TypeScript Variables and Type System

Type Annotations

TypeScript lets you annotate variables with their types. The compiler infers types when you don't annotate explicitly.

let name: string = "Alice";
let age: number = 25;
let isActive: boolean = true;
let data: null = null;
let value: undefined = undefined;

// Type inference — TypeScript figures it out
let city = "London";      // inferred as string
let count = 42;           // inferred as number

Arrays and Tuples

const numbers: number[] = [1, 2, 3, 4, 5];
const names: Array = ["Alice", "Bob"];

// Tuple — fixed-length array with known types
const point: [number, number] = [10, 20];
const entry: [string, number] = ["score", 95];

Union Types

let id: string | number;
id = "abc-123";  // OK
id = 42;         // OK
// id = true;    // ❌ Error

function printId(id: string | number) {
    if (typeof id === "string") {
        console.log(id.toUpperCase());
    } else {
        console.log(id.toFixed(2));
    }
}

Literal Types and Type Aliases

type Direction = "up" | "down" | "left" | "right";
type Status = "pending" | "approved" | "rejected";

let move: Direction = "up";   // OK
// move = "diagonal";          // ❌ Error

type Point = { x: number; y: number };
const origin: Point = { x: 0, y: 0 };

any, unknown, never

let anything: any = "hello";   // disables type checking
anything = 42;                 // OK

let safe: unknown = getData(); // must narrow before using
if (typeof safe === "string") {
    console.log(safe.toUpperCase());
}

function fail(msg: string): never {  // never returns
    throw new Error(msg);
}