Basic types and inference

let name: string = 'Asha';
let age: number = 29;
let isActive: boolean = true;
let big: bigint = 10n;
// In practice, let inference do this: let name = 'Asha';

const marks: number[] = [78, 92];          // or Array<number>
const names: string[] = [];
const point: [number, number] = [18.52, 73.85];  // a tuple: fixed length and types
const entry: [string, number] = ['rice', 40];
marks.push('ninety');  // Error: string is not assignable to number

function addGst(amount: number, rate: number = 18): number {
  return amount * (1 + rate / 100);
}
function greet(name: string, title?: string): string {   // ? = optional
  return title ? `${title} ${name}` : name;
}
function logVisit(page: string): void {   // void: returns nothing useful
  console.log('visited', page);
}
const double = (n: number): number => n * 2;

type Method = 'GET' | 'POST' | 'PUT' | 'DELETE';
function request(method: Method, url: string) { /* … */ }
request('POST', '/orders');   // OK
request('POTS', '/orders');   // Error: '"POTS"' is not assignable to type 'Method'.

function findUser(id: number): { name: string } | undefined { /* … */ return undefined; }
const user = findUser(7);
console.log(user.name);    // Error: 'user' is possibly 'undefined'.
console.log(user?.name);   // OK

const data: unknown = JSON.parse('{"amount": 5}');
data.amount;                       // Error: 'data' is of type 'unknown'.
if (typeof data === 'object' && data !== null && 'amount' in data) {
  console.log((data as { amount: number }).amount);  // allowed after checking
}