Create type from an Array parameter and use as a type in a mapping function

Question

How do I map an array parameter to another parameter type.

An example of what I want is below:


enum Item {
   A = 'A',
   B = 'B',
   C = 'C'
}
type ItemType = `${Item}`

type MappingFn = ( dependencies ) => string;

const createConfig = (dependencies: ItemType[], mappingFn: MappingFn) => {
 //
}

createConfig([ Item.A, Item.B ], (dependencies) => {
 // dependencies should be typed as Record<A | B, any>
  // ie I can access 'dependencies.A' or 'dependencies.B' but error with 'dependencies.C'
} );

jcalz · Accepted Answer · 2023-01-04 16:18:58Z

1

Your createConfig() function should be generic in the type K corresponding to the literal types of the elements of the dependencies parameter, constrained to ItemType if you want:

const createConfig = <K extends ItemType>(
  dependencies: K[],
  mappingFn: (dependencies: Record<K, any>) => string
) => {
  //
}

Let's see it in action:

createConfig(['A', 'B'], (dependencies) => {
  dependencies.A; // okay, any
  dependencies.B; // okay, any
  dependencies.C; // error, property C not known to exist
  return "wantsAString"
});

Looks good.

Playground link to code

edited Jan 4, 2023 at 16:18

answered Jan 4, 2023 at 16:07

jcalz

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4 Comments

Ewan Over a year ago

Yeah this is similar to what I did. However, I have issue that it allows all keys not just the ones specified. I've updated my example as it's not 'string'.

jcalz Over a year ago

Okay, updated. Hopefully the question stabilizes soon.

Ewan Over a year ago

Thanks. It's weird I can see your example works as expected, but my actual implementation doesn't. Although it is a bit more complicated as I have another generic passed in the function. So then when I call the function K is expected to be defined.

jcalz Over a year ago

That's a different issue, see this SO question/answer.

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