TypeScript exact key value mappings

key-value-mappings.ts

/* Tested with TypeScript 2.4.2 */

// If you create an object like this, although the keys are correctly inferred, all of the values will be of type string
const obj = {
  FOO: 'FOO', // FOO: string
  BAR: 'BAR', // BAR: string
};

// You can easily fix this by defining a type for the key values
type KeyValue = 'FOO' | 'BAR';

// You can then use this to define the exact keys and values in an object
const exactObj: {[K in KeyValue]: K} = {
  FOO: 'FOO', // FOO: 'FOO'
  BAR: 'BAR', // BAR: 'BAR'
};