Generalizing Function Signatures

function isAvailable<Obj>(
  obj: Obj,
  key: string | number | symbol
): key is keyof Obj {
  return key in obj;
}

function loadLanguage(collection: Languages, lang: string) {
  if (isAvailable(collection, lang)) {
    // lang is keyof Languages
    collection[lang]; // access ok!
  }
}

function selectElement(collection: AllowedElements, elem: string) {
  if (isAvailable(collection, elem)) {
    // elem is keyof AllowedElements
    collection[elem]; // access ok
  }
}