Consider following code:

type Name = 'foo' | 'bar'

type ProviderFuncFromString = (data: string) => { name: Name; data: string }

type ProviderFuncFromNumber = (numb: number) => { name: Name; data: string }

function providerFuncFromStringFactory(name: Name): ProviderFuncFromString {
  return (data) => ({ name, data })
}

function providerFuncFromNumberFactory(name: Name): ProviderFuncFromNumber {
  return (numb) => ({ name, data: numb.toString() })
}

type ProviderMap = {
  foo: ProviderFuncFromString
  bar: ProviderFuncFromNumber
}

const providerMap: ProviderMap = {
  foo: providerFuncFromStringFactory('foo'),
  bar: providerFuncFromNumberFactory('bar'),
}

There are two duplications here:

• Both ProviderMap and providerMap say that the provider func for foo is from string, and that provider func for bar is from number
• In providerMap the keys foo and bar are duplicated both in the key and value.

How can I remove both of these duplications?

I don’t want to use any manual type assertions like as keyword as I want to maintain type-safety provided by TypeScript.

I know that for simple string union types I can deduplicate type and const like this:

export const NameVal = ['foo', 'bar']

export type NameType = (typeof NameVal)[number]

Here foo and bar are defined only once, providing us both with type and runtime const.

However, I don’t know how to apply this “trick” to more advanced types.

I tried e.g.:

type providerMapType = (typeof providerMap)[keyof typeof providerMap]

but then the type becomes

type providerMapType = ProviderFuncFromString | ProviderFuncFromNumber

and not the desired

type providerMapType = {
  foo: ProviderFuncFromString
  bar: ProviderFuncFromNumber
}

As for the duplication within providerMap of foo and bar, I was trying something like this:

function getFromStringFactory(name: Name): [Name, ProviderFuncFromString] {
  return [name, providerFuncFromStringFactory(name)]
}

function getFromNumberFactory(name: Name): [Name, ProviderFuncFromNumber] {
  return [name, providerFuncFromNumberFactory(name)]
}

const providerMap2 = Object.fromEntries([
  getFromStringFactory('foo'),
  getFromNumberFactory('bar'),
])

but here providerMap2 evaluates to const providerMap2: any.