Generic typing for some utility functions

Question

I have some utility functions that I want to add generic types to in typescript but can't figure out how to do it.

(These are based on the functionality of ramda but I don't want to use ramda in this project.)

pick :

function pick(keys, object) {
  const result = Object.assign(
    {},
    ...keys.map(k => (k in object ? { [k]: object[k] } : {}))
  );
  return result;
}

reject :

function reject(keys, object) {
  const result = Object.assign(
    {},
    ...Object.keys(object)
      .filter(k => !keys.includes(k))
      .map(k => ({ [k]: object[k] }))
  );
  return result;
}
}

... I'd like to use them like this and have typescript work out the resultant type :

type Item = {title: string, description: string}

const test: { [key: string]: Item } = {
  a: { title: 'TitleA', description: 'DescriptionA' },
  b: { title: 'TitleB', description: 'DescriptionB' },
  c: { title: 'TitleC', description: 'DescriptionC' },
  d: { title: 'TitleD', description: 'DescriptionD' }
};

const test1 = pick(['a', 'c'], test) // {a: { title: 'TitleA', description: 'DescriptionA' }, c: { title: 'TitleC', description: 'DescriptionC' }}
const test2 = reject(['a', 'c'], test) // {b: { title: 'TitleB', description: 'DescriptionB' },d: { title: 'TitleD', description: 'DescriptionD' }}

Does anyone have a suggestion of how to add generic typing to these pick and reject functions?

Answer 1

You can use two generic type parameters, one for the object itself and one for the picked keys. Then you can add a type boundary to the keys to be a subset of the object keys. The return type is then a mapped type of the keys looked up on the object:

 function pick<O, K extends keyof O>(keys: K[], object: O): { [T in K]: O[T] }

The reject function can be typed similarily:

 function reject<O, K extends keyof O>(keys: K[], object: O): { [T in keyof O]: T extends K ? undefined : O[T] }