How do I update a type on the fly using lodash.set?

[nandorojo]

🍩 Feature Request

Is your feature request related to a problem?

I have a function that updates the type of an object. It turns milliseconds into date objects.

Here's the idea:

const object = {
  lastUpdated: 1616779693036,
  createdAt: 1616779693019
}

const objectWithDates = msToDates(object, ['lastUpdated'])

typeof objectWithDates.lastUpdated // this should be Date

typeof objectWithDates.createdAt // should be number, since I didn't pass it in the array

Describe the solution you'd like

A way to change the type of each field in the object, dynamically, based on the array of paths I pass.

Describe alternatives you've considered

This is my code currently:

import set from 'lodash.set'
import get from 'lodash.get'
import type { F } from 'ts-toolbelt'

function msToDates<
  Obj extends object,
  Path extends string
>(obj: Obj, paths: F.AutoPath<Obj, Path>[]) {
  const next: Obj = { ...obj }

  paths.forEach((path) => {
    const ms = get(obj, path) 
    set(next, path, new Date(ms)) 
  })

  return next
}

This works well. However, the ReturnType of the ms values I pass to paths is still a number, not a Date.

Teachability, Documentation, Adoption, Migration Strategy

This might already be possible, curious if you have an idea how to do it @millsp. Thank you!

[millsp]

Hey @nandorojo , that is because set is mutating and TS cannot keep track of mutations. Instead of a foreach, you should look into returning something like https://lodash.com/docs/4.17.15#mapValues. However, I'm not sure how the types work there. If your object is going to contain other fields than numbers, you might want to implement your own mapped type.

[nandorojo]

Is there a way to map a type from one to another, where certain fields are changed from number to Date?

type NumToDate<O, Paths> something like that?

[millsp]

Yes, I'll post the solution in a few minutes

[nandorojo]

Sweet, I figure I can use that as the return type of my function

[millsp]

Sorry, but I realized that the O.P utilities are behaving in a dissociative way (ie. on par with unions). So, I am not able to make the example work right now. It should have been as simple as this:

/* eslint-disable */

import type { S, F, O } from "ts-toolbelt";

declare function msToDates<O extends object, P extends string>(
  obj: O,
  paths: F.AutoPath<O, P>[]
): O.P.Update<O, S.Split<P, ".">, Date>;

const object = {
  lastUpdated: 1616779693036,
  createdAt: 1616779693019,
  subProperty: {
    milliseconds: 2123154541221
  }
};

const objectWithDates = msToDates(object, [
  "lastUpdated",
  "subProperty.milliseconds"
]);

typeof objectWithDates.lastUpdated;
typeof objectWithDates.createdAt;

I'm going to mark this issue as a bug, because I think most people will want the O.P tools to be associative instead. Associativity is much harder to accomplish, while dissociation could just be done by distributing the path before hand. If you run the example, you will see that unions are produced (dissociative), it defeats the purpose in your use-case and probably any other use-case. Sorry about this!

[nandorojo]

@millsp thanks so much for the details!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.