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Introducing unintentional dependencies on rxjs-compat is something that I see developers doing every now and then. Let’s have a look at rxjs-compat to see what it is, how it works and how depending upon it can be avoided.

If you’re only interested in avoiding the dependency, skip to the TL;DR at the bottom of the article.

So what is it?

In RxJS version 6, breaking changes made the library simpler:

  • the prototype-patching operators were removed; and
  • the export locations were rearranged so that each export was available from only a single location.

Those changes made the library easier to maintain, document and explain, but they created a burden for developers with large RxJS-version-5 codebases: pretty much all RxJS-related code would need to be modified.

The rxjs-compat package was created so that developers could upgrade to RxJS version 6 without having to modify code. It re-implements the prototype-patching operators and makes available all of the RxJS-version-5 export locations.

How does it work?

The RxJS-version-6 distribution includes files for all of the version-5 export locations. However, the imports within those files redirect to rxjs-compat.

For example, let’s look at the of observable creator and the mapTo operator.

In RxJS version 5, you can choose to import everything:

import Rx from "rxjs/Rx";
const answer = Rx.Observable.of(6 * 9).mapTo(42);

Or you can choose to patch Observable with only the creators and operators you need:

import { Observable } from "rxjs/Observable";
import "rxjs/add/observable/of";
import "rxjs/add/operator/mapTo";
const answer = Observable.of(6 * 9).mapTo(42);

Or you can choose to directly import only the creators and operators you need:

import { of } from "rxjs/observable/of";
import { mapTo } from "rxjs/operator/mapTo";
const answer = * 9), 42);

With rxjs-compat installed, all of the above snippets will work with RxJS version 6 — despite their using version-5 export locations.

So how does this work? Well, the RxJS-version-6 package includes all of the files imported in the snippets:


But each of those files contains nothing apart from the import of an rxjs-compat file. For example, rxjs/Observable.js looks like this:

"use strict";
function __export(m) {
for (var p in m)
if (!exports.hasOwnProperty(p)) exports[p] = m[p];
Object.defineProperty(exports, "__esModule", { value: true });

And rxjs-compat/observable/of.js looks like this:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
var rxjs_1 = require("rxjs");
exports.of = rxjs_1.of;

So importing of from a version-5 export location, imports of from rxjs-compat which imports everything from rxjs.

How can it be avoided? — TL;DR

If an RxJS-version-5 export location us used — without having rxjs-compat installed — an error something like this is effected:

ERROR in ~/project/node_modules/rxjs/observable/of.js
Module not found: Error: Can't resolve 'rxjs-compat/observable/of'
in '~/project/node_modules/rxjs/observable'

Which is a little confusing — unless you know how rxjs-compat works — so it’s understandable that developers sometimes resort to installing rxjs-compat to resolve the error. It’s likely to continue to cause confusion, too, as there are so many snippets — on Stack Overflow and elsewhere — that use version-5 export locations.

The preferred way to resolve the error is to import only from RxJS-version-6 export locations. Fortunately, there are only a few:

import { /* classes, creators, schedulers */ } from "rxjs";
import { ajax } from "rxjs/ajax";
import { /* pipeable operators */ } from "rxjs/operators";
import { TestScheduler } from "rxjs/testing";
import { webSocket } from "rxjs/webSocket";

So, in RxJS version 6, the snippets above should be rewritten as:

import { of } from "rxjs";
import { mapTo } from "rxjs/operators";
const answer = of(6 * 9).pipe(mapTo(42));

To help avoid the problem, I’ve added a TSLint rule — rxjs-no-compat — to rxjs-tslint-rules that bans imports from RxJS-version-5 export locations.

This post is also published on my personal blog:

RxJS core team member; front-end developer; mentor; speaker; open-source contributor

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