flow-object-values-by-union-key.js

// @flow

type Key = 'foo' | 'bar';

/**
* We must define an object type with an indexer that uses the same union type.
* Otherwise Flow will throw an error when we try accessing the value by a key
*/
const titles: {[Key]: string} = {
  foo: 'This is a foo',
}

function getTitle(key: Key) {
  if (key in titles) {
    return titles[key];
  }
}