const doSomething = () => {}
const doSomethingElse = () => {}
const doMagic = () => {}
let cases = {
  'a': doSomething,
  'b': doSomethingElse,
  'c': doMagic
};
let key = ???;
if (containsKey(cases, key)) {
  cases[key]();
} else {
  console.log("I don't know what to do!");
};