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til / Pick with keyof typeof in TypeScript

In TypeScript, we might want to get the keys of an object as a type for some purpose.

const transportation = {
  AIRPLANE: 'airplane',
  BICYCLE: 'bicycle',
  CAR: 'car',
  TRAIN: 'train',

type TransportationType = keyof typeof transportation

If we want to limit the options, we can’t use the Pick utility directly on this type, even though it looks like we’ve created a string union.

type OnlyCarAndBicycle = Pick<TransportationType, 'CAR' | 'BICYCLE'>
// TypeScript error:
// Type '"CAR"' does not satisfy the constraint '...'
// Type '"BICYCLE"' is not assignable to type '...'

Instead, we should use typeof transportation, filter our options, then use keyof.

type OnlyCarAndBicycle = keyof Pick<typeof transportation, 'CAR' | 'BICYCLE'>

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