typesript

index.ts

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

  }

}