lets discuss 40 + frequently asked angular and JavaScript interview questions 

JavaScript Interview Questions

1. What are closures in JavaScript?

• Closures allow an inner function to remember variables from its outer function even after the outer function has finished executing.

function outer() {

  let count = 0;

  return function inner() {

    count++;

    return count;

  };

}

const counter = outer();

console.log(counter()); // 1

console.log(counter()); // 2

• Closures power callbacks, event listeners, and async programming all of which are used in Angular.

2. Difference between var, let, and const?

• var : functionscoped, can be redeclared.
• let : blockscoped, reassignable.
• const: blockscoped, cannot be reassigned.

3. What is event delegation?

• Instead of attaching event listeners to multiple child elements, you attach one to the parent. The event bubbles up and gets handled at the parent level.

•  Saves memory and improves performance—super important in large apps.

4. Difference between == and ===?

• ==  loose equality (performs type conversion).
• ===  strict equality (compares type + value).
• Always use === to avoid bugs.

5. What is the event loop in JavaScript?

• The event loop is what makes JavaScript asynchronous. It constantly checks the call stack and the callback queue, allowing nonblocking code execution.

6. What are Promises and async/await?

• Promise :represents a future value (resolved or rejected).
• async/await: syntactic sugar for writing cleaner asynchronous code.

async function fetchData() {

  const res = await fetch('/api/data');

  return res.json();

}

7. What is hoisting?

• Hoisting means JavaScript moves variable and function declarations to the top of their scope before code execution.

8. Difference between null and undefined?

• null : intentional “no value.”
• undefined : variable declared but not assigned.

9. What are arrow functions, and how are they different?

• Arrow functions don’t have their own this. They inherit it from their surrounding scope.
• Useful in Angular callbacks and RxJS streams.

10. What is the difference between synchronous and asynchronous JavaScript?

• Synchronous : one task at a time.
• Asynchronous : tasks can run in the background (timers, API calls).

11. What is a callback function?

• A callback is a function passed as an argument and executed later.

12. What is a prototype in JavaScript?

• Every JS object has a prototype, which enables inheritance. Angular’s TypeScript classes compile down to prototypebased JS.

13. What are higherorder functions?

• Functions that take other functions as arguments or return them.
• Examples: map(), filter(), reduce().

14. Explain deep copy vs shallow copy.

• Shallow copy: copies only toplevel properties.
• Deep copy  means copies nested objects as well.

Angular Interview Questions

15. What is Angular, and how does it differ from AngularJS?

• AngularJS (1.x) :JavaScriptbased, MVC, slower.
• Angular (2+) :TypeScriptbased, componentdriven, faster.

16. What are the main building blocks of Angular?

• Modules
• Components
• Templates
• Directives
• Services
• Pipes

17. What is Dependency Injection in Angular?

DI means Angular creates and injects services instead of you creating them manually. Makes code testable and modular.

18. Explain Angular data binding.

• Interpolation : {{ name }}
• Property binding : [src]="imageUrl"
• Event binding :(click)="onClick()"
• Twoway binding : [(ngModel)]="username"

19. What are lifecycle hooks in Angular?

• ngOnInit() : runs after component loads
• ngOnChanges() : detects input property changes
• ngOnDestroy() :cleanup before component is destroyed

for more details on 8 life cycle hooks angular 

20. How do Angular components communicate?

• @Input() - Parent - Child
• @Output() -Child -Parent
• Services with RxJS :Share data across components

21. What are Angular directives?

• Structural : *ngIf, *ngFor
• Attribute : [ngClass], [ngStyle]

22. What is Angular Routing?

• Routing allows navigation between views. Features include:
• Route parameters (:id)
• Lazy loading
• Route guards

23. What are Observables in Angular?

• Observables (via RxJS) handle async data streams (HTTP requests, events, sockets).
• Unlike promises, they can emit multiple values over time.

24. What is Angular Ivy?

• Ivy is the rendering engine that makes Angular apps smaller and faster.

25. What is Angular Universal?

• It enables ServerSide Rendering (SSR) for SEO and performance.

26. How does Angular handle forms?

• Templatedriven forms : simpler.
• Reactive forms:  more control, better validation.

27. What are Angular Pipes?

• Transform data in templates. Examples: date, currency, async.
• You can also build custom pipes.

28. What is lazy loading in Angular?

• Loads modules only when needed, reducing initial bundle size.

29. What are Guards in Angular routing?

• Guards like CanActivate and CanDeactivate control route access.

30. What is ViewEncapsulation?

• Controls how CSS styles are applied:
• Emulated (default)
• None
• ShadowDOM

31. What are standalone components in Angular 17+?

• Standalone components don’t need NgModules, making apps simpler.

32. What are Angular Signals?

• Signals are new reactive primitives for managing state, simpler than RxJS in some cases.

33. What are Angular Zones?

• NgZone tracks async tasks and triggers change detection.

34. What is AheadofTime (AOT) compilation?

• AOT compiles Angular templates at build time & faster runtime performance.

35. Difference between templatedriven and reactive forms?

• Template driven: simple, less code.
• Reactive: scalable, great for complex forms.

36. How do you improve performance in Angular apps?

• Lazy loading
• OnPush change detection
• TrackBy in *ngFor
• Caching API responses

37. Difference between Subject, BehaviorSubject, and ReplaySubject in RxJS?

• Subject: plain multicast observable.
• BehaviorSubject: holds last value.
• ReplaySubject: replays multiple past values.

38. How do you handle state management in Angular?

• RxJS services
• NgRx
• Akita
• Signals (newer alternative)

39. Difference between Reactive and Imperative programming?

• Reactive: declarative, uses streams (RxJS, signals).
• Imperative : stepbystep instructions.

40. How do you make Angular apps SEOfriendly?

• Angular Universal (SSR)
• Prerendering
• Meta tags
• Lazyloaded images

•  We all know that == is equality operator is used to compare two values in JavaScript.
• === operator is strict equality operator
• Both == & === are used to compare values in JavaScript. then what is the difference between == and === lets discuss in detail
• == compares two values by converting them to same data types, 10 == ''10" will be true because string type of 10 converted in to number.
• === compares two values without converting them to same data type. 10 === "10"
• Use our JavaScript compiler to test your snippets
  

 'splice()' and 'slice()' are two array techniques in JavaScript that are often confused with each other. In order to resolve these ambiguities, we will compare the two methods below step by step.

1. Creating a Slice: The Simple Version

Purpose

Evaluates only a small part of an array without changing it.

Usage

array.slice(startIndex, endIndex);

Main Points

• Produces a new array consisting of the elements from startIndex to endIndex (not inclusive).
• Changes made to the original array are not visible.
• If indexes are negative, they count from the end.
• If no arguments are given, then array.slice() makes a shallow copy of the entire array.

Example

const fruits = ['apple', 'banana', 'cherry', 'date'];

const sliced = fruits.slice(1, 3);

console.log(sliced); // ['banana', 'cherry']

console.log(fruits); // ['apple', 'banana', 'cherry', 'date'] (that's how it remained)

2. splice(): Add/Remove Elements from an Array

Purpose

Alters the original array, inserts or deletes elements.

Syntax

array.splice(startIndex, deleteCount, item1, item2,...);

Key Points

• Changes the original array.
• Returns an array followed by the removed elements (if any).
• Deleting, adding or changing elements on the spot are all possible.

Examples

a. Deleting Elements

const numbers = [1, 2, 3, 4, 5];

const removed = numbers.splice(1, 2); // Start removing from position 1 and remove 2 from that

console.log(removed); // [2, 3]

console.log(numbers); // [1, 4, 5]

b. Adding Elements

const colors = ['red', 'blue'];

colors.splice(1, 0, 'green'); // Add 'green' after index 1 without deleting anything

console.log(colors); // ['red', 'green', 'blue']

c. Replacing Elements

const letters = ['a', 'b', 'c'];

letters.splice(1, 1, 'x'); // Replace 'b' with 'x' at index 1

console.log(letters); // ['a', 'x', 'c']

3. The Key Differences

Feature	slice()	splice()
Mutates Original	No	Yes
Return Value	New array of extracted elements	Array of removed elements (if any)
Parameters	(start, end)	(start, deleteCount, items...)
Use Case	Copy a piece of an array	Add/remove elements in place

• Slice Use it when you need to take a part from the array out without changing the original.
• Example: Generate a copy of an array:
• const copy = arr.slice();
• Splice Use it when you need to remove, add, or adjust an array.
• Example: Update a list dynamically, such as editing a to-do list.

5. Pitfalls of Using These Two Methods

Mixed Parameters Puzzle

• Slice (1, 3) obtains elements from indices 1 and 2 (excluding index 3).
• Splice (1, 3) starts at index 1 and removes three elements.

Splice's Mutability

• Splice() changes the original array, so always make sure you don't need to preserve the original data before using it.

Summary

• Slice: copy parts of an array without changing the original.
• Splice: can edit an array directly, deleting, introducing or replacing parts.

• JavaScript prime number program
• In this example will learn about how to find the number is prime or not.
• WHATS IS PRIME NUMBER
• A number greater than 1 with exactly two factors is called prime number.
• Number which is divisible by 1 and itself is prime number. If it is divisible by any other number(other than 1 and itself) then it's not a prime number.
• Consider an example of number 5 ,Which has only two factors  1 and 5. This means 5 is a prime number. Lets take one more example Number 6, which has more than two factors, i.e 1,2,3. This means 6 is not a prime number.
• The first ten prime numbers are 2,3,5,7,11,13,17,19,23,29.
• Note: It should be noted that 1 is non-prime number, because in the above defination already mentioned A number greater than 1 with 2 factors called prime number..
• 
  
• Here we have simple program to print the prime numbers in java script.