Useful Typescript

useful.ts

// returns an optional type of properties of T where type satisfies U
export type KeysOfValueType<T, U> = {
  [K in keyof T]: T[K] extends U ? K : never
}[keyof T]

// returns an optional type of properties of T where T[prop]=string
export type StringsOf<T> = KeysOfValueType<T, string>
  
// example of getting only string values from an object in a generic
// https://github.com/microsoft/TypeScript/issues/44532
function innerLowerCase<
  T, K extends keyof T,
>(obj: T & Record<string, string | number | Record<string, unknown>> & Record<K, string>, key: K) {
  const objString = obj[key]
  return objString.toLowerCase()
}