RxJS v6 testing breaking with ESBuild bundling (Jest testing)

[devversion]

Command

test

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

main

Description

Currently, using a builder that leverages ESBuild under the hood. e.g. the Jest builder- results in runtime breakages when RxJS v6 is used together with the TestScheduler code.

  it('should work', () => {
    new TestScheduler((a, b) => expect(a).toEqual(b)).run(({cold, expectObservable}) => {
      expectObservable(cold('-t', {t: 'hi'})).toBe('-t', {t: 'hi'});
    });
  });

Root cause:

RxJS v6 does not have exports conditions, so the mainFields will be respected. The ESBuild code path
does not match the ES2015+ main fields, but instead matches just module- causing the ES5 distribution
to be bundled. This ends up breaking at runtime. If the es2015 main field would have matched- no error
would have occurred.

This seems to happen because, the ES5 distribution applies mixins for the ColdObservable class using a @PURE side-effect call, while in ES2015 it's not marked as side-effect free.

ES5 (breaking)

ES2015

Minimal Reproduction

https://github.com/devversion/rxjs-v6-esbuild-cli-breaking

Exception or Error

● AppComponent › should work

    expect(received).toEqual(expected) // deep equality

    - Expected  - 5
    + Received  + 5

      Array [
        Object {
    -     "frame": 1,
    +     "frame": 0,
          "notification": Notification2 {
    -       "error": undefined,
    -       "hasValue": true,
    -       "kind": "N",
    -       "value": "hi",
    +       "error": [TypeError: observable.logSubscribedFrame is not a function],
    +       "hasValue": false,
    +       "kind": "E",
    +       "value": undefined,
          },
        },
      ]

      at TestScheduler2.TestScheduler.run.cold.cold [as assertDeepEqual] (src/app/app.component.spec.ts:35:43)
      at node_modules/rxjs/_esm5/internal/testing/TestScheduler.js:122:23
          at Array.filter (<anonymous>)
      at TestScheduler2.flush (node_modules/rxjs/_esm5/internal/testing/TestScheduler.js:120:43)
      at TestScheduler2.run (node_modules/rxjs/_esm5/internal/testing/TestScheduler.js:346:18)
      at src/app/app.component.spec.ts:35:55
      at _ZoneDelegate.invoke (node_modules/zone.js/fesm2015/zone.js:368:26)
      at ProxyZoneSpec.onInvoke (node_modules/zone.js/fesm2015/zone-testing.js:273:39)
      at _ZoneDelegate.invoke (node_modules/zone.js/fesm2015/zone.js:367:52)
      at _Zone.run (node_modules/zone.js/fesm2015/zone.js:129:43)
      at Object.wrappedFunc (node_modules/zone.js/fesm2015/zone-testing.js:740:30)

### Your Environment

```text
Angular CLI: 16.1.0
Node: 16.14.2
Package Manager: npm 8.19.2
OS: linux x64

Angular: 16.1.1
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1601.0
@angular-devkit/build-angular   16.1.0
@angular-devkit/core            16.1.0
@angular-devkit/schematics      16.1.0
@angular/cli                    16.1.0
@schematics/angular             16.1.0
rxjs                            6.6.7
typescript                      5.1.3

Anything else relevant?

Possible solutions:

• For RxJS v6, we should add es2015 as condition.
• We should consider not optimizing when bundling for tests?

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