Jest mocking rxjs' lastValueFrom

Question

I have a controller which used rxjs' lastValueFrom method, and I want to mock the method so that the actual method won't be called. My code for the controller is:

async doSomething(request) {
  ...
  const data = await lastValueFrom(
    ...
  );
  ...
}

For the test, I've tried multiple ways, which is:

import rxjs from 'rxjs';

jest.spyOn(rxjs, 'lastValueFrom').mockReturnValueOnce(
  new Promise((resolve, reject) => {
    resolve(true);
  }),
);
// This gives me error: Cannot spyOn on a primitive value; undefined given

import { lastValueFrom } from 'rxjs';

lastValueFrom = jest.fn().mockReturnValueOnce(
  new Promise((resolve, reject) => {
    resolve(true),
);
// This gives me error: Cannot assign to 'lastValueFrom' because it is an import.

import rxjs from 'rxjs';

rxjs.lastValueFrom = jest.fn().mockReturnValue(
  new Promise((resolve, reject) => {
    resolve(true);
  }),
);
// This gives me error: Cannot set property 'lastValueFrom' of undefined

Answer 1

If you're using TypeScript I'm afraid the only way is mocking the import 'rxjs'.

jest.mock('rxjs', () => {
  const original = jest.requireActual('rxjs');

  return {
    ...original,
    lastValueFrom: () =>
      new Promise((resolve, reject) => {
        resolve(true);
      }),
  };
});

import { of, lastValueFrom } from 'rxjs';

test('test', () => {
  lastValueFrom(of(1)).then((val) => {
    expect(val).toStrictEqual(true);
  });
});

Live demo: https://stackblitz.com/edit/node-6vjbxb?file=index.spec.ts

With just ES* seems like you could do: How to spy on a default exported function with Jest?