Is it possible to make the type property depend on the value of name? I tired this, but it doesn't work:
type Attribute<T> = {
name: keyof T;
type: T[keyof T] extends string ? 'a' : 'b';
};
type Item = {
id: string;
date: number;
};
const att1: Attribute<Item> = {
name: 'id',
type: 'a', // this should work
};
const att2: Attribute<Item> = {
name: 'id',
type: 'b', // this should not work
};
const att3: Attribute<Item> = {
name: 'date',
type: 'b', // this should work
};
The problem is that T[keyof T] extends string is expanded to all types that T[keyof T] can assume, and at least one of them doesn't extend string, so the condition is always false.
One solution is to use a mapped type to produce an object type whose values are all the allowed types, then index that with keyof T again to get a union of the individual types out:
type Attribute<T> = {
[K in keyof T]: {
name: K,
type: T[K] extends string ? 'a' : 'b';
}
}[keyof T]
I'm not sure this is the simplest solution, but it does seem to work.
