TypeScript Notes

This note covers the main TypeScript topics from this codebase with simple explanations, examples, and common use cases.

1. Installation and Setup

npm init -y
npm i -D typescript
npx tsc --init

What each command does:

npm init -y creates a package.json file.
npm i -D typescript installs TypeScript as a development dependency.
npx tsc --init creates a tsconfig.json file.
npx tsc compiles .ts files into .js files.

If tsconfig.json contains "outDir": "./dist", compiled JavaScript files will be created inside the dist folder.

Example scripts:

"scripts": {
  "build": "npx tsc",
  "dev": "npx tsc --watch",
  "start": "node dist/index.js"
}

"scripts": {
  "dev":"tsx watch src/index.ts",
  "build":"tsc",
  "start":"node dist/index.js"
}

{
    "watch":["src"],
    "ext":"ts",
    "exec":"tsx src/server.ts"
}

 "scripts": {
    "dev": "nodemon",
    "build": "tsc",
    "start": "node dist/server.js"
  }

Notes:

build compiles once.
dev with --watch recompiles automatically when files change.
start runs the compiled JavaScript.

For third-party libraries, sometimes you also install types:

npm i -D @types/library-name

Use case:

Use @types/... when a package does not ship its own TypeScript types.
Example: @types/node is commonly used in Node.js projects.
Some libraries like axios already include their own types, so extra @types packages are not needed.

2. Type Inference

Type inference means TypeScript automatically understands a type from the value you assign.

let score = 100;        // inferred as number
let username = "nishan"; // inferred as string

Why it is useful:

You write less code.
TypeScript still gives autocomplete and error checking.

Use case:

Use inference when the type is obvious from the value.

3. Type Annotation

Type annotation means you explicitly write the type yourself.

let age: number = 22;
let isLoggedIn: boolean = true;
let city: string = "Kathmandu";

Why it is useful:

Makes your intention clear.
Helps when the value is not assigned immediately.

Use case:

Use annotation for function parameters, return types, complex objects, and variables declared before assignment.

4. Basic Types

let fullName: string = "Nishan";
let marks: number = 95;
let passed: boolean = true;

Use case:

These are the most common types for daily programming.

5. Union Types

A union means a value can be one of multiple types.

let id: string | number;

id = 101;
id = "EMP-101";

Literal unions are also common:

let apiStatus: "success" | "error" | "loading" = "success";

Why it is useful:

It models real-world situations where data can come in more than one form.

Use case:

API status values
IDs that may be numbers or strings
Inputs that accept different valid shapes

6. Type Narrowing

Type narrowing means TypeScript starts with a broad type and then narrows it to a smaller, safer type after checks.

function showAge(age: number | string) {
  if (typeof age === "number") {
    return age.toFixed(0);
  }

  return age.toUpperCase();
}

Why it is useful:

It lets you safely use properties and methods for the correct type.

Use case:

Very helpful when working with unions, user input, and API data.

7. Common Type Guards

Type guards are checks that help TypeScript narrow a type.

`typeof`

Best for primitive values like string, number, and boolean.

function printValue(value: string | number) {
  if (typeof value === "string") {
    console.log(value.toUpperCase());
    return;
  }

  console.log(value.toFixed(2));
}

`instanceof`

Best for classes.

class AdminUser {
  login() {
    console.log("admin login");
  }
}

class NormalUser {
  signup() {
    console.log("normal signup");
  }
}

function handleUser(user: AdminUser | NormalUser) {
  if (user instanceof AdminUser) {
    user.login();
  }
}

`in` operator

Best for checking object properties.

type Fish = { swim: () => void };
type Bird = { fly: () => void };

function move(animal: Fish | Bird) {
  if ("swim" in animal) {
    animal.swim();
    return;
  }

  animal.fly();
}

Truthy and falsy checks

function showMessage(msg?: string) {
  if (msg) {
    return `Message: ${msg}`;
  }

  return "No message provided";
}

User-defined type guard

type PersonalDetails = {
  name: string;
  age: number;
  city: string;
  phone: number;
};

function isPersonalDetails(obj: unknown): obj is PersonalDetails {
  return (
    typeof obj === "object" &&
    obj !== null &&
    "name" in obj &&
    "age" in obj &&
    "city" in obj &&
    "phone" in obj
  );
}

Use case:

Use type guards whenever TypeScript needs proof before letting you access properties safely.

8. Custom Type Definition

You can create your own reusable types with type or interface.

Using `type`

type User = {
  name: string;
  age: number;
  email: string;
};

Using `interface`

interface Product {
  title: string;
  price: number;
}

Use case:

Use custom types to keep code clean and reusable.
Useful for objects, API responses, function contracts, and component props.

9. Type Assertion

Type assertion tells TypeScript, "trust me, I know this value's type."

let response: any = "success";
let responseLength: number = (response as string).length;

Another common example:

type Book = {
  name: string;
};

const rawBook = '{"name":"The Great Gatsby"}';
const bookObj = JSON.parse(rawBook) as Book;

Important:

Type assertion does not change the real runtime value.
It only changes what TypeScript believes.

Use case:

DOM elements
JSON.parse
values from third-party libraries

Be careful:

Wrong assertions can cause runtime errors.

10. `any` vs `unknown`

`any`

let value: any = "typescript";
value = 10;
value = [1, 2, 3];
value.toUpperCase(); // no TypeScript error, but can fail at runtime

`unknown`

let data: unknown = "typescript";

if (typeof data === "string") {
  console.log(data.toUpperCase());
}

Difference:

any disables type safety.
unknown keeps type safety until you check the value.

Use case:

Prefer unknown for external or unsafe data.

11. Interfaces and Types

Both interface and type can describe object shapes.

interface Admin {
  username: string;
  email: string;
}

type Customer = {
  username: string;
  email: string;
};

Main difference:

interface is best for object and class contracts.
type is more flexible because it can also represent unions, tuples, and primitives.

Examples where type is stronger:

type Status = "success" | "error";
type Point = [number, number];

Methods inside an interface

interface Singer {
  start(): void;
  stop(): void;
}

Index signature

interface StringDictionary {
  [key: string]: string;
}

Interface merging

interface UserAccount {
  email: string;
}

interface UserAccount {
  password: string;
}

After merging:

const user: UserAccount = {
  email: "[email protected]",
  password: "secret"
};

Interface extends

interface A {
  a: string;
}

interface B {
  b: string;
}

interface C extends A, B {
  c: number;
}

Can a class implement a type?

Yes, a class can implement:

an interface
an object-shaped type

Example:

type PersonShape = {
  name: string;
  greet(): void;
};

class Person implements PersonShape {
  constructor(public name: string) {}

  greet() {
    console.log(`Hello, ${this.name}`);
  }
}

A class cannot implement a union like this:

type Result = "success" | "error";

That is because a union is not a single object contract.

12. Objects in TypeScript

type UserProfile = {
  readonly id: number;
  name: string;
  email?: string;
};

const profile: UserProfile = {
  id: 1,
  name: "Nishan"
};

Concepts:

readonly means the property cannot be changed after creation.
? means the property is optional.

Use case:

readonly is useful for database IDs.
optional properties are useful when some data may or may not exist.

13. Intersection Types with Optional Fields

Intersection combines multiple types into one.

type ContactInfo = {
  email?: string;
  phone?: string;
};

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

type EmployeeProfile = EmployeeInfo & ContactInfo;

const employee: EmployeeProfile = {
  id: 1,
  name: "Nishan",
  email: "[email protected]"
};

Use case:

Use intersections when you want to combine reusable pieces of data.

14. Utility Types

TypeScript has built-in utility types that save time.

`Partial<T>`

Makes all properties optional.

type Book = {
  title: string;
  price: number;
};

function updateBook(updates: Partial<Book>) {
  console.log(updates);
}

Use case:

Update forms
patch APIs

`Required<T>`

Makes all properties required.

type DraftUser = {
  name?: string;
  email?: string;
};

type FinalUser = Required<DraftUser>;

Use case:

Final validation before saving data

`Pick<T, K>`

Selects only some properties.

type Nishan = {
  name: string;
  age: number;
  city: string[];
};

type NishanBasic = Pick<Nishan, "name" | "age">;

Use case:

Show only selected fields in a UI or response

`Omit<T, K>`

Removes some properties.

type NishanWithoutCity = Omit<Nishan, "city">;

Use case:

Remove secret or unnecessary fields before sharing data

15. Functions

Always type function parameters, and usually type return values for important functions.

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

Optional parameter:

function order(type?: string) {
  if (type) {
    return `You ordered a ${type} pizza.`;
  }

  return "You ordered a pizza.";
}

Use case:

Typed functions catch mistakes early and improve autocomplete.

16. Arrays

Normal array syntax

const names: string[] = ["nishan", "dhakal"];
const numbers: number[] = [1, 2, 3, 4, 5];

Generic array syntax

const ratings: Array<number> = [3, 4, 4.5, 5];

Array of objects

type Order = {
  name: string;
  price: number;
};

const orders: Order[] = [
  { name: "pizza", price: 10 },
  { name: "burger", price: 5 },
  { name: "pasta", price: 8 }
];

Use case:

Use typed arrays when all values follow the same structure.

17. Tuples

Tuples are fixed-length arrays where each position has a specific type.

let user: [string, number, boolean?];

user = ["nishan", 22];
user = ["nishan", 22, true];

Named tuple example:

let bottle: [price: number, size: "small" | "medium" | "large"];
bottle = [10, "medium"];

Use case:

Coordinates
API return pairs
small fixed data sets

18. Enum

Enums let you define a group of named constant values.

enum Size {
  Small,
  Medium,
  Large
}

let bottleSize: Size = Size.Medium;

Use case:

Predefined status, direction, or size values

19. OOP in TypeScript

Class and object

class Person {
  name: string;
  age: number;

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

  display() {
    console.log(`Name: ${this.name}, Age: ${this.age}`);
  }
}

const p1 = new Person("Nishan", 22);
p1.display();

What is `this`?

Inside a class method, this usually refers to the current object instance.

this.name
this.age

What is an instance?

An instance is the actual object created from a class blueprint.

const p1 = new Person("Nishan", 22);

Here p1 is an instance of Person.

Access modifiers

class Employee {
  public name: string;
  private empId: number;
  protected department: string;

  constructor(name: string, empId: number, department: string) {
    this.name = name;
    this.empId = empId;
    this.department = department;
  }
}

Meaning:

public can be accessed anywhere.
private can be used only inside the same class.
protected can be used inside the same class and child classes.

Getter and setter

class Bottle {
  private _capacity: number;

  constructor(capacity: number) {
    this._capacity = capacity;
  }

  get capacity(): number {
    return this._capacity;
  }

  set capacity(value: number) {
    if (value <= 0) {
      throw new Error("Capacity must be positive");
    }

    this._capacity = value;
  }
}

Use case:

Control how values are read and updated.

Static members

Static members belong to the class itself, not to each object.

class MathUtils {
  static PI = 3.14;
}

console.log(MathUtils.PI);

Use case:

shared constants
helper methods

Abstract class

An abstract class is a base class that cannot be created directly and can force child classes to implement methods.

abstract class Drink {
  abstract serve(): void;
}

class Tea extends Drink {
  serve(): void {
    console.log("Serving tea");
  }
}

Use case:

shared blueprint for related classes

20. Generics

Generics let you write reusable code while keeping type safety.

Generic function

function wrapInArray<T>(value: T): T[] {
  return [value];
}

wrapInArray(5);
wrapInArray("hello");
wrapInArray({ name: "Nishan", age: 20 });

Multiple generic types

function pair<T, U>(a: T, b: U): [T, U] {
  return [a, b];
}

Generic interface

interface Box<T> {
  value: T;
}

const numberBox: Box<number> = {
  value: 123
};

Generic API response

interface ApiResponse<T> {
  status: number;
  data: T;
}

const response: ApiResponse<{ msg: string }> = {
  status: 200,
  data: { msg: "success" }
};

Use case:

reusable helpers
API responses
data wrappers
components and utility functions

21. Typed Web Requests

TypeScript is very useful when calling APIs.

import axios, { type AxiosResponse } from "axios";

interface Todo {
  userId: number;
  id: number;
  title: string;
  completed: boolean;
}

const apiResponse: AxiosResponse<Todo> = await axios.get(
  "https://jsonplaceholder.typicode.com/todos/1"
);

Why it is useful:

You get autocomplete for API response fields.
You catch incorrect property access at compile time.

Use case:

frontend and backend API integrations

22. Best Practices

Prefer type inference when the type is obvious.
Use type annotations when clarity matters.
Prefer unknown over any.
Use unions for flexible input.
Use narrowing before accessing type-specific methods.
Use interfaces or type aliases for reusable structures.
Use generics for reusable typed functions.
Use utility types to avoid repeating code.
Use type assertion only when you are confident.

23. Quick Revision Summary

Type inference: TypeScript guesses the type.
Type annotation: You write the type yourself.
Union: one value, many possible types.
Narrowing: reduce a broad type to a specific one.
Type guards: checks like typeof, instanceof, and in.
Type assertion: tell TypeScript what a value is.
Interface/type: reusable custom type definitions.
Utility types: Partial, Required, Pick, Omit.
Generics: reusable type-safe code.
Tuples: fixed position arrays.
OOP: classes, instances, this, getters/setters, static, abstract.

This file is meant to be a simple reference. You can keep adding your own examples from the src folder as you learn more.

TypeScript Notes

1. Installation and Setup

2. Type Inference

3. Type Annotation

4. Basic Types

5. Union Types

6. Type Narrowing

7. Common Type Guards

typeof

instanceof

in operator

Truthy and falsy checks

User-defined type guard

8. Custom Type Definition

Using type

Using interface

9. Type Assertion

10. any vs unknown

any

unknown

11. Interfaces and Types

Methods inside an interface

Index signature

Interface merging

Interface extends

Can a class implement a type?

12. Objects in TypeScript

13. Intersection Types with Optional Fields

14. Utility Types

Partial<T>

Required<T>

Pick<T, K>

Omit<T, K>

15. Functions

16. Arrays

Normal array syntax

Generic array syntax

Array of objects

17. Tuples

18. Enum

19. OOP in TypeScript

Class and object

What is this?

What is an instance?

Access modifiers

Getter and setter

Static members

Abstract class

20. Generics

Generic function

Multiple generic types

Generic interface

Generic API response

21. Typed Web Requests

22. Best Practices

23. Quick Revision Summary

`typeof`

`instanceof`

`in` operator

Using `type`

Using `interface`

10. `any` vs `unknown`

`any`

`unknown`

`Partial<T>`

`Required<T>`

`Pick<T, K>`

`Omit<T, K>`

What is `this`?