JavaScript Fundamentals Every Angular Developer Must Know for Interviews (2026)

 

JavaScript Fundamentals Every Angular Developer Must Know for Interviews (2026)

If you're preparing for an Angular interview, especially for a Senior Angular Developer role, strong JavaScript fundamentals are essential. Many interviewers spend a significant portion of the interview testing JavaScript concepts because Angular is built on top of JavaScript and TypeScript.

In this article, we'll cover the most important JavaScript interview topics with practical examples.

1. var vs let vs const

These are used to declare variables, but they behave differently.

var name = "Manikandan";
let age = 35;
const country = "India";
Featurevarletconst
ScopeFunctionBlockBlock
ReassignYesYesNo
HoistedYesYesYes

Example

if (true) {
  var a = 10;
  let b = 20;
}

console.log(a); // 10
console.log(b); // Error

2. Hoisting

JavaScript moves declarations to the top during execution.

console.log(name);

var name = "Mani";

Output:

undefined

Internally, JavaScript treats it as:

var name;

console.log(name);

name = "Mani";

3. Closures

A closure allows a function to remember variables from its outer scope even after the outer function has completed execution.

function counter() {

  let count = 0;

  return function() {
    count++;
    return count;
  };
}

const increment = counter();

console.log(increment()); // 1
console.log(increment()); // 2

Interview Definition

A closure is a function that retains access to variables from its lexical scope even when executed outside that scope.

4. Scope

JavaScript has three main scopes:

Global Scope

let name = "Mani";

Function Scope

function test() {
  let age = 35;
}

Block Scope

if (true) {
  let city = "Chennai";
}

5. Event Loop

JavaScript is single-threaded but can handle asynchronous operations using the Event Loop.

console.log("1");

setTimeout(() => {
  console.log("2");
}, 0);

console.log("3");

Output:

1
3
2

Execution Flow:

Call Stack
↓
Web APIs
↓
Callback Queue
↓
Event Loop

6. == vs ===

Double Equals

5 == "5"

Output:

true

Performs type conversion.

Triple Equals

5 === "5"

Output:

false

Checks both value and type.

7. Promises

Promises represent future values of asynchronous operations.

const promise = new Promise((resolve, reject) => {
  resolve("Success");
});

promise.then(result => {
  console.log(result);
});

Promise States:

  • Pending

  • Fulfilled

  • Rejected

8. Async/Await

Modern and cleaner way to handle asynchronous operations.

Using Promise

getUsers()
  .then(data => {
    console.log(data);
  });

Using Async/Await

async function loadUsers() {
  const data = await getUsers();
  console.log(data);
}

9. Callbacks

A callback is a function passed as an argument to another function.

function greet(callback) {

  console.log("Hello");

  callback();
}

greet(() => {
  console.log("Mani");
});

Output:

Hello
Mani

10. Higher-Order Functions

A higher-order function accepts or returns another function.

function execute(callback) {
  callback();
}

Examples:

  • map()

  • filter()

  • reduce()

11. map() vs forEach()

forEach()

arr.forEach(x => {
  console.log(x);
});

Returns:

undefined

map()

const result = arr.map(x => x * 2);

Returns:

new array

12. filter() vs find()

filter()

Returns multiple matching items.

const result = users.filter(user => user.age > 18);

find()

Returns the first matching item.

const result = users.find(user => user.id === 1);

13. Understanding this

The value of this depends on how a function is called.

const person = {

  name: "Mani",

  greet() {
    console.log(this.name);
  }

};

person.greet();

Output:

Mani

14. Arrow Functions vs Regular Functions

Regular Function

function greet() {
}

Has its own this.

Arrow Function

const greet = () => {
};

Inherits this from its parent scope.

Angular developers commonly use arrow functions.

15. Destructuring

Object Destructuring

const user = {
  name: "Mani",
  age: 35
};

const { name, age } = user;

Array Destructuring

const arr = [1, 2, 3];

const [a, b] = arr;

16. Spread Operator

Used to copy or merge arrays and objects.

const arr1 = [1, 2];
const arr2 = [...arr1, 3];

Result:

[1, 2, 3]

Object Example:

const user = {
  ...oldUser,
  name: "Kumar"
};

17. Rest Operator

Collects multiple values into a single array.

function sum(...numbers) {
  console.log(numbers);
}

sum(1, 2, 3, 4);

Output:

[1, 2, 3, 4]

18. Shallow Copy vs Deep Copy

Shallow Copy

const copy = { ...user };

Nested objects remain shared.

Deep Copy

const copy = structuredClone(user);

Creates a completely independent copy.

19. Debouncing

Useful for search boxes.

Without Debouncing:

m → API Call
ma → API Call
man → API Call
mani → API Call

With Debouncing:

debounceTime(500)

Only one API call is executed after the user stops typing.

20. Throttling

Limits function execution to fixed intervals.

Example:

Scroll Event

Instead of executing hundreds of times per second, throttle executes at a controlled rate.

21. Memory Leaks

Memory leaks occur when resources are not released properly.

Example:

setInterval(() => {
  console.log("Running");
}, 1000);

Solution:

clearInterval(intervalId);

Angular Example:

ngOnDestroy() {
  this.subscription.unsubscribe();
}

22. null vs undefined

undefined

let value;

Output:

undefined

null

let value = null;

Represents an intentional empty value.

23. Prototypes

JavaScript uses prototypes for inheritance.

function Person(name) {
  this.name = name;
}

Person.prototype.greet = function() {
  console.log("Hello");
};

All Person objects share the same greet() method.

24. Currying

Transforms a function with multiple parameters into a series of functions.

function sum(a) {
  return function(b) {
    return a + b;
  };
}

console.log(sum(10)(20));

Output:

30

Most Important JavaScript Topics for Angular Interviews

Before attending your next Angular interview, make sure you understand:

✅ var, let, const
✅ Scope
✅ Hoisting
✅ Closures
✅ Event Loop
✅ Promises
✅ Async/Await
✅ Callbacks
✅ this Keyword
✅ Arrow Functions
✅ map/filter/reduce
✅ Destructuring
✅ Spread & Rest Operators
✅ Debounce & Throttle
✅ Memory Leaks
✅ Prototypes
✅ Deep Copy vs Shallow Copy
✅ == vs ===
✅ Event Bubbling & Capturing
✅ Microtasks & Macrotasks

Mastering these concepts will significantly improve your confidence and performance in Angular, JavaScript, and Frontend interviews.

Happy Learning and Best of Luck with Your Interviews! 🚀

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