Is it possible to build dynamic string type based on other custom string type in typescript?

I have prepared simple code to represent the issue. I have a type that accepts a bunch of string, let’s called it Names:

type Names = "field1" | "field2";

Now I need to build a type that will accept also a bunch of new strings based on the Names type.

type Names = "field1" | "field2";

type FieldsNames =
  | "field1_created"
  | "field1_updated"
  | "field1_deleted"
  | "field2_created"
  | "field2_updated"
  | "field2_deleted";

As you can see, for each string (field1, field2) I need to build a type that accepts another three types of string. This already can grow really fast.

Is there a way to for example, extract keys from Names type and based on that, create new custom types? If so how to do it with typescript?

>Solution :

You need to use distributive conditional types:

type Names = "field1" | "field2";

type Prefix = 'created' | 'updated' | 'deleted'

type FieldsNames =
    | "field1_created"
    | "field1_updated"
    | "field1_deleted"
    | "field2_created"
    | "field2_updated"
    | "field2_deleted";

type Builder<T extends string> = T extends any ? `${T}_${Prefix}` : never

// Result is the same as FieldsNames
type Result = Builder<Names>

type Assert<T, _ extends T> = T

type Test = Assert<Result, FieldsNames> // ok

Playground

T extends any – turns on distributivity