TypeScript

Core Concepts

TypeScript adds a static type system on top of JavaScript. Types are erased at compile time — the output is plain JS.

Utility Types

Partial<T>

Makes all properties optional:

interface User { id: number; name: string }
type PartialUser = Partial<User> // { id?: number; name?: string }

Pick<T, K> and Omit<T, K>

type UserPreview = Pick<User, 'id' | 'name'>
type UserWithoutId = Omit<User, 'id'>

Record<K, V>

const map: Record<string, number> = { a: 1, b: 2 }

Generics

function identity<T>(value: T): T {
  return value
}

Type Narrowing

function process(input: string | number) {
  if (typeof input === 'string') {
    // input is narrowed to string here
    return input.toUpperCase()
  }
  return input * 2
}

satisfies operator

Validates a value against a type without widening it:

const config = {
  port: 3000,
  host: 'localhost',
} satisfies Record<string, string | number>
// config.port is still number, not string | number