Basic TypeScript for JavaScript Programmers

1. Basic Type Annotation

   let helloWorld: string = "Hello World";

   function (nums: Array<number>): Promise<number>{
   }

2. Interfaces

• Declaring and using interfaces.
• Extending interfaces.
• Declaration merging.

3. Types

• Declaring types.
• Using intersections.
• Function types.

4. When to Use type vs interface

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

type Admin = User & {
  role: string;
};

interface User {
  name: string;
  age: number;
}

interface Admin extends User {
  role: string;
}


interface User {
  name: string;
}

interface User {
  age: number;
}

// Resulting type: { name: string; age: number }

🧠 When to Use What?
Use interface when:

• You're working with object-oriented code.
• You want to take advantage of declaration merging.

• You're defining public APIs or libraries.
  Use type when:

• You need to define union, tuple, or primitive types.

• You want to create complex compositions using intersections.

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5. Type Inference

TypeScript can automatically infer types based on the assigned value:

let message = "Hello"; // inferred as string
let count = 42;        // inferred as number

You can mention that explicit typing is optional when the context provides enough information.

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6. Union and Literal Types

type Status = "success" | "error" | "loading";

function showStatus(status: Status) {
  console.log(status);
}

Useful for modeling finite states.

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7. Optional and Readonly Properties

interface User {
  name: string;
  age?: number; // optional
  readonly id: number; // cannot be changed
}

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8. Enums

enum Role {
  Admin,
  User,
  Guest
}

const userRole: Role = Role.Admin;

Great for defining named constants.

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9. Generics

function identity<T>(arg: T): T {
  return arg;
}

const output = identity<string>("Hello");

Generics allow you to write reusable and type-safe code.

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10. Type Assertions

let someValue: unknown = "this is a string";
let strLength = (someValue as string).length;

Useful when you know more about the type than TypeScript does.

Reference:-

1. https://www.typescriptlang.org/docs/handbook