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Step by Step Project TutorialJavaScript Unicode Character Value Finder — charCodeAt & fromCharCode Project 2026JavaScript Weather App Project — Fetch API & DOM Complete Tutorial 2026JavaScript Cheat Sheet 2026 — Complete Quick ReferenceImportant JavaScript Concepts Every Developer Must Know in 2026JavaScript One Shot Notes 2026 — Complete JS in One PageJavaScript Quick Revision Notes — Interview Prep in 30 Minutes 2026Abstraction in JavaScript OOP – Hide Complexity, Expose SimplicityJavaScript Classes Tutorial – ES6 Class Syntax, Inheritance & MoreJavaScript Constructor Functions – new Keyword, Prototype & OOP PatternsEncapsulation in JavaScript – Private Fields, Getters, Setters & Data ProtectionJavaScript Inheritance – extends, super, and Prototype-Based InheritanceObject-Oriented Programming in JavaScript – OOP Concepts Complete GuideJavaScript OOP Projects – Build Real-World Apps with Classes & OOPPolymorphism in JavaScript – Method Overriding, Duck Typing & Many FormsJavaScript Prototype Chain – How Property Lookup Works in JavaScriptJavaScript Prototypes – Prototype-Based Inheritance, Object.create & prototype PropertyJavaScript Advanced Practice Problems — 15+ Interview-Level Challenges 2026JavaScript Assignments — 10 Project-Based Learning Tasks 2026JavaScript Beginner Practice Problems — 20+ Coding Exercises 2026JavaScript Coding Challenges — 25 Interview-Style Problems 2026JavaScript Intermediate Practice Problems — 20+ Coding Challenges 2026JavaScript MCQs — 50 Multiple Choice Questions with Answers 2026JavaScript Regex Character Classes – [abc], [^abc], Ranges & ShortcutsJavaScript Regex Email Validation – Pattern, Edge Cases & Best PracticesJavaScript Regex Flags – g, i, m, s, u, y Flags Explained with ExamplesJavaScript Regex Groups – Capturing, Non-Capturing & Named GroupsJavaScript Number Validation with Regex — Complete Patterns & Examples 2026JavaScript Password Validation with Regex — Strength Checker & Patterns 2026JavaScript Regex Quantifiers – *, +, ?, {n,m}, Greedy vs Lazy MatchingJavaScript Regex Basics – Regular Expressions Tutorial for BeginnersJavaScript Regex Projects — 10 Practical Mini-Projects with Code 2026JavaScript URL Validation with Regex — Complete Patterns & Examples 2026JavaScript Username Validation with Regex — Patterns & Form Examples 2026JavaScript TutorialJavaScript Lazy Loading — Images, Modules & Components Complete Guide 2026JavaScript Performance Optimization — Speed Up Your Code Complete Guide 2026JavaScript Unit Testing with Jest — Complete Guide for Beginners 2026
Complete JavaScript interview guide 2026. Covers 50 most asked questions on closures, hoisting, promises, prototypes, async/await, event loop, DOM, and ES6+ features.
Your complete JavaScript interview preparation guide. These are the most frequently asked questions — from fresher-level to senior developer interviews at top companies.
Section 1: JavaScript Fundamentals
Q1: What is JavaScript? How is it different from Java?
Answer:
- JavaScript is a dynamic, interpreted, prototype-based scripting language primarily for web.
- Java is statically typed, compiled to bytecode, class-based OOP language.
- No actual relation beyond similar syntax style.
| Feature | JavaScript | Java |
|---|---|---|
| Typing | Dynamic | Static |
| Execution | Interpreted/JIT | Compiled (JVM) |
| OOP | Prototype-based | Class-based |
| Threading | Single-threaded | Multi-threaded |
| Use case | Web frontend/backend | Enterprise/Android |
Q2: What are the data types in JavaScript?
Answer: JavaScript has 8 data types:
Primitive (7):
js example
typeof 42 // "number"
typeof "hello" // "string"
typeof true // "boolean"
typeof undefined // "undefined"
typeof null // "object" ← bug! (primitive)
typeof Symbol() // "symbol"
typeof 9007199254740993n // "bigint"Non-Primitive (1):
js example
typeof {} // "object"
typeof [] // "object"
typeof function(){} // "function" (subtype of object)Output
"number" "string" "boolean" "undefined" "object" "symbol" "bigint" "object" "object" "function"
Q3: What is the difference between == and ===?
js example
console.log(1 == "1"); // true (type coercion)
console.log(1 === "1"); // false (no coercion)
console.log(null == undefined); // true
console.log(null === undefined); // false
console.log(NaN == NaN); // false
console.log(NaN === NaN); // falseOutput
true false true false false false
Rule: Always use === unless you have a specific reason for ==.
Q4: var vs let vs const
js example
// var: function-scoped, hoisted as undefined, re-declarable
var x = 1;
var x = 2; // OK
// let: block-scoped, hoisted into TDZ, not re-declarable
let y = 1;
// let y = 2; // SyntaxError
// const: block-scoped, must be initialized, not reassignable
const z = 1;
// z = 2; // TypeError| Feature | var | let | const |
|---|---|---|---|
| Scope | Function | Block | Block |
| Hoisting | undefined | TDZ | TDZ |
| Re-declare | ✅ | ❌ | ❌ |
| Re-assign | ✅ | ✅ | ❌ |
| window prop | ✅ | ❌ | ❌ |
Q5: What is undefined vs null?
js example
let a; // undefined — variable declared but not assigned
let b = null; // null — explicitly "no value"
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object" (JS bug)
console.log(null == undefined); // true
console.log(null === undefined); // falseOutput
"undefined" "object" true false
Q6: What is NaN? How to check for it?
js example
console.log(NaN === NaN); // false — NaN is not equal to itself!
console.log(typeof NaN); // "number"
console.log(isNaN("hello")); // true (coerces first — use carefully)
console.log(Number.isNaN("hello")); // false (strict — doesn't coerce)
console.log(Number.isNaN(NaN)); // trueOutput
false "number" true false true
Best Practice: Always use Number.isNaN() over isNaN().
Section 2: Functions
Q7: What is a closure?
Answer: A closure is a function that retains access to its lexical scope even after the outer function has returned.
Example
function counter() {
let count = 0;
return {
inc: () => ++count,
get: () => count
};
}
const c = counter();
c.inc(); c.inc(); c.inc();
console.log(c.get()); // 3 — count is private, persists!
Output
3
Q8: What is hoisting?
js example
console.log(x); // undefined (var hoisted)
var x = 5;
greet(); // "Hello!" (function hoisted fully)
function greet() { console.log("Hello!"); }
// console.log(y); // ReferenceError (TDZ — let/const)
let y = 10;Output
undefined Hello!
Q9: Arrow functions vs Regular functions
Example
// Regular function
function regular() {
return this;
}
// Arrow function — inherits 'this' from outer scope
const arrow = () => this;
const obj = {
name: "JS",
regular: function() { return this.name; },
arrow: () => this?.name // 'this' is outer context
};
console.log(obj.regular()); // "JS"
console.log(obj.arrow()); // undefined (or global.name)
Output
JS undefined
Differences:
| Feature | Regular | Arrow |
|---|---|---|
this | Dynamic | Lexical |
arguments | Yes | No |
new | Yes | No (not constructor) |
| Syntax | Verbose | Concise |
Q10: What are higher-order functions?
Example
// A function that takes a function as argument or returns one
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
const evens = numbers.filter(n => n % 2 === 0);
const sum = numbers.reduce((acc, n) => acc + n, 0);
console.log(doubled); // [2, 4, 6, 8, 10]
console.log(evens); // [2, 4]
console.log(sum); // 15
Output
[2, 4, 6, 8, 10] [2, 4] 15
Q11: What is this in JavaScript?
Example
const obj = {
name: "WoHoTech",
greet() {
console.log(this.name); // "WoHoTech"
},
greetArrow: () => {
console.log(this?.name); // undefined
}
};
obj.greet();
obj.greetArrow();
function show() { console.log(this); }
show(); // window (non-strict) or undefined (strict)
show.call({x:1}); // {x:1}
Output
WoHoTech undefined
Q12: call, apply, bind
Example
function introduce(role, company) {
return `${this.name} is ${role} at ${company}`;
}
const person = { name: "Priya" };
console.log(introduce.call(person, "Developer", "Google"));
console.log(introduce.apply(person, ["Designer", "Apple"]));
const boundFn = introduce.bind(person, "Engineer");
console.log(boundFn("Microsoft"));
Output
Priya is Developer at Google Priya is Designer at Apple Priya is Engineer at Microsoft
Section 3: Scope & Prototype
Q13: What is the scope chain?
js example
const global = "G";
function outer() {
const a = "A";
function inner() {
const b = "B";
console.log(b, a, global); // inner → outer → global
}
inner();
}
outer();Output
B A G
Q14: What is the prototype chain?
js example
function Animal(name) {
this.name = name;
}
Animal.prototype.speak = function() {
return `${this.name} makes a sound`;
};
const dog = new Animal("Rex");
console.log(dog.speak());
console.log(dog.hasOwnProperty("name")); // true
console.log(dog.hasOwnProperty("speak")); // false (on prototype)Output
Rex makes a sound true false
Q15: What does the new keyword do?
Example
function Person(name) {
this.name = name;
}
// new does 4 things:
// 1. Creates empty object: {}
// 2. Sets [[Prototype]] to Person.prototype
// 3. Executes Person with this = new object
// 4. Returns the object (unless explicit object returned)
const p = new Person("Alice");
console.log(p.name);
console.log(p instanceof Person);
Output
Alice true
Section 4: ES6+ Features
Q16: Destructuring
Example
// Array destructuring
const [a, b, ...rest] = [1, 2, 3, 4, 5];
console.log(a, b, rest); // 1 2 [3, 4, 5]
// Object destructuring
const { name, age = 25, city: location } = { name: "Raj", city: "Delhi" };
console.log(name, age, location); // Raj 25 Delhi
Output
1 2 [3, 4, 5] Raj 25 Delhi
Q17: Spread vs Rest
Example
// Spread — expand iterables
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]
const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 }; // { a: 1, b: 2 }
// Rest — collect remaining
function sum(first, ...rest) {
return rest.reduce((acc, n) => acc + n, first);
}
console.log(sum(1, 2, 3, 4)); // 10
Output
10
Q18: Template Literals
Example
const name = "JavaScript";
const year = 2026;
// Multi-line
const html = `
<h1>${name}</h1>
<p>Year: ${year}</p>
`;
// Tagged templates
function highlight(strings, ...values) {
return strings.reduce((result, str, i) =>
result + str + (values[i] ? `<b>${values[i]}</b>` : ""), "");
}
const greeting = highlight`Hello ${name}! Welcome to ${year}.`;
console.log(greeting);
Output
Hello <b>JavaScript</b>! Welcome to <b>2026</b>.
Q19: Optional Chaining & Nullish Coalescing
js example
const user = {
profile: {
address: null
}
};
// Without optional chaining: throws error
// user.profile.address.city
// With optional chaining
console.log(user?.profile?.address?.city); // undefined
console.log(user?.profile?.phone?.number); // undefined
console.log(user?.settings?.theme ?? "light"); // "light"Output
undefined undefined light
Q20: Map vs Object vs Set vs Array
| Feature | Map | Object | Set | Array |
|---|---|---|---|---|
| Key types | Any | String/Symbol | — | Index |
| Order | Insertion | Not guaranteed | Insertion | Index |
| Size | .size | Object.keys().length | .size | .length |
| Iteration | Easy | Need Object.entries | Easy | Easy |
| Use when | Key-value, any key | Record/config | Unique values | Ordered list |
Example
const map = new Map();
map.set({ id: 1 }, "object key"); // objects as keys!
map.set(true, "boolean key");
const set = new Set([1, 2, 2, 3, 3]);
console.log([...set]); // [1, 2, 3] — duplicates removed
Output
[1, 2, 3]
Section 5: Async JavaScript
Q21: What is a Promise?
Example
const p = new Promise((resolve, reject) => {
setTimeout(() => resolve("Done!"), 1000);
});
p.then(val => console.log(val))
.catch(err => console.log(err));
Output
Done!
Q22: async/await syntax
js example
async function loadUser(id) {
try {
const user = await getUser(id); // awaits Promise
const posts = await getPosts(user.id);
return { user, posts };
} catch (err) {
console.error("Failed:", err.message);
}
}Q23: Event Loop — What is the output?
Example
console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
Output
1 4 3 2
Section 6: OOP in JavaScript
Q24: Class Syntax
js example
class Animal {
#sound; // private field
constructor(name, sound) {
this.name = name;
this.#sound = sound;
}
speak() {
return `${this.name}: ${this.#sound}!`;
}
static create(name, sound) {
return new Animal(name, sound);
}
}
class Dog extends Animal {
constructor(name) {
super(name, "Woof");
this.tricks = [];
}
learn(trick) {
this.tricks.push(trick);
return this;
}
}
const d = new Dog("Rex");
d.learn("sit").learn("stay");
console.log(d.speak());
console.log(d.tricks);Output
Rex: Woof! ["sit", "stay"]
Q25: Inheritance via Prototype
js example
function Shape(color) { this.color = color; }
Shape.prototype.describe = function() {
return `${this.color} shape`;
};
function Circle(color, radius) {
Shape.call(this, color); // inherit properties
this.radius = radius;
}
Circle.prototype = Object.create(Shape.prototype); // inherit methods
Circle.prototype.constructor = Circle;
Circle.prototype.area = function() {
return Math.PI * this.radius ** 2;
};
const c = new Circle("red", 5);
console.log(c.describe());
console.log(c.area().toFixed(2));Output
red shape 78.54
Section 7: Common Interview Traps
Q26: typeof null
js example
console.log(typeof null); // "object" — famous JS bug!Output
"object"
Q27: Floating Point Precision
js example
console.log(0.1 + 0.2); // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3); // false!
// Fix:
console.log(Math.abs(0.1 + 0.2 - 0.3) < Number.EPSILON); // true
console.log((0.1 + 0.2).toFixed(1) === "0.3"); // trueOutput
0.30000000000000004 false true true
Q28: Implicit Type Coercion
Example
console.log(true + 1); // 2
console.log(false + 1); // 1
console.log("5" - 3); // 2
console.log("5" + 3); // "53"
console.log([] + {}); // "[object Object]"
console.log(+[]); // 0
Output
2 1 2 "53" "[object Object]" 0
Q29: Object Reference vs Primitive
js example
let a = 5;
let b = a;
b = 10;
console.log(a); // 5 (primitives copied by value)
let obj1 = { x: 1 };
let obj2 = obj1;
obj2.x = 99;
console.log(obj1.x); // 99 (objects copied by reference)Output
5 99
Q30: Array Methods Chaining
Example
const students = [
{ name: "Alice", score: 85 },
{ name: "Bob", score: 72 },
{ name: "Carol", score: 91 },
{ name: "Dave", score: 68 }
];
const topStudents = students
.filter(s => s.score >= 80)
.sort((a, b) => b.score - a.score)
.map(s => `${s.name}: ${s.score}`);
console.log(topStudents);
Output
["Carol: 91", "Alice: 85"]
Most Asked Interview Tips 🎯
- Know your data types — especially
typeof null === "object". - Explain closures with a counter example.
- Draw the event loop when explaining async.
- Use const by default — shows modern JS knowledge.
- Mention Edge Cases — null checks, empty arrays, NaN.
- Know HOFs — map, filter, reduce are always asked.
- Prototype chain — draw it out if asked.
thisrules — regular vs arrow in callbacks.
Interview Preparation Checklist
- [ ] Data types and typeof
- [ ] var / let / const differences
- [ ] Hoisting (var, let, function declarations)
- [ ] Closures (definition + 3 use cases)
- [ ] Scope chain (lexical scoping)
- [ ] Prototype and prototype chain
- [ ]
thiskeyword and its 4 rules - [ ] call, apply, bind
- [ ] Event loop (sync → microtask → macrotask)
- [ ] Promises (states, .all, .race, .allSettled, .any)
- [ ] async/await (+ error handling)
- [ ] ES6: destructuring, spread/rest, template literals
- [ ] map, filter, reduce
- [ ] Shallow vs deep copy
- [ ] Type coercion (
==traps) - [ ] Classes and inheritance
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