Basic types, inference and type annotations — TypeScript — Types, Generics and Real-World Patterns

1. Basic types, inference and type annotations

Basic★ Must knowLesson 1 of 9

typescript

let name: string = "Divakar";
let age = 34;                    // inferred as number — annotate only when inference can't help
let active: boolean = true;
let ids: number[] = [1, 2, 3];   // or Array<number>
let pair: [string, number] = ["qty", 5];   // tuple

let anything: any = 4;           // turns type checking OFF — avoid
let input: unknown = JSON.parse("{}");     // safe "any": must narrow before use
function fail(msg: string): never { throw new Error(msg); }  // never returns
function log(msg: string): void { console.log(msg); }

let maybe: string | undefined;   // with strictNullChecks, undefined must be explicit
const status = "open" as const;  // literal type "open", not string

Type	Meaning
`any`	Opt out of checking — errors spread silently
`unknown`	Anything, but you must check the type before using it
`never`	No value possible (functions that throw, exhaustive checks)
`void`	Function returns nothing useful
`null` / `undefined`	Separate types under `strict`

Interview tip: "Prefer unknown over any for data from outside (JSON, errors in catch) — it forces you to validate."