type PaletteColors = keyof typeof palette type Palette = { [Color in PaletteColors]: string }

function isKeyOfMcuParams(x: string): x is keyof McuParams { switch (x) { case 'foo': case 'bar': case 'baz': return true; default: return false; } }

keyof is a keyword in TypeScript which accepts a given object type and returns a union type of its keys. These are equivalent: type StatusKey = keyof { online: string; offline: string; busy: string; dnd: string; } type StatusKey = 'online' | 'offline' | 'busy' | 'dnd'

type A = keyof Person["favouriteNumbers"]; // string | number type B = keyof Person["address"]; // "street" | "postcode"