Setup & first types.
Three commands to typed JavaScript.

# 1. Install the TypeScript compiler globally
npm install -g typescript

# 2. Verify it worked
tsc --version
Version 5.7.2

# 3. Create a file and compile it
echo 'const msg: string = "hi"; console.log(msg);' > hello.ts
tsc hello.ts

# 4. Run the output (plain JavaScript)
node hello.js
hi

// ─── The primitives ───────────────────────────
let name: string = 'Alice';
let age: number = 28;
let isActive: boolean = true;

// ─── Arrays ───────────────────────────────────
let tags: string[] = ['ts', 'js'];
let scores: Array<number> = [90, 85, 77];
// string[] and Array are the same thing

// ─── Objects (inline shape) ───────────────────
let user: { name: string; age: number } = {
name: 'Alice',
age: 28
};

// ─── Function parameters and return ───────────
function greet(name: string): string {
return Hello, ${name}!`;
}
// ↑ param ↑ return type

// ─── Optional and default params ──────────────
function greet2(name: string, greeting?: string): string {
return ``${greeting ?? 'Hello'}, ${name}!;
}
// ↑ optional — string | undefined

// ─── The compiler catches mistakes ────────────
let count: number = 5;
count = 'five'; // ❌ Type 'string' is not assignable to type 'number'

function add(a: number, b: number): number {
return a + b;
}
add('1', '2'); // ❌ Argument of type 'string' is not assignable to parameter of type 'number'

// ─── Inference: no annotation needed ──────────
let age = 28; // inferred: number
let name = 'Alice'; // inferred: string
let isActive = true; // inferred: boolean
let tags = ['ts', 'js']; // inferred: string[]
let user = { name: 'A', age: 28 };
// inferred: { name: string; age: number }

// ─── Return type inference ────────────────────
function add(a: number, b: number) {
return a + b; // inferred return: number
}
// You don't have to write ": number" — TS figures it out

// ─── When inference is NOT enough ─────────────
// Function parameters: always annotate, no initial value
function greet(name) { // ❌ Parameter 'name' implicitly has an 'any' type
return Hello,${name}`;
}

// Empty arrays: annotate to give them a type
let items = []; // inferred: any[] — dangerous
let items2: string[] = []; // ✅ typed

// ─── The rule ─────────────────────────────────
// Annotate: parameters, exported return types, empty collections
// Infer: local variables, obvious returns, most things

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

function greet(user: User): string {
return Hello, ${user.name}!`;
}

const alice: User = { name: 'Alice', age: 28 };
console.log(greet(alice));

// interface: GONE — it only existed for type checking
// : User on the param: GONE
// : string on the return: GONE
// : User on the const: GONE

function greet(user) {
return Hello,${user.name}!`;
}

const alice = { name: 'Alice', age: 28 };
console.log(greet(alice));