TypeScript Advanced Types Cheat Sheet

type IsString<T> = T extends string ? true : false;type A = IsString<'hi'>; // truetype B = IsString<42>;   // false// Distributive conditional types over unionstype ToArray<T> = T extends any ? T[] : never;type C = ToArray<string | number>; // string[] | number[]// infer keyword extracts a type from within a conditionaltype ElementType<T> = T extends (infer U)[] ? U : T;type D = ElementType<number[]>; // number

type Readonly2<T> = { readonly [K in keyof T]: T[K] };type Partial2<T> = { [K in keyof T]?: T[K] };// Key remapping with 'as' (TS 4.1+)type Getters<T> = {  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];};interface Person { name: string; age: number; }type PersonGetters = Getters<Person>;// { getName: () => string; getAge: () => number }// Filtering keys with 'never'type NonFunctionKeys<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];

type Direction = 'top' | 'bottom' | 'left' | 'right';type Margin = `margin-${Direction}`;// 'margin-top' | 'margin-bottom' | 'margin-left' | 'margin-right'type EventName<T extends string> = `on${Capitalize<T>}`;type ClickEvent = EventName<'click'>; // 'onClick'// Combined with mapped types for typed event handlerstype Handlers<T extends string> = { [K in T as EventName<K>]: () => void };

interface Circle { kind: 'circle'; radius: number; }interface Square { kind: 'square'; side: number; }type Shape = Circle | Square;function area(shape: Shape): number {  switch (shape.kind) { // narrows the union based on the discriminant    case 'circle': return Math.PI * shape.radius ** 2;    case 'square': return shape.side ** 2;    default:      const _exhaustive: never = shape; // compile error if a case is missed      throw new Error('Unhandled shape');  }}