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Here's all you need to get started ๐
1.Components
โข Functional Components
โข Class Components
โข JSX (JavaScript XML) Syntax
2.Props (Properties)
โข Passing Props
โข Default Props
โข Prop Types
3.State
โข useState Hook
โข Class Component State
โข Immutable State
4.Lifecycle Methods (Class Components)
โข componentDidMount
โข componentDidUpdate
โข componentWillUnmount
5.Hooks (Functional Components)
โข useState
โข useEffect
โข useContext
โข useReducer
โข useCallback
โข useMemo
โข useRef
โข useImperativeHandle
โข useLayoutEffect
6.Event Handling
โข Handling Events in Functional Components
โข Handling Events in Class Components
7.Conditional Rendering
โข if Statements
โข Ternary Operators
โข Logical && Operator
8.Lists and Keys
โข Rendering Lists
โข Keys in React Lists
9.Component Composition
โข Reusing Components
โข Children Props
โข Composition vs Inheritance
10.Higher-Order Components (HOC)
โข Creating HOCs
โข Using HOCs for Reusability
11.Render Props
โข Using Render Props Pattern
12.React Router
โข <BrowserRouter>
โข <Route>
โข <Link>
โข <Switch>
โข Route Parameters
13.Navigation
โข useHistory Hook
โข useLocation Hook
State Management
14.Context API
โข Creating Context
โข useContext Hook
15.Redux
โข Actions
โข Reducers
โข Store
โข connect Function (React-Redux)
16.Forms
โข Handling Form Data
โข Controlled Components
โข Uncontrolled Components
17.Side Effects
โข useEffect for Data Fetching
โข useEffect Cleanup
18.AJAX Requests
โข Fetch API
โข Axios Library
Error Handling
19.Error Boundaries
โข componentDidCatch (Class Components)
โข ErrorBoundary Component (Functional
Components)
20.Testing
โข Jest Testing Framework
โข React Testing Library
21. Best Practices
โข Code Splitting
โข PureComponent and React.memo
โข Avoiding Reconciliation
โข Keys for Dynamic Lists
22.Optimization
โข Memoization
โข Profiling and Performance Monitoring
23. Build and Deployment
โข Create React App (CRA)
โข Production Builds
โข Deployment Strategies
Frameworks and Libraries
24.Styling Libraries
โข Styled-components
โข CSS Modules
25.State Management Libraries
โข Redux
โข MobX
26.Routing Libraries
โข React Router
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1.Components
โข Functional Components
โข Class Components
โข JSX (JavaScript XML) Syntax
2.Props (Properties)
โข Passing Props
โข Default Props
โข Prop Types
3.State
โข useState Hook
โข Class Component State
โข Immutable State
4.Lifecycle Methods (Class Components)
โข componentDidMount
โข componentDidUpdate
โข componentWillUnmount
5.Hooks (Functional Components)
โข useState
โข useEffect
โข useContext
โข useReducer
โข useCallback
โข useMemo
โข useRef
โข useImperativeHandle
โข useLayoutEffect
6.Event Handling
โข Handling Events in Functional Components
โข Handling Events in Class Components
7.Conditional Rendering
โข if Statements
โข Ternary Operators
โข Logical && Operator
8.Lists and Keys
โข Rendering Lists
โข Keys in React Lists
9.Component Composition
โข Reusing Components
โข Children Props
โข Composition vs Inheritance
10.Higher-Order Components (HOC)
โข Creating HOCs
โข Using HOCs for Reusability
11.Render Props
โข Using Render Props Pattern
12.React Router
โข <BrowserRouter>
โข <Route>
โข <Link>
โข <Switch>
โข Route Parameters
13.Navigation
โข useHistory Hook
โข useLocation Hook
State Management
14.Context API
โข Creating Context
โข useContext Hook
15.Redux
โข Actions
โข Reducers
โข Store
โข connect Function (React-Redux)
16.Forms
โข Handling Form Data
โข Controlled Components
โข Uncontrolled Components
17.Side Effects
โข useEffect for Data Fetching
โข useEffect Cleanup
18.AJAX Requests
โข Fetch API
โข Axios Library
Error Handling
19.Error Boundaries
โข componentDidCatch (Class Components)
โข ErrorBoundary Component (Functional
Components)
20.Testing
โข Jest Testing Framework
โข React Testing Library
21. Best Practices
โข Code Splitting
โข PureComponent and React.memo
โข Avoiding Reconciliation
โข Keys for Dynamic Lists
22.Optimization
โข Memoization
โข Profiling and Performance Monitoring
23. Build and Deployment
โข Create React App (CRA)
โข Production Builds
โข Deployment Strategies
Frameworks and Libraries
24.Styling Libraries
โข Styled-components
โข CSS Modules
25.State Management Libraries
โข Redux
โข MobX
26.Routing Libraries
โข React Router
โข Reach Router
React โค๏ธ for more
Web Development Projects โฌ๏ธ
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๐ JavaScript Interview Questions with Answers โ Part 2
11. What is the typeof operator?
The typeof operator is used to determine the data type of a value.
Example:
Important Interview Point:
Output:
This is a historical behavior in JavaScript.
12. What are template literals?
Template literals are strings created using backticks ``.
They allow you to easily embed variables and expressions using
Example:
Output:
Benefits:
Easy string interpolation
Supports multi-line strings
Allows expressions inside strings
13. What are JavaScript operators?
Operators are symbols used to perform operations on values.
Common Types:
Arithmetic:
Comparison:
Logical:
Assignment:
Example:
14. What is the ternary operator?
The ternary operator is a short way of writing an
Syntax:
Example:
Output:
Equivalent if...else:
Interview Tip: Use ternary operators for simple conditions. Avoid deeply nested ternaries because they reduce readability.
15. What is variable hoisting?
Hoisting is JavaScript's behavior of processing declarations before executing the code in their scope.
With var:
Output:
The declaration is hoisted, but the assignment happens later.
With let and const:
This results in a
Function declarations are also hoisted:
16. What is scope in JavaScript?
Scope determines where a variable can be accessed in a program.
Example:
Main Types:
Global Scope
Function Scope
Block Scope
Module Scope
Understanding scope is essential for closures and avoiding variable conflicts.
17. What are global, function, and block scope?
Global Scope
A variable declared outside functions or blocks can generally be accessed throughout the script.
Function Scope
Variables declared with
11. What is the typeof operator?
The typeof operator is used to determine the data type of a value.
Example:
console.log(typeof "Hello"); // "string"
console.log(typeof 100); // "number"
console.log(typeof true); // "boolean"
console.log(typeof undefined); // "undefined"
Important Interview Point:
console.log(typeof null);
Output:
"object" This is a historical behavior in JavaScript.
12. What are template literals?
Template literals are strings created using backticks ``.
They allow you to easily embed variables and expressions using
${}.Example:
const name = "Ajay";
const age = 25;
console.log(`My name is ${name} and I am ${age} years old.`);
Output:
My name is Ajay and I am 25 years old.Benefits:
Easy string interpolation
Supports multi-line strings
Allows expressions inside strings
13. What are JavaScript operators?
Operators are symbols used to perform operations on values.
Common Types:
Arithmetic:
+ - * / % **Comparison:
== === != !== > < >= <=Logical:
&& || !Assignment:
= += -= *= /=Example:
const a = 10;
const b = 5;
console.log(a + b); // 15
console.log(a > b); // true
14. What is the ternary operator?
The ternary operator is a short way of writing an
if...else statement.Syntax:
condition ? valueIfTrue : valueIfFalse;
Example:
const age = 20;
const result = age >= 18 ? "Adult" : "Minor";
console.log(result);
Output:
AdultEquivalent if...else:
if (age >= 18) {
result = "Adult";
} else {
result = "Minor";
}Interview Tip: Use ternary operators for simple conditions. Avoid deeply nested ternaries because they reduce readability.
15. What is variable hoisting?
Hoisting is JavaScript's behavior of processing declarations before executing the code in their scope.
With var:
console.log(x);
var x = 10;
Output:
undefined The declaration is hoisted, but the assignment happens later.
With let and const:
console.log(x);
let x = 10;
This results in a
ReferenceError. let and const are hoisted but remain in the Temporal Dead Zone (TDZ) until their declaration is reached.Function declarations are also hoisted:
greet();
function greet() {
console.log("Hello");
}
16. What is scope in JavaScript?
Scope determines where a variable can be accessed in a program.
Example:
function test() {
let message = "Hello";
console.log(message);
}
test();message is accessible inside the function but not outside it.Main Types:
Global Scope
Function Scope
Block Scope
Module Scope
Understanding scope is essential for closures and avoiding variable conflicts.
17. What are global, function, and block scope?
Global Scope
A variable declared outside functions or blocks can generally be accessed throughout the script.
let name = "Deepak";
function greet() {
console.log(name);
}
Function Scope
Variables declared with
var inside a function are accessible throughout that function.โค1
function test() {
var age = 25;
console.log(age);
}age
cannot be accessed outside
test()
.
Block Scope
let
and
const
are block-scoped.
if (true) {
let x = 10;
const y = 20;
console.log(x, y);
}x
and
y
cannot be accessed outside the
if
block.
18. What is strict mode ("use strict")?
Strict mode enables a stricter set of JavaScript rules and helps catch certain programming mistakes.
Example:
"use strict";
x = 10;
This produces a
ReferenceError
because
x
was not declared.
Without strict mode, older JavaScript behavior could create a global variable in some situations.
Benefits:
Catches common mistakes
Prevents accidental global variables
Makes some unsafe operations throw errors
Helps write cleaner code
19. What are comments in JavaScript?
Comments are text ignored by the JavaScript engine. They are used to explain code or temporarily disable code.
Single-Line Comment:
// This is a comment
console.log("Hello");
Multi-Line Comment:
/*
This is a
multi-line comment
*/
console.log("Hello");
Why Use Comments?
โ Explain complex logic
โ Improve code readability
โ Help other developers understand the code
โ Document important decisions
20. What are JavaScript modules?
Modules allow you to split JavaScript code into separate, reusable files.
They help organize large applications and prevent unnecessary global variables.
Export:
// math.js
export function add(a, b) {
return a + b;
}
Import:
// app.js
import { add } from "./math.js";
console.log(add(10, 20));
Types of Exports:
Named exports
Default exports
Default Export:
export default function greet() {
console.log("Hello");
}Important Interview Point:
ES Modules use
import
and
export
. They are the standard module system for modern JavaScript.
โค๏ธ Double Tap For Part 3
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๐ JavaScript Interview Questions with Answers โ Part 4
31. How do you create objects in JavaScript?
There are several ways to create objects in JavaScript.
1. Object Literal
2. new Object()
3. Constructor Function
4. Class
Interview Tip:
Object literals are usually preferred for simple objects, while classes or constructor functions are useful when creating many similar objects.
32. What is object destructuring?
Object destructuring allows you to extract properties from an object and store them in variables.
Example:
Output:
John
25
Rename Variables:
Default Value:
33. What is the spread operator (...)?
The spread operator expands the elements of an iterable or properties of an object.
Array Example:
Output:
[1, 2, 3, 4, 5]
Object Example:
Common Uses:
โ Copy arrays
โ Merge arrays
โ Copy objects
โ Merge objects
โ Pass values to functions
34. What is the rest operator?
The rest operator (...) collects multiple values into a single array or object.
Example:
Output: 60
Here,
Important Interview Point:
The same
Spread โ expands values:
Rest โ collects values:
35. What are default parameters?
Default parameters allow you to provide a default value when an argument is not passed or is undefined.
Example:
Output: Hello Guest
If a value is provided:
Multiple Defaults:
36. What is optional chaining (?. )
Optional chaining allows you to safely access nested properties without throwing an error when an intermediate value is null or undefined.
Without Optional Chaining:
With Optional Chaining:
31. How do you create objects in JavaScript?
There are several ways to create objects in JavaScript.
1. Object Literal
const user = {
name: "John",
age: 25
};2. new Object()
const user = new Object();
user.name = "Frey";
user.age = 35;
3. Constructor Function
function User(name, age) {
this.name = name;
this.age = age;
}
const user = new User("John", 25);4. Class
class User {
constructor(name, age) {
this.name = name;
this.age = age;
}
}
const user = new User("John", 25);Interview Tip:
Object literals are usually preferred for simple objects, while classes or constructor functions are useful when creating many similar objects.
32. What is object destructuring?
Object destructuring allows you to extract properties from an object and store them in variables.
Example:
const user = {
name: "John",
age: 25,
city: "New York"
};
const { name, age } = user;
console.log(name);
console.log(age);Output:
John
25
Rename Variables:
const { name: userName } = user;
console.log(userName);Default Value:
const { country = "India" } = user;
console.log(country);33. What is the spread operator (...)?
The spread operator expands the elements of an iterable or properties of an object.
Array Example:
const numbers = [1, 2, 3];
const newNumbers = [...numbers, 4, 5];
console.log(newNumbers);
Output:
[1, 2, 3, 4, 5]
Object Example:
const user = {
name: "John",
age: 25
};
const updatedUser = {
...user,
city: "New York"
};Common Uses:
โ Copy arrays
โ Merge arrays
โ Copy objects
โ Merge objects
โ Pass values to functions
34. What is the rest operator?
The rest operator (...) collects multiple values into a single array or object.
Example:
function sum(...numbers) {
return numbers.reduce((total, num) => total + num, 0);
}
console.log(sum(10, 20, 30));Output: 60
Here,
...numbers collects all arguments into an array.Important Interview Point:
The same
... syntax has different purposes:Spread โ expands values:
const arr2 = [...arr1];Rest โ collects values:
function test(...args) {} 35. What are default parameters?
Default parameters allow you to provide a default value when an argument is not passed or is undefined.
Example:
function greet(name = "Guest") {
console.log(`Hello ${name}`);
}
greet();Output: Hello Guest
If a value is provided:
greet("Deepika"); โ Hello Deepika Multiple Defaults:
function createUser(name = "Guest", age = 18) {
console.log(name, age);
}36. What is optional chaining (?. )
Optional chaining allows you to safely access nested properties without throwing an error when an intermediate value is null or undefined.
Without Optional Chaining:
const user = {};
console.log(user.address.city); // ErrorWith Optional Chaining:
console.log(user.address?.city); // undefined
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Example with Function:
The function is called only if
Common Use: Very useful when working with API responses where some properties may be missing.
37. What is nullish coalescing (??)
The nullish coalescing operator returns the right-hand value when the left-hand value is null or undefined.
Example:
Output: Guest
Important Difference From ||
Interview Tip: Use
38. What are object methods?
An object method is a function stored as a property of an object.
Example:
Output: Hello Deepak
Another Example:
39. What is method chaining?
Method chaining means calling multiple methods one after another on the same object or result.
Example:
Output: TPIRCSAVAJ
Array Example:
Output: [20,40]
Common Uses: Array processing, String manipulation, Promise chains, Libraries such as jQuery
40. What is object freezing and sealing?
JavaScript provides
Object.freeze()
Prevents: Adding properties, Removing properties, Changing existing properties
Object.seal()
Prevents: Adding properties, Removing properties
But existing properties can still be modified.
Key Difference:
Object.freeze(): Cannot add, delete, or modify properties
Object.seal(): Cannot add or delete properties, but can modify existing ones
๐ฅ Interview Tip: Both methods are shallow โ nested objects can still be modified unless they are separately frozen/sealed.
โค๏ธ Double Tap For Part 5
user.getName?.();The function is called only if
getName exists and is callable.Common Use: Very useful when working with API responses where some properties may be missing.
37. What is nullish coalescing (??)
The nullish coalescing operator returns the right-hand value when the left-hand value is null or undefined.
Example:
const username = null;
console.log(username ?? "Guest");
Output: Guest
Important Difference From ||
|| considers all falsy values: console.log(0 || 100); โ 100?? only checks null and undefined: console.log(0 ?? 100); โ 0 Interview Tip: Use
?? when 0, false, or "" are valid values that should not be replaced.38. What are object methods?
An object method is a function stored as a property of an object.
Example:
const user = {
name: "Deepak",
greet() {
console.log(`Hello ${this.name}`);
}
};
user.greet();Output: Hello Deepak
Another Example:
const calculator = {
add(a, b) { return a + b; },
multiply(a, b) { return a * b; }
};
console.log(calculator.add(10, 20));39. What is method chaining?
Method chaining means calling multiple methods one after another on the same object or result.
Example:
const result = "javascript"
.toUpperCase()
.split("")
.reverse()
.join("");
console.log(result);
Output: TPIRCSAVAJ
Array Example:
const result = [1, 2, 3, 4, 5]
.filter(num => num % 2 === 0)
.map(num => num * 10);
console.log(result);
Output: [20,40]
Common Uses: Array processing, String manipulation, Promise chains, Libraries such as jQuery
40. What is object freezing and sealing?
JavaScript provides
Object.freeze() and Object.seal() to restrict modifications to objects.Object.freeze()
Prevents: Adding properties, Removing properties, Changing existing properties
const user = { name: "Deepak", age: 25 };
Object.freeze(user);
user.age = 30;
user.city = "Pune";
console.log(user); // unchangedObject.seal()
Prevents: Adding properties, Removing properties
But existing properties can still be modified.
const user = { name: "Deepak", age: 25 };
Object.seal(user);
user.age = 30;
console.log(user.age); // 30Key Difference:
Object.freeze(): Cannot add, delete, or modify properties
Object.seal(): Cannot add or delete properties, but can modify existing ones
๐ฅ Interview Tip: Both methods are shallow โ nested objects can still be modified unless they are separately frozen/sealed.
โค๏ธ Double Tap For Part 5
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๐ JavaScript Interview Questions with Answers โ Part 5
41. How do arrays work in JavaScript?
An array is an ordered collection of values. JS arrays can hold different data types and use zero-based indexing.
Example:
Important Points:
โข Index starts at 0
โข Arrays are objects in JavaScript
โข Arrays can grow or shrink dynamically
โข Arrays can contain mixed data types
42. What is the difference between map() and forEach()?
Both iterate over an array, but used differently.
map()
Creates and returns a new array.
forEach()
Executes a function for each element but does not return a new array.
Key Difference:
โข map(): Returns a new array. Used for transformation. Can be chained.
โข forEach(): Returns undefined. Used for side effects.
43. What is filter()?
Creates a new array with elements that pass a condition. Original array is not modified.
Example:
44. What is reduce()?
Processes an array and produces a single accumulated value.
Example:
Common Uses: Calculate totals, averages, count items, group data, build objects.
Interview Tip: Understand the accumulator and current value arguments.
45. What is find()?
Returns the first element that satisfies a condition. Returns undefined if none match.
Example:
find() vs filter(): find = first match, filter = all matches.
46. What is findIndex()?
Returns the index of the first element that satisfies a condition. Returns -1 if none match.
Example:
47. What is some()?
Checks if at least one element satisfies a condition. Returns Boolean.
Example:
48. What is every()?
Checks if all elements satisfy a condition. Returns Boolean.
Example:
some() vs every(): some = at least one, every = all.
49. What is the difference between slice() and splice()?
slice()
Returns a portion without modifying the original.
splice()
Adds, removes, or replaces elements and modifies the original.
41. How do arrays work in JavaScript?
An array is an ordered collection of values. JS arrays can hold different data types and use zero-based indexing.
Example:
const items = ["Apple", 25, true];
console.log(items[0]); // Apple
console.log(items.length); // 3
Important Points:
โข Index starts at 0
โข Arrays are objects in JavaScript
โข Arrays can grow or shrink dynamically
โข Arrays can contain mixed data types
42. What is the difference between map() and forEach()?
Both iterate over an array, but used differently.
map()
Creates and returns a new array.
const numbers = [1, 2, 3];
const doubled = numbers.map(num => num * 2); // [2, 4, 6]
forEach()
Executes a function for each element but does not return a new array.
numbers.forEach(num => console.log(num * 2));
Key Difference:
โข map(): Returns a new array. Used for transformation. Can be chained.
โข forEach(): Returns undefined. Used for side effects.
43. What is filter()?
Creates a new array with elements that pass a condition. Original array is not modified.
Example:
const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.filter(num => num % 2 === 0); // [2, 4]
44. What is reduce()?
Processes an array and produces a single accumulated value.
Example:
const numbers = [10, 20, 30];
const total = numbers.reduce((sum, num) => sum + num, 0); // 60
Common Uses: Calculate totals, averages, count items, group data, build objects.
Interview Tip: Understand the accumulator and current value arguments.
45. What is find()?
Returns the first element that satisfies a condition. Returns undefined if none match.
Example:
const numbers = [10, 20, 30, 40];
const result = numbers.find(num => num > 20); // 30
find() vs filter(): find = first match, filter = all matches.
46. What is findIndex()?
Returns the index of the first element that satisfies a condition. Returns -1 if none match.
Example:
const numbers = [10, 20, 30, 40];
const index = numbers.findIndex(num => num > 20); // 2
47. What is some()?
Checks if at least one element satisfies a condition. Returns Boolean.
Example:
const numbers = [1, 3, 5, 8];
const result = numbers.some(num => num % 2 === 0); // true
48. What is every()?
Checks if all elements satisfy a condition. Returns Boolean.
Example:
const numbers = [2, 4, 6, 8];
const result = numbers.every(num => num % 2 === 0); // true
some() vs every(): some = at least one, every = all.
49. What is the difference between slice() and splice()?
slice()
Returns a portion without modifying the original.
const numbers = [1, 2, 3, 4, 5];
const result = numbers.slice(1, 4); // [2, 3, 4]
splice()
Adds, removes, or replaces elements and modifies the original.
const numbers = [1, 2, 3, 4, 5];
numbers.splice(1, 2); // removes 2 elements at index 1
console.log(numbers); // [1, 4, 5]
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Key Difference:
โข slice(): Does not modify original. Extracts elements. Returns copied portion.
โข splice(): Modifies original. Adds/removes/replaces. Returns removed elements.
50. What are push(), pop(), shift(), and unshift()?
Methods that modify arrays.
โข push(): Add to END โ
โข pop(): Remove from END โ
โข unshift(): Add to START โ
โข shift(): Remove from START โ
Quick Memory Trick:
push/pop = END, unshift/shift = START
โค๏ธ Double Tap For Part 6
โข slice(): Does not modify original. Extracts elements. Returns copied portion.
โข splice(): Modifies original. Adds/removes/replaces. Returns removed elements.
50. What are push(), pop(), shift(), and unshift()?
Methods that modify arrays.
โข push(): Add to END โ
arr.push(3) โข pop(): Remove from END โ
arr.pop() โข unshift(): Add to START โ
arr.unshift(0) โข shift(): Remove from START โ
arr.shift() Quick Memory Trick:
push/pop = END, unshift/shift = START
โค๏ธ Double Tap For Part 6
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