Extension method Observable<T> in Angular6 typescript is a powerful feature that allows

Question

Extension method Observable<T> in Angular6 typescript is a powerful feature that allows

I've been attempting to create an extension method for Observable

import { Observable } from 'rxjs/Observable';

declare module 'rxjs/Observable' {
  interface Observable<T> {
    customFilter<T>(this: Observable<T>): Observable<T>;
  }
}

function customFilter<T>(this: Observable<T>) {
  return this;
}

Observable.prototype.customFilter = customFilter;

However, I keep encountering a compilation error

ERROR in src/app/utils/observable.extensions.ts(13,1): error TS2693: 'Observable' only refers to a type, but is being used as a value here.

Here is my tsconfig.app.json configuration:

{
  "extends": "../tsconfig.json",
  "compilerOptions": {
    "outDir": "../out-tsc/app",
    "baseUrl": "./",
    "module": "es6",
    "types": [],
    "paths": {
      "@angular/*": [
        "../node_modules/@angular/*"
      ]
    }
  },
  "exclude": [
    "test.ts",
    "**/*.spec.ts"
  ]
}

Does anyone have any insights on how to resolve this issue?

typescript angular6 extension-methods

Answer 1

Answer №1

This particular situation is quite intricate. Within the rxjs/Observable module, you will find:

export * from 'rxjs-compat/Observable';

Your addition of an exported interface named Observable interferes with the export * statement's ability to re-export Observable from rxjs-compat/Observable. It almost mimics placing

export interface Observable<T> { ... }

directly within the rxjs/Observable module. (It might be beneficial if the compiler could signal this as an error occurrence. There was a previously noted issue, but unfortunately, it was closed without a clear explanation as to why the error detection was not feasible.) Consequently, the sole definition of Observable that gets exported from rxjs/Observable happens to be the interface established in your modification; its essence remains undefined.

To rectify this, consider modifying your augmentation to target rxjs/internal/Observable (where the original declaration of the Observable class exists) instead of rxjs/Observable. This adjustment guarantees that your interface aligns appropriately with the initial class.

Answer 2

This particular situation is quite intricate. Within the rxjs/Observable module, you will find:

export * from 'rxjs-compat/Observable';

Your addition of an exported interface named Observable interferes with the export * statement's ability to re-export Observable from rxjs-compat/Observable. It almost mimics placing

export interface Observable<T> { ... }

directly within the rxjs/Observable module. (It might be beneficial if the compiler could signal this as an error occurrence. There was a previously noted issue, but unfortunately, it was closed without a clear explanation as to why the error detection was not feasible.) Consequently, the sole definition of Observable that gets exported from rxjs/Observable happens to be the interface established in your modification; its essence remains undefined.

To rectify this, consider modifying your augmentation to target rxjs/internal/Observable (where the original declaration of the Observable class exists) instead of rxjs/Observable. This adjustment guarantees that your interface aligns appropriately with the initial class.

Extension method Observable<T> in Angular6 typescript is a powerful feature that allows

Answer №1

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