TypeScript Generic to change all types of a particular type to another

Question

I'm trying to come up with a typescript type that takes an object and sets all keys of type number to be string. The following doesn't work:

export type ChangeTypeOfKeys<T extends object> = {
  [key in keyof T]: key extends number ? string : T[key]
}

const c: ChangeTypeOfKeys<{ a: number, b: boolean }> = {
    a: 'ok',
    b: false,
}

The resulting type should be { a: string, b: boolean }, but instead I get a TS error:

Type 'string' is not assignable to type 'number'.(2322)
input.tsx(5, 29): The expected type comes from property 'a' which is declared here on type 'ChangeTypeOfKeys<{ a: number; b: boolean; }>'

TS playground link

I think the issue is related to how I'm using key in key in keyof T, since replacing key extends number with another argument works:

export type ChangeTypeOfKeys<T extends object, Replace> = {
  [key in keyof T]: Replace extends number ? string : T[key]
}

const c: ChangeTypeOfKeys<{ a: number, b: boolean }, number> = {
    a: 'ok',
    b: 'ok',
}

TS playground link

Not sure if there are any special properties with key that I'm missing here. TIA!

Sorry, do you want to change the property's key type or the property's value type? — Connor Low, Sep 14 '21 at 23:48
It was the property's value type, but you figured it out :) Thanks a million! — tic, Sep 15 '21 at 00:15

Connor Low · Accepted Answer · 2021-09-15T05:05:54.210

To get the value of property referenced by key, do this:

export type ChangeTypeOfKeys<T extends object> = {
  [key in keyof T]: T[key] extends number ? string : T[key]
  //                ^^^^^^
}

Playground Link

The type of key will always be string in the example you gave. To be absolutely clear, in your code:

key refers to the name of a property,
T[key] refers to the property named key on type T.

_{Also note, extends object allows for [] types (among others), which might not be what you want. Try extending from {} or Record<string, any> instead.}

TypeScript Generic to change all types of a particular type to another

1 Answers1