Using a JS object to configure a class property type in TypeScript

Question

Let's say I have an object of factories like this one:

const factories = { foo: () => 'some string', bar: () => 123, }; 

I would like to use this object to type a class property. The goal for this class is to have a property with the same keys than above, but with the return type of the factories instead of the factories themselves:

class MyClass { myItems: ???; // What type here to have the below working? } const instance = new MyClass(); instance.myItems.foo; // Should be a string, not a function instance.myItems.bar; // Should be a number, not a function 

In short, I don't want that:

class MyClass { myItems: typeof factories; } 

But something like the below:

class MyClass { myItems: Record<keyof typeof factories, ReturnType<typeof factories>>; } 

But of course, the above does not work, with this error:

TS2344: Type '{ foo: () => string; bar: () => number; }' does not satisfy the constraint '(...args: any) => any'.   Type '{ foo: () => string; bar: () => number; }' provides no match for the signature '(...args: any): any'.

Do you have any help with that?

Thanks in advance!

Answer 1

Here's a way you could do it:

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZgQ2FEAnAlgUwjAvDAbwCgZ4QQAuGACgEo8A+GAcghAFtMZoMwBzZgBoSMAEYJUVOoxgBGAEwBmYQF8A3ESJQAngAcuAMSQpU2gDwAVJvmm4mFjVr2HjabQDUEAGwCu2S9aEIgDaANIw6GAwANaY2iBwMBYAulQWYckwmAAeUJhgACY4RshuZpFwmKgwAKpMAPy1MFRgmABuVRrqmqCQsG3efhBUJSYeg-46+gnwrhjYgcSkcORUzHnQQiLiknJKqhpAA

const factories = { foo: () => 'some string', bar: () => 123, }; type Factory<T> = () => T; type FactoryValues<T> = { [K in keyof T]: T[K] extends Factory<infer U> ? U : never; }; const values: FactoryValues<typeof factories> = { foo: 'test', bar: 123, };