Enforce shape of object values while treating keys still as const's

July 3, 2024

I have a map of filters defined like this:

type TFiltersNext = { [key: string]: { type: 'date' | 'enum' } };

const filtersNext: TFiltersNext = {
  foo: { type: 'date' }
};

I want to define valid filter names in a type like this:

type TFilterNameNext = keyof typeof filtersNext;

However this isn’t working as this test doesn’t cause issues:

const filterName: TFilterNameNext = 'bar';

As we see, bar is not a valid filter name.

I recreated this in TS Playground here – https://www.typescriptlang.org/play/?#code/C4TwDgpgBAKgYgSwDbAgJwM4DkIA9hQC8UA3lANoDWEIAXFBsGggHYDmAuvWaJPQOQATAIap+UAD5R+EFgFcAtuIC+UZQG4AUJoDGAexaMoAM2SpMOfPXhn02PAWIlNUE3r3covCAJFi1mhqa3rCIKOhYwgoQlo5Q1CB6xl7gEEkmthYOWvqGBKbhaJHR1mHmxTEORNLG7sL86kA

If I remove the : TFiltersNext from const filtersNext: TFiltersNext = { it works, but then I lose the enforcement of the shape of the values.

Anyone know whats going on?

>Solution :

Use the satisfies keyword:

type TFiltersNext = { [key: string]: { type: 'date' | 'enum' } };

const filtersNext = {
  foo: { type: 'date' }
} satisfies TFiltersNext;

type TFilterNameNext = keyof typeof filtersNext;
const filterName: TFilterNameNext = 'fooa';

Then you’ll get error and get the explicit keys. You cannot explicitly tell the type of the object is :TFiltersNext because its keys are declared to be anything ([key: string])