Typescript generic parameters assertion

Question

I want to do a compile-time assertion on the following code:

interface FormFields {
  [K: string]: string | boolean | number;
}

function FormTextInput<
  FieldName extends keyof Fields,
  Fields extends FormFields
>(fieldName: FieldName) { ... }

// should throw compile-time error:
FormTextInput<'someNumberField', { someNumberField: number }>('someNumberField')

// should NOT throw error:
FormTextInput<'someStringField', { someStringField: string }>('someStringField')

so that FormTextInput always throws an error if FieldName refers to a non-string value of Fields

Is it possible do to this at compile-time with Typescript? I have seen some docs on asserts ( https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions ) but it doesn't seem it was meant to be used for this scenario

Aleksey L. · Accepted Answer · 2020-04-23 13:06:00Z

1

You can define helper utility which picks keys with string value from source type:

type PickKeysWithStringValue<T> =
  { [P in keyof T]: T[P] extends string ? P : never }[keyof T];

type Test = PickKeysWithStringValue<{ foo: string, bar: number }> // results in "foo"

function FormTextInput<Fields extends FormFields>(fieldName: PickKeysWithStringValue<Fields>) {  }

// throws compile-time error:
FormTextInput<{ someNumberField: number }>('someNumberField')

// doesn't throw error:
FormTextInput<{ someStringField: string }>('someStringField')

Playground

edited Apr 23, 2020 at 13:06

answered Apr 23, 2020 at 13:00

Aleksey L.

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5 Comments

Hoffmann Over a year ago

works great! But question: Is there a way to improve the error message? it just says Type '"someNumberField"' does not satisfy the constraint '"someStringField"'

Hoffmann Over a year ago

Also I am sure this came up before, but if I change the order of the generics in the function to <Fields, FieldName> why can't TS infer the type of the second generic from the arguments of the function. Would be nice to not need to specify the fieldName twice

Aleksey L. Over a year ago

Why do you need generic type parameter for field name?

Hoffmann Over a year ago

I am doing some magic so the user can only pass fieldNames that match a certain type in Fields. To do that I need it as a generic

Aleksey L. Over a year ago

Can you add an example on playground?

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