Top 30 JavaScript Interview Questions and Answers for Technical Leads (2026)
JavaScript remains the foundation of modern web and hybrid mobile application development. Whether you're preparing for a Technical Lead, Senior Developer, Angular, Ionic, React, or Full Stack interview, mastering these core JavaScript concepts is essential.
This guide covers the most frequently asked JavaScript interview questions along with concise professional answers.
1. What is the difference between var, let, and const?
Feature| var| let| const
Scope| Function| Block| Block
Reassignment| Yes| Yes| No
Redeclaration| Yes| No| No
Hoisting| Yes| Yes| Yes
Best Practice: Prefer "const" by default and use "let" only when reassignment is required.
---
2. What is Hoisting?
Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope during compilation.
console.log(a);
var a = 10;
Output:
undefined
---
3. What is a Closure?
A closure allows a function to access variables from its outer scope even after the outer function has completed execution.
function counter() {
let count = 0;
return function() {
count++;
return count;
};
}
Common Uses
- Data encapsulation
- Event handlers
- Callbacks
- Module patterns
---
4. What is the JavaScript Event Loop?
JavaScript is single-threaded. The Event Loop enables asynchronous operations without blocking the main thread.
Components
- Call Stack
- Web APIs
- Callback Queue
- Microtask Queue
- Event Loop
Priority
1. Call Stack
2. Microtask Queue (Promises)
3. Callback Queue (setTimeout, setInterval)
---
5. Explain the Call Stack
The Call Stack is a mechanism used by JavaScript to keep track of function execution.
function first() {
second();
}
function second() {
console.log("Hello");
}
first();
Execution occurs in a Last-In-First-Out (LIFO) manner.
---
6. What are Promises?
Promises represent the eventual completion or failure of an asynchronous operation.
States
- Pending
- Fulfilled
- Rejected
fetch(url)
.then(response => response.json())
.then(data => console.log(data));
---
7. Promise vs Async/Await
Promise
fetch(url)
.then(res => res.json())
.then(data => console.log(data));
Async/Await
const response = await fetch(url);
const data = await response.json();
Async/Await provides cleaner and more readable asynchronous code.
---
8. Difference Between == and ===
"5" == 5
Output:
true
"5" === 5
Output:
false
Use "===" to avoid unexpected type coercion.
---
9. What is the this Keyword?
The "this" keyword refers to the object that invokes a function.
const user = {
name: "John",
show() {
console.log(this.name);
}
};
---
10. Arrow Functions vs Regular Functions
Arrow functions provide shorter syntax and do not create their own "this" context.
const add = (a, b) => a + b;
---
11. What is Callback Hell?
Callback Hell occurs when multiple asynchronous operations are nested inside one another.
login(() => {
getUser(() => {
getOrders(() => {
processOrder();
});
});
});
Solution
- Promises
- Async/Await
---
12. What is Debouncing?
Debouncing delays execution until the user stops triggering an event.
Use Cases
- Search bars
- Autocomplete
- Input validation
---
13. What is Throttling?
Throttling limits the number of times a function can execute within a given time period.
Use Cases
- Scroll events
- Resize events
- Infinite scrolling
---
14. Deep Copy vs Shallow Copy
Shallow Copy
const copy = Object.assign({}, obj);
Deep Copy
const copy = structuredClone(obj);
Deep copies create completely independent objects.
---
15. Explain Map, Filter, and Reduce
Map
Transforms data.
numbers.map(x => x * 2);
Filter
Returns matching records.
numbers.filter(x => x > 10);
Reduce
Aggregates data.
numbers.reduce((a, b) => a + b);
---
16. What is Destructuring?
Destructuring extracts values from arrays or objects.
const { name, age } = user;
---
17. What is the Spread Operator?
const newArray = [...oldArray];
Uses
- Cloning arrays
- Merging arrays
- Copying objects
---
18. What is the Rest Operator?
function sum(...numbers) {
return numbers.reduce((a, b) => a + b);
}
Rest parameters allow variable numbers of arguments.
---
19. What is Currying?
Currying transforms a function with multiple arguments into nested functions.
const add = a => b => a + b;
---
20. What is Memoization?
Memoization improves performance by caching previously computed results.
Benefits
- Faster execution
- Reduced calculations
- Improved performance
---
21. What is a Prototype?
Every JavaScript object inherits properties and methods from its prototype.
Examples:
Object.prototype
Array.prototype
---
22. What is the Prototype Chain?
When a property is not found on an object, JavaScript searches its prototype chain until it finds the property or reaches null.
---
23. What is Scope?
Types of Scope
- Global Scope
- Function Scope
- Block Scope
let value = 10;
---
24. What is the Temporal Dead Zone (TDZ)?
TDZ refers to the period between entering a scope and initializing a variable declared with "let" or "const".
console.log(a);
let a = 10;
This results in a runtime error.
---
25. Difference Between Synchronous and Asynchronous Execution
Synchronous
a();
b();
c();
Asynchronous
fetch(url);
Asynchronous operations do not block execution.
---
26. What is the Module Pattern?
The Module Pattern helps encapsulate private variables and expose public APIs.
const userModule = (() => {
let name = "John";
return {
getName() {
return name;
}
};
})();
---
27. What is Garbage Collection?
JavaScript automatically removes objects that are no longer referenced, freeing memory.
Benefits
- Automatic memory management
- Reduced memory leaks
---
28. What Causes Memory Leaks?
Common causes include:
- Global variables
- Unremoved event listeners
- Unclosed timers
- Retained object references
---
29. What is Optional Chaining?
Optional chaining safely accesses nested properties.
user?.address?.city
It prevents runtime errors when properties are undefined.
---
30. Null vs Undefined
Undefined| Null
Variable declared but not assigned| Intentional empty value
let a;
let b = null;
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