How to test redux-saga delay

A good way to test Redux Saga calls is by using the call effect. In this case, you can slightly refactor your saga as follows:

import { delay } from 'redux-saga';
import { call } from 'redux-saga/effects';

export function* incrementAsync() {
  yield call(delay, 1000);
}

You would then test this like so:

import { delay } from 'redux-saga';
import { call } from 'redux-saga/effects';

describe('incrementAsync', () => {
  it('should incrementAsync()', () => {
    const generator = incrementAsync();

    expect(generator.next().value).toEqual(call(delay, 1000));
  });
});

This works because the result of the yield to call is a simple object, describing a call to the delay function. No need for any mocks :)

There is also of course the great redux-saga-test-plan helper library. Using that, your test would become the following:

import { testSaga } from 'redux-saga-test-plan';
import { delay } from 'redux-saga';
import { call } from 'redux-saga/effects';

describe('incrementAsync', () => {
  it('should incrementAsync()', () => {
    testSaga(incrementAsync)
      .next()
      .call(delay, 1000)
      .next()
      .isDone();
  });
});

If you check delay saga effect code you can see that it is a bound function:

export const delay = call.bind(null, delayUtil)

so if you import delay in two different modules it will be two different functions that have no visual difference.

You can check this in the codesandbox example (see test tab):

const testFunction = () => {};

describe("example bound functions equality test", () => {
  it("Two bound functions are not equal", () => {
    expect(testFunction.bind(this))
      .not.toEqual(testFunction.bind(this));
  });
});

Result is:

To test your saga you should mock your delay effect (if you are using Jest);

import { delay } from "redux-saga";
import { incrementAsync } from "../sagas";

jest.mock("redux-saga");

describe("incrementAsync Saga test", () => {
  it("should incrementAsync", () => {
    const generator = incrementAsync();
    expect(generator.next().value).toEqual(delay(1000));
  });
});