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.

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.

6. Union and Literal Types

type Status = "success" | "error" | "loading"; function showStatus(status: Status) { console.log(status); } 

Useful for modeling finite states.

7. Optional and Readonly Properties

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

8. Enums

enum Role { Admin, User, Guest } const userRole: Role = Role.Admin; 

Great for defining named constants.

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.

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