📘 TypeScript Notes for Interview Preparation

📌 What is TypeScript?

TypeScript is a superset of JavaScript that adds:

• Static Typing
• Interfaces
• Classes
• Compile-time Error Checking
• Modern ES Features

TypeScript code is compiled into JavaScript using:

tsc filename.ts

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🚀 Why Use TypeScript?

JavaScript	TypeScript
Dynamically Typed	Statically Typed
Runtime Errors	Compile-time Errors
Hard to Debug	Easy to Debug
No Interfaces	Supports Interfaces

✔ Large Scale Applications ✔ Code Maintainability ✔ Better IntelliSense ✔ Refactoring

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🔹 Basic Types

let id: number = 101;
let name: string = "Raushan";
let isActive: boolean = true;
let data: any = 10;
let list: number[] = [1,2,3];
let tuple: [number, string] = [1,"Hello"];

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🔹 Special Types

Any

let value: any = 10;
value = "Hello";

Unknown

let userInput: unknown;

Void

function greet(): void {
   console.log("Hello");
}

Never

function error(): never {
   throw new Error("Error");
}

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🔹 Type Inference

let num = 100; // inferred as number

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🔹 Functions

function add(a:number, b:number): number {
   return a+b;
}

Optional Parameter

function show(name:string, age?:number){}

Default Parameter

function greet(name:string="Guest"){}

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🔹 Interfaces

interface Employee {
   id:number;
   name:string;
}

let emp:Employee = {
   id:1,
   name:"John"
}

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🔹 Type Alias

type User = {
   id:number;
   name:string;
}

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🔹 Union & Intersection Types

Union (|)

let id: number | string;

Intersection (&)

type Admin = User & {
   role:string;
}

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🔹 Enums

enum Role {
   Admin,
   User,
   Guest
}

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🔹 Classes

class Person {
   name:string;

   constructor(name:string){
      this.name=name;
   }
}

Access Modifiers

• public
• private
• protected
• readonly

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⭐ Generics (Very Important)

function getData<T>(value:T):T {
   return value;
}

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🔹 Modules

Export

export class Test{}

Import

import {Test} from './test';

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🔹 Type Assertions

let value:any="Hello";
let length:number=(value as string).length;

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🔹 Decorators

function Log(target:any){}

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🔹 Interface vs Type

Interface	Type
Used for Object Structure	Used for anything
Extendable	Flexible
Less Complex	More Powerful

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🔹 tsconfig.json

{
 "target": "ES6",
 "module": "commonjs",
 "strict": true,
 "outDir": "./dist"
}

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📌 Most Asked Interview Topics

✔ Interfaces ✔ Generics ✔ Union Types ✔ Enums ✔ Type vs Interface ✔ Any vs Unknown ✔ Access Modifiers ✔ Type Inference ✔ Modules ✔ Decorators ✔ tsconfig.json

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