Set generic type of a value on an object without specifying key type

Question

I have data structure like this:

const endpoints = {
  async Login: (params) => { ... }
  async Register: (params) => { ... }
}

Now I want to specify that every item in this object must accept params object and return a promise.

I can do something like this:

interface EndpointMap {
  [endpointName: string]: (
    params: Record<string, any>
  ) => Promise<any>;
}

This works well. But there's a downside to this.

If I do elsewhere that for example endpoint : keyof typeof endpoints the result would only be string. If I remove the EndpointMap interface, I'd get a String Union of all the keys on the endpoint object. Much better!

Is there a way to have the best of both worlds?

Thanks!

Answer 1

You can achieve this by creating endpoints using identity function:

interface EndpointMap {
  [endpointName: string]: (
    params: Record<string, any>
  ) => Promise<any>;
}

const createEndpoints = <TMap extends EndpointMap>(map: TMap) => map;

const endpoints = createEndpoints({
  login: async (params) => ({}),
  register: async (params) => ({})
});

/*
  type of 'endpoints' variable is: 
  {
      login: (params: Record<string, any>) => Promise<{}>;
      register: (params: Record<string, any>) => Promise<{}>;
  }
*/

We've declared identity function with EndpointMap constraint on generic type parameter so typescript will verify that passed parameter has appropriate structure. Additionally parameter type will be inferred and valid keys' type won't be widened to string, so:

type Keys = keyof typeof endpoints; // will be "login" | "register"

Playground

Answer 2

One way to achieve this is instead of giving keys as type string define the keys in the type EndpointMap:

type EndpointMap = {
    [key in 'Login' | 'Register']: (
        params: any
    ) => Promise<any>
}

const endpoints: EndpointMap = {
    Login: (params) => { ... },
    Register: (params) => {...  }
}

// Valid
const oneKey: keyof typeof endpoints = 'Login'

// Type '"Random"' is not assignable to type '"Login" | "Register"'
const otherKey: keyof typeof endpoints = 'Random'