OverrideProperties changes undefined as optional

[CarinaChenot]

Considering this:

import { OverrideProperties } from 'type-fest';

type Original = {
  foo: string;
  bar: string;
};

type Modified = {
  foo: string | undefined;
  bar: string;
};

const test: OverrideProperties<Original, Modified> = { bar: '' };

foo is typed as foo?: string instead of foo: string | undefined

This is particularly problematic when using exactOptionalPropertyTypes flag

[sindresorhus]

Are you on the latest type-fest version?

I added a test for your case and it is passing: 2547a00

[CarinaChenot]

@sindresorhus Indeed, issue went away with 4.10.0 my bad, thank you!

[mwg-ofx]

Whatever changed between 4.9.0 and 4.10.0 related to OverrideProperties was a breaking change that should have happened with a major version bump. It doesn't matter if the behaviour of OverrideProperties was technically buggy, the fact is that there's plenty of code out there relying on the old behaviour, and upgrading to a newer release of the v4 series has been made more difficult by this breakage.