Problem with generic types in Typescript

Previously, my code was functioning properly with 

"typescript": "3.8.3"
. However, after upgrading to Angular 10 along with 
"typescript": "4.0.8"
, I started encountering TypeScript errors:

handleEventResponse<T extends any>(event: CustomEvent, { resolve, reject }): T {
    let response: T = {} as T;
    if (Array.isArray(event.data)) {
      response = event.data as any;
      resolve(response);
    } else {
      Object.assign(response, event.data);
      if (response.status === 'ok') {
        resolve(response);
      } else {
        reject(response.error);
      }
    }

    return response;
}

The compiler is showing the following errors :

Compiling TypeScript sources through ngc
ERROR: src/lib/services/custom-events.service.ts:3

362       if (response.status === 'ok') {
                       ~~~~~~
src/lib/services/custom-events.service.ts:365:25 -

365         reject(response.error);
                            ~~~~~

I would greatly appreciate any suggestions on how to rectify these issues.

typescripttypestypescript-genericsangular10

Answer №1

Issue

The method in question is using a generic type that is constrained to any:

handleEventData<T extends any>(...) { ... }

A change introduced in TypeScript version 3.9 affects this behavior:

Type Parameters Constrained with any are no longer treated as any

In earlier TypeScript versions, treating a type parameter limited to any as any was allowed.

function exampleFunc<T extends any>(arg: T) {
  arg.randomProperty; // no error!
}

This oversight has been rectified in TypeScript 3.9, now generating errors for such operations.

function foo<T extends any>(arg: T) {
 arg.unknownProperty;
  //  ~~~~~~~~~~~~~~~
  // Property 'unknownProperty' does not exist on type 'T'.
}

As a result, type T is now considered as unknown. Operating on a value typed as unknown lacks type safety. In your code, the existence of status or error cannot be guaranteed.

Resolution

To address this, you must indicate to TypeScript that status and error could potentially exist within type T. Here's an illustration:

interface ResponseData {
  status: <your type>;
  error: <your type>;
}

handleEventData<T extends ResponseData>(...) { ... }

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