so great is that it is very flexible, and lots of people use it. Many people can help you if you get stuck. JavaScript is used for things, which is why people like to use it. We put together a list of JavaScript Interview Questions and Answers to help people who are just starting with JavaScript and people who have been using it for a while to learn more about JavaScript and get ready for job interviews. Explore our JavaScript Course Syllabus to begin your journey in web development.

JavaScript Interview Questions for Freshers

1. What are the different data types in JavaScript?

JavaScript has different types of values that help store and manage data. These are mainly divided into primitive and non-primitive types.

• String – text data (e.g., “Hello”)
• Number – numeric values (e.g., 10, 3.5)
• BigInt – large integers
• Boolean – true or false values
• Undefined – variable declared but not assigned
• Null – empty or no value
• Symbol – unique identifier
• Object – stores complex data like arrays, functions, and objects

2. What is the main difference between == and === operators?

The difference lies in value comparison and type checking.

• == compares only values (it performs type conversion if needed)
• === compares both value and data type (strict comparison)

Example:

• “5” == 5 → true
• “5” === 5 → false

3. Explain var, let, and const.

JavaScript provides three ways to declare variables:

• var → function-scoped, can be redeclared and updated
• let → block-scoped, can be updated but not redeclared in the same scope
• const → block-scoped, cannot be reassigned once declared

4. What is Hoisting?

Hoisting is JavaScript’s default behavior, where variable and function declarations are moved to the top of their scope before execution.

However:

• Only declarations are hoisted, not initializations
• var is hoisted with an undefined value
• let and const are hoisted but not initialized

5. What is the difference between null and undefined?

This is a common interview question based on value assignment.

• undefined → It indicates that a variable exists but has no value yet.
• null → intentional absence of value (assigned by developer)

6. What is a Closure?

A closure is an important concept in JavaScript functions.

• A function that remembers variables from its outer scope.
• Even when the outer function has already finished running.
• Helps in data privacy and function factories.
  

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7. Explain the this keyword.

The this keyword refers to the object that is currently executing the function.

• In a method → refers to the object
• In a function → depends on strict or non-strict mode
• In the constructor → refers to the new object created

8. What are Arrow Functions?

Arrow functions simplify function writing with cleaner syntax.

• Introduced in ES6
• Shorter syntax than regular functions
• Do not have their own this
• Cannot be used as constructors

9. What is an IIFE in JavaScript?

A Promise is used to handle asynchronous operations in JavaScript.

• It represents a future result of an operation
• It has 3 states:
  • Pending
  • Fulfilled
  • Rejected
• Helps avoid callback hell

10. What is a Higher-Order Function?

These methods control the value of this.

• call() → invokes a function immediately with arguments.
• apply() → similar to call(), but arguments are passed as an array.
• bind() → returns a new function with bound this.

11. What is the DOM?

Event delegation is a smart way to handle events efficiently.

• Uses event bubbling.
• Add one event handler to the parent container.
• Handles events for child elements automatically.
• Improves performance in large applications.

12. Explain the Event Loop.

An IIFE is a function that runs immediately after it is defined.

It is commonly used to:

• Create a private scope.
• Avoid polluting global variables.
• Execute code once.

13. What are Promises and their states?

A higher-order function works with other functions.

• Takes another function as an argument
• OR returns a function as output
• Examples: map(), filter(), reduce()

14. What is the difference between call(), apply(), and bind()?

The DOM (Document Object Model) represents a web page.

• It is a tree-like structure of HTML elements.
• JavaScript can manipulate it dynamically.
• Used to change the content, style, and structure of pages.

15. What is Event Delegation?

The Event Loop is the mechanism that enables asynchronous behavior in JavaScript.

• JavaScript runs in a single thread.
• The event loop manages the execution of tasks.
• Handles callback queue and call stack.
• Ensures non-blocking behavior.

JavaScript Interview Questions for Experienced 

1. Explain the Event Loop and its components (Macrotasks vs. Microtasks).

JavaScript is single-threaded, but it can handle asynchronous operations using the Event Loop. The Event Loop continuously checks the call stack and task queues to decide what to execute next.

• Macrotasks (Task Queue):
  • Includes setTimeout, setInterval, DOM events.
  • Execution happens once the current task and microtasks finish.
• Microtasks (Microtask Queue):
  • Includes Promises (.then, .catch, .finally)
  • Always executed before macrotasks

Execution order:

• Call Stack → Microtasks → Macrotasks

This priority ensures faster handling of Promises compared to timers.

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2. How do you detect and prevent memory leaks?

Memory leaks occur when memory is not released even after it is no longer needed, leading to performance issues.

Common causes:

• Unremoved event listeners
• Unused timers (setInterval, setTimeout)
• Global variables holding references

Prevention & Detection:

• Remove event listeners when not needed
• Clear timers using clearInterval / clearTimeout
• Avoid unnecessary global variables
• Use Chrome DevTools (Memory tab):
  • Take heap snapshots
  • Identify detached DOM nodes
  • Track objects not being garbage collected

3. What is the main difference between prototypal and classical inheritance?

JavaScript uses Prototypal Inheritance, even though ES6 introduced class syntax.

• Prototypal Inheritance:
  • Objects inherit directly from other objects
  • Uses prototype chain
  • Flexible and dynamic
• Classical Inheritance (ES6 syntax):
  • Looks like traditional OOP (Java, C++)
  • Uses class and extends
  • Internally still based on prototypes (syntactic sugar)

4. How would you implement a custom debounce and throttle function?

Debounce and throttle are techniques used to control how often a function runs, especially during frequent events like scrolling or typing.

• Debouncing:
  • Delays execution until the event stops triggering
  • Useful for search input or resize events
• Throttling:
  • Ensures execution at fixed time intervals
  • Useful for scroll tracking or button clicks

5. Explain the “Temporal Dead Zone” (TDZ).

The Temporal Dead Zone refers to the time between entering a block and declaring a variable using let or const.

• Variables are hoisted but not initialized.
• Accessing them before declaration causes a ReferenceError.
• Helps avoid accidental usage before initialization.

6. How can you reduce expensive reflows and repaints in web development?

Minimize reflows and repaints by consolidating DOM updates, steering clear of constant layout alterations, and distinguishing reads from writes. Refrain from changing styles repeatedly within loops, and employ transform and opacity for animations as they do not cause layout recalculations, ensuring smooth performance.

7. How can lexical scoping be implemented in practice?

Lexical scoping works because functions store a reference to their outer scope when they are defined, allowing them to access those variables later.

8. What are Generators, and where would you use them?

Generators are special functions that can pause and resume execution using yield.

• Use function* syntax
• Return an iterator
• Useful for:
  • Infinite data streams
  • Lazy evaluation
  • Custom iterators
  • Managing async flows

9. Explain Web Workers and their role in performance.

Web Workers allow JavaScript to run tasks in background threads.

• Run scripts separately from the main UI thread.
• Useful for heavy computations.
• Prevent UI freezing.
• Communicate using postMessage().

10. How do WeakMap and WeakSet differ from Map and Set?

Weak collections help with memory management.

• WeakMap:
  • Keys must be objects
  • Weak references (can be garbage collected)
• WeakSet:
  • Stores only objects
  • Weakly held references

Benefit:

Prevent memory leaks by not blocking garbage collection.

11. Is JavaScript pass-by-value or pass-by-reference?

JavaScript is pass-by-value.  For primitives, a duplicate of the value is sent, meaning modifications do not impact the original. For objects, a reference copy is passed, so changing properties impacts the original, but reassigning does not.

12. What is memoization, and how do you implement it?

Memoization is a performance optimization technique.

• Stores results of expensive function calls.
• It returns the cached result when the same input is used.
• Avoids repeated calculations.
• Best suited for pure functions.

13. How does “Tree Shaking” work in module bundling?

Tree Shaking helps optimize applications by removing dead code.

• Used by tools like Webpack and Rollup.
• Works with ES6 import/export.
• Eliminates dead code.
• Reduces file size and improves performance.

14. What is the main difference between shallow copy and deep copy?

• Shallow Copy:
  • Copies only top-level properties.
  • Nested objects are still referenced.
• Deep Copy:
  • Copies all nested objects.
  • Fully independent copy.

15. Explain common JavaScript Design Patterns.

Design patterns offer proven solutions for common design challenges.

• Singleton → Only one instance exists.
• Observer → Event-based communication.
• Factory → Creates objects without exposing logic.
  

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Conclusion

JavaScript is a skill to learn right now. Companies need JavaScript jobs that pay well. JavaScript works across many industries. Start with the basics of JavaScript practice often, and pick up JavaScript frameworks along the way. It takes time to master JavaScript. If you stay consistent with JavaScript, you will get there. Go through these JavaScript Interview Questions and Answers before your interview. They help you review what you know about JavaScript and spot anything you might have missed in JavaScript. If you want results faster, good JavaScript training helps a lot. Join our Placement and Training Institute in Chennai to learn JavaScript the right way and get the support you need to land your first job or grow in your career with JavaScript.