是否可以在打字稿的对象中使用泛型？

Question

I'm trying to set the type of one object value based on the type of another value and I'm curious if this is possible. Given:

type KeyType = 'string' | 'number'
type ValueType = { string: string, number: number }

type TestObject<T extends KeyType> = {
  key: T
  args: ValueType[T]
}

const test1: TestObject = { key: 'string', value: 'hello' } // should be ok
const test2: TestObject = { key: 'number', value: 2 } // should be ok
const test3: TestObject = { key: 'string', value: 2 } // should fail, value should be string

is there a way to do this in typescript? I can only get this to work if I pass the generic to the type declaration. However I know in functions its possible for this to be inferred.

Answer 1

is it possible to use generics in an object in typescript?

Yes. Here is the working code (you were close):

export type KeyType = 'string' | 'number'
type ValueType = { string: string, number: number }

type TestObject<T extends KeyType> = {
  key: T
  value: ValueType[T]
}

const testA: TestObject<'string'> = { key: 'string', value: 'hello' } // should be ok
const testB: TestObject<'number'> = { key: 'number', value: 2 } // should be ok
const testC: TestObject<'string'> = { key: 'string', value: 2 } // Error: value needs to be string

Showing the error:

More

You need to specify the generic when using a generic as a type annotation eg TestObject<'number'>