Mocha helpers for testing big


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210Jul 14, 2020Apr 14, 2018Minified + gzip package size for @bigtest/mocha in KB


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Convergent Mocha functions for testing against asynchronous states


import { describe, beforeEach, it } from '@bigtest/mocha';
import { expect } from 'chai';

describe('clicking my button', () => {
  beforeEach(() => $button.click());

  // repeatedly asserts until passing
  it('shows a loading indicator', () => {

  // repeatedly asserts it is passing until the timeout
  it.always('does not navigate away', () => {

Convergent Assertions

Typically, when testing asynchronous states (such as rendered content in an application) your tests need to run at just the right moment so that they are executed in the correct context. If your tests run too soon, they will fail; if they run too slow, well then you just have slow tests.

Converging on a state means asserting against a state until the assertion passes, in which case you have successfully converged on that state!

This package uses @bigtest/convergence to repeatedly run assertions until they pass, or until the timeout has expired. Performing tests in this way allows them to pass the moment the desired state is achieved. This results in very fast tests when testing asynchronous things.

Read the @bigtest/convergence docs on convergences for more info as to why converging on a desired state is better than trying to time it properly.

How does it work?

For the most part, you write tests in the exact same way that you're used to writing tests with Mocha. The only difference is that this package wraps Mocha's it in a convergence helper so that any assertions you write using it become convergent assertions that allow you to easily test asynchronous states.

This package also wraps the Mocha hooks before, after, beforeEach, and afterEach to support automatically timing and running returned Convergence instances from @bigtest/convergence.

Writing Tests

Because convergent assertions are run repeatedly until they pass, it is highly recommended that you do not perform any side-effects in your assertions. This will result in your side-effect being run multiple, perhaps even hundreds of times.

For this reason, you should keep your side-effect producing code in hooks and out of your assertions. These "pure assertions" also help your tests be more readable and explicit.

:no_entry_sign: do not do this:

describe('my button', () => {
  it('shows a loading indicator after clicking', () => {
    // this will be called every time the assertion runs

    // it might take a few milliseconds for any side-effects to
    // happen, so this might fail the first time and cause this entire
    // assertion to run again, thus clicking the button again

:white_check_mark: do this:

describe('clicking my button', () => {
  // keep side-effects inside hooks
  beforeEach(() => $button.click());

  // a pure assertion has no side-effects; even if it fails, it can
  // be run again and again without consequence
  it('shows a loading indicator', () => {

Asserting that something has not happened

Another common scenario is asserting that something has not happened. If you were to test for this normally (or even with a convergent assertion above) the test could potentially pass successfully before a side-effect has time to even happen.

In these scenarios, you want to converge when the state meets an expectation for a given period of time. In other words, "if this assertion remains true for X amount of time, this test is considered to be passing."

@bigtest/mocha provides an it.always method to do just this. This method will run the assertion throughout the entire timeout period ensuring it never fails. When the assertion does fail, the test fails. If the assertion never fails, it will pass just after the timeout period.

describe('clicking my button', () => {
  beforeEach(() => $button.click());

  // the default timeout for it.always is 100ms
  it.always('does not navigate away for at least 1 second', () => {

Convergent Hooks

Sometimes you may attempt to perform an async task to find it fails due to a preconceived state not being met. For example, you can't click a button if it doesn't exist in the DOM. You may use @bigtest/convergence to converge on these states and return convergences inside of your hooks. The hooks provided by @bigtest/mocha will automatically set the timeout and run returned Convergence instances.

describe('clicking my button', () => {
  // @bigtest/mocha will wait for a returned Convergence to converge
  // before continuing with the assertions
  beforeEach(() => new Convergence()
    .once(() => expect($button).to.exist)
    .do(() => $button.click()));

  it('shows a loading indicator', () => {

Check out the @bigtest/convergence API docs for working with the Convergence class.

Pausing Tests

Pausing tests can be very useful when debugging. It allows you to investigate your application during a critical moment in your testing suite. Mocha does not have a convinient way to pause tests, but @bigtest/mocha helps alleviate this by providing an it.pause method. It works by setting the current timeout to 0 and gives Mocha a promise that never resolves. This effectively pauses the entire suite until you remove it.pause and restart your tests.

describe('clicking my button', () => {
  beforeEach(() => $button.click());

  // if the class is never set, this test will fail; it.pause allows
  // us to investigate the app at this point in the test suite
  it.pause('shows a loading indicator', () => {

If you find any bugs or have a feature request, please open an issue on github!

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