esbuild is unable to tree-shake rxjs under node platform

Discussed in https://github.com/ReactiveX/rxjs/discussions/6785

^{Originally posted by Conaclos January 28, 2022}
Hi!

I discovered that esbuild is not able to tree-shake rxjs package when bundling under node platform.
esbuild priorizes exports condition 'node' over main-fields.

Demonstration

$ cat index.js
export { Subject } from 'rxjs'

$ esbuild --bundle index.js --platform=node | wc -l
9654

$ # Specifying main-fields does not change the output
$ esbuild --bundle index.js --platform=node --main-fields=es2015,module,main | wc -l
9654

$ # browser bundle is lighter
$ esbuild --bundle index.js --platform=browser | wc -l
1161

Note that we need to bundle for node since our project and most of our dependencies imports node-specific modules.

How to solve the issue?

rxjs should not use the condition "node".
It should use the condition "require":

  ...
  "exports": {
    ".": {
      "require": "./dist/cjs/index.js",
      "es2015": ...,
      "default": ...
    },
    ....

instead of:

  ...
  "exports": {
    ".": {
      "node": "./dist/cjs/index.js",
      "es2015": ...,
      "default": ...
    },
    ....
```</div>

Start by locating the package exports configuration for rxjs and compare the node and require conditions shown in the issue. Reproduce the index.js esbuild commands for node and browser platforms, then verify that the node bundle tree-shakes the Subject import to the expected smaller output.