JavaScript with a static type system layered on top, compiled (or transpiled) down to plain JS. Nearly the default choice for new JS projects of any size now.

Setup

npm install -D typescript
npx tsc --init

Run directly during development without a manual compile step via tsx or ts-node:

npm install -D tsx
npx tsx src/index.ts

Core syntax

// basic types
let age: number = 30;
let name: string = "Ada";
let active: boolean = true;
let tags: string[] = ["a", "b"];
let tuple: [string, number] = ["x", 1];

// interfaces vs type aliases
interface User {
  id: number;
  name: string;
  email?: string; // optional
}

type ID = string | number; // union type

// functions
function add(a: number, b: number): number {
  return a + b;
}

// generics
function firstOf<T>(items: T[]): T | undefined {
  return items[0];
}

// enums
enum Role { Admin, Editor, Viewer }

Narrowing

function printLength(value: string | string[]) {
  if (Array.isArray(value)) {
    console.log(value.length); // TS knows it's an array here
  } else {
    console.log(value.length); // TS knows it's a string here
  }
}

Utility types worth knowing

type PartialUser = Partial<User>;      // all fields optional
type UserName = Pick<User, "name">;    // subset of fields
type WithoutEmail = Omit<User, "email">;
type ReadonlyUser = Readonly<User>;

tsconfig.json essentials

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true
  }
}

strict: true is the single highest-value setting, it turns on strictNullChecks, noImplicitAny, and friends together.

Popular use

Almost everywhere plain JavaScript is used today, React/Next.js apps, Node backends (Express, NestJS), and CLI tooling, increasingly defaults to TypeScript rather than JS.

Common gotchas

• Types are erased at compile time, they don't exist at runtime, so you can't typeof a custom type or interface.
• any disables checking entirely for that value, prefer unknown when you genuinely don't know the type, and narrow it before use.
• A .ts file importing a .js-only library sometimes needs @types/<package> (npm i -D @types/lodash, for example) for type info.