Mapping Object Keys in TypeScript

Often you want to iterate over the keys of an object and perform a function on either the keys or their values (or both). TypeScript isn't able to infer the type of the key without a little help. Let's take the example of taking in a user input of a dog and saving that dog to a database.

The Problem

const doggoInput = {
  name: 'Pedro',
  breed: 'mutt',
  onDoorbell() {
    alert('woof woof!')
  }
}

const doggo = new Dog()

Object.keys(doggoInput).forEach(key => {
  // TypeScript can't infer that key is a key of doggo
 doggo[key] = doggoInput[key] 
})

The Solution

(Object.keys(doggoInput) as unknown as keyof typeof doggoInput).forEach(key => {
  // TypeScript knows key is type 'name' | 'breed' | 'onDoorbell'
  doggo[key] = doggoInput[key]
})

And viola! We've fixed our TypeScript problem without ts-ignore!