How can I filter an object by another’s values?

[delucis]

🤔 Question

Describe your question

Say we have an interface with some fields:

interface Data {
  x: string;
  y: string;
}

and a function that takes an object to select fields from that interface:

function select(opts: { x: boolean, y: boolean }) {
  const data: Data = getData() // from somewhere
  const o = {};
  for (const key in data) {
    if (opts[key] === true) o[key] = data[key];
  }
  return o;
}

How could we type the return such that it excludes fields in Data that are not true in the options object provided to select?

Thanks!

Search tags, topics

#typescript #object #filter #exclude

[millsp]

It isn't possible so easily. Though it might be possible through writing you own custom type. First of all, start writing the type that will perform this:

interface Data {
    x: string;
    y: string;
}

type SelectIfTrue<O extends object, O1 extends object> =
    O.Pick<O, O.SelectKeys<O1, true>>

type t0 = SelectIfTrue<Data, {x: true, y: false}> // works

Then, you integrate the type to your function by casting its result:

// you will have to use `as const`. so we use a generic parameter
function select<O extends object>(opts: O) {
    const data: Data = {} as Data // from somewhere
    const o = {}

    for (const key in data)
        if (opts[key] === true) o[key] = data[key]

    return o as SelectIfTrue<Data, O> // casting the result
}

const t1 = select({x: true, y: false})			// not working
const t2 = select({x: true, y: false} as const) // works

I find this cumbersome. It is good to try to summarize TypeScript types. Even though this library can allow you to do such computations, you should not try to compute absolutely everything. Instead, your select should spit a simpler type out:

type t2 = O.Optional<Data>

[delucis]

@pirix-gh Thank you! Perhaps I should be less precise, it just seemed like it would be nice given the strict coupling between input and output. Thanks also for these great tools! 😃

[delucis]

Oh, although I notice that at least with the strict compiler option the as const assertion isn’t necessary, which was the main thing I disliked as it’s user-facing. Nice! Thanks again for your help.