RxReact/Jest Helpers

Development Sponsored By:

RxReact/Jest Helpers are simple tools for testing RxJS observables. This can be useful any project, not just React.

Installation

In your project:

npm install @rxreact/jest-helpers --save

yarn add @rxreact/jest-helpers

RxJS and Jest are peer dependencies and need to be installed separately.

Then import the library:

import { watchSignal } from "@rxreact/jest-helpers";

Basic Usage

This library adds a function for making an observable hot, and adds two matchers to Jest to checking if an observable has emitted a value.

If you have used expect(jest.fn()).toHaveBeenCalled() and expect(jest.fn()).toHaveBeenCalledWith(value), these helpers should feel familiar.

Checking if a observable emitted

To check if an observable emitted any value:

await expect(of("a")).toEmit();

Checking if a observable emitted a specific value

await expect(of("a")).toEmitValue("a");

Checking that a observable did not emit anything

await expect(never()).not.toEmit();

Checking that a observable did not emit a specific value

await expect(of("a")).not.toEmitValue("b");

Watching a cold observable

Often observables are cold - they will forget any values they might have emitted before something subscribes to them. To get around this, you can use watchSignal to get a hot version of the observable with shareReplay, which will record everything the observable emits.

watchSignal cleans up after itself after every test to prevent memory leaks. So, make sure to only call watchSignal in a beforeEach or a test body.

import { Subject } from "rxjs";
import { watchSignal } from "./jest-helpers";

it("watches a cold signal", async () => {
  const subject$ = new Subject<string>();
  const hot$ = watchSignal(subject$);

  // Hasn't emitted yet
  await expect(hot$).not.toEmit();

  // Make subject$ emit a value.
  subject$.next("a");

  // A cold subject loses all values.
  await expect(subject$).not.toEmitValue("a");

  // A hot watched signal remembers values.
  await expect(hot$).toEmitValue("a");
});

Loading the library

To use the expect(observable$).toEmit() and expect(observable$).toEmitValue(value) functions in your test, you must import the library in the test file.

If you need watchSignal (and you probably will), simply importing it will do the job:

import { watchSignal } from "@rxreact/jest-helpers";

If you have a test that only tests hot signals, you will still need to import the library to you test file to get the jest extensions:

import "@rxreact/jest-helpers";

Awaiting Expectations

.toEmit() / toEitValue() work by waiting 100 ms for the observable to emit, to account for asynchronous observables. So you must await every expectation, to make sure you catch failures:

await expect(o$).toEmit();