TypeError: Object.method is not a function when using jest-CodePudding

I am trying to put together a basic implementation of an observable class in a js file

export class Observable {
constructor(emitter){
    return this._emitter = emitter
}

foo(){
    return'bar'
}

subscribe(next, error = {}, complete = {}){
    if (typeof next === "function") {
        return this._emitter({
            next, 
            error: error,
            complete: complete
        })
    }
    else {
        return this._emitter(next)
    }
}
}

with a corresponding jest file

import { Observable } from "./observable";

describe("Observable", () => {
    test('foo', () => {
        let fakeAsyncData$ = new Observable();
        expect(fakeAsyncData$.foo()).toBe('bar')
    })

    test('Observable subscriptions next returns value in pipe', () => {
        let test_value = "testing"

        let fakeAsyncData$ = new Observable(observer => {
            observer.next(test_value);
        });

        fakeAsyncData$.subscribe({
            next(val) { expect(val).toBe(test_value) }
        });
    });
});

For some reason the second test is failing due to subscribe not being a function. Which seems to be incorrect. Here is the entire jest output

 FAIL  ./observable.test.js
  Observable
    ✓ foo (21 ms)
    ✕ Observable subscriptions next returns value in pipe (2 ms)

  ● Observable › Observable subscriptions next returns value in pipe

    TypeError: fakeAsyncData$.subscribe is not a function

      14 |         });
      15 |
    > 16 |         fakeAsyncData$.subscribe({
         |                        ^
      17 |             next(val) { expect(val).toBe(test_value) }
      18 |         });
      19 |     });

      at Object.subscribe (observable.test.js:16:24)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 passed, 2 total
Snapshots:   0 total
Time:        0.592 s, estimated 1 s
Ran all test suites.
npm ERR! Test failed.  See above for more details.

What have I managed to screw up here?

CodePudding user response：

Remove the return keyword from the constructor. You are returning the result of the assignment this._emitter = emitter which is a function, as calling console.log(typeof fakeAsyncData$) will tell you.

Objects instantiated via a constructor do not need to explicitly return that instance. Doing so with any non-primitive value will return that value instead. Primitive return values are ignored - see MDN.