How to define a type for a template in TypeScript?

Question

I'm going to define a type where I can store the response from various API that has similar responses.

The response looks like

type TResponse<E> = {
   Code: number;
   Message: string;
   Result: {
      [i: string]:  Array<E>;
      Count: number;
   };
   Status: string;
}

[i: string] can be anything except for Count depending on the API design, for example, Rows , Values , Logs , etc...
This for now gives me an error, because of the Count property conflicts with [i: string] .

PS I'm using the typescript version ^3.9.7

Answer 1

You can use an intersection type:

type TResponse<E> = {
   Code: number;
   Message: string;
   Result: {
      [i: string]: Array<E>;
    } & {
      Count: number;
   };
   Status: string;
}

declare const response: TResponse<string>;
response.Result.Count; // number
response.Result.abc; // string[]

Note that this is slightly unsafe, which is why TS requires that all properties are assignable to its index signature:

declare const key: string;
response.Result[key]; // string[] even though key could be Count

Also, trying to create objects like this requires a type assertion:

const response2: TResponse<string> = {
   Code: 0,
   Message: '',
   // Type '{ Count: number; }' is not assignable to type ...
   Result: {
      Count: 1
   },
   Status: ''
}

const response3 = {
   Code: 0,
   Message: '',
   Result: {
      Count: 1
   },
   Status: ''
} as TResponse<string>

Playground link