Can RxJS 5 and a newer release coexist in one bundle during a zero-downtime migration?

Short Answer

Yes, RxJS 5 and a newer release can coexist in a single bundle during a zero-downtime migration, but only if the bundler keeps them as distinct packages. Without explicit configuration, bundlers will deduplicate them, causing runtime failures because RxJS 6+ operators cannot safely consume RxJS 5 Observable instances.

Confirmed Facts

• RxJS 6 introduced pipeable operators and a changed Observable prototype. RxJS 5 Observables lack Symbol.observable and the pipe method expected by modern operators.
• Legacy rxjs/add/... patch imports mutate the Observable prototype globally. If both versions share a bundle, these patches can corrupt the newer version’s behavior.
• Webpack 4 and earlier deduplicate dependencies by default. To preserve two copies, you must alias one version via package.json.

Required Steps

1. Alias the legacy version in package.json to force separate resolution:

   "dependencies": {
     "rxjs": "^7.8.0",
     "rxjs5": "npm:rxjs@5.7.2"
   }
   

2. Import explicitly in code that must run on the old version:

   import { Observable } from 'rxjs5';
   import 'rxjs5/add/operator/map';
   

3. Adapt at boundaries when data flows from RxJS 5 to RxJS 6:

   import { from } from 'rxjs';
   const adapted$ = from(rxjs5Observable); // wraps in RxJS 6 Observable
   

4. Verify bundle separation with webpack-bundle-analyzer or by searching the minified output for two distinct rxjs substrings.

Verification Checklist

• Both versions appear as separate directories in node_modules.
• Bundle analyzer shows two RxJS entries; minified code contains both rxjs5 and rxjs6 markers.
• No rxjs/add imports in code targeting the new version.
• Integration tests confirm RxJS 5 streams work in isolation and RxJS 6 streams work in isolation, with adapters at the boundary.

Missing Diagnostic Detail

If your bundler is not Webpack (e.g., Rollup, Parcel), the aliasing syntax differs. Confirm which bundler you are using so the aliasing configuration can be adjusted accordingly.