Dave Bernhard

Web Developer

📚 Reading: The Riddle-Master of Hed by Patricia A. McKillip

Is writing tests hard? Or does your code suck?

Tips for testing React components with Mocha, Enzyme and Sinon


Spark|First lit on January 24
11 min read
Table of contentsTDD light, or Diet TDDEasy-to-test React componentsTips for using EnzymeMounting / Rendering a wrapperSimulating events and calling functions passed as propsEvents and casting typesEvents that call React hooksSpying, mocking and stubbing with SinonSpiesFakesStubsTreat tests as source code*fin*

Well ok then, you got me. Maybe that title is overly spicy.It was probably the capsicum that just tipped it. Lesson learned. I'm sure your code is gorgeous - intelligent and a real looker - and, to be honest, writing tests can be difficult, and it's not always our fault. I'm not a fan of blaming tools, but given the choice, I'd use Jest and React Testing Library over Mocha and Enzyme, even though I'm less familiar with both. That said, allow me to show you the voluptuously grassed hill that I'm prepared to die on:

Code that is easy to test is better than code that is difficult to test. If you're struggling to write a test, you should improve the source code, not your testing knowledge.

I'm not going to claim that this opinion is novel. I'm sure I've read something similar myself some time ago. But it's something I've arrived at through my own experience of writing tests, and so in that sense, it is original. And besides, I believe it to be worth reiterating. Not least because it's a tenet that I am frequently guilty of forgetting myself.

TDD light, or Diet TDD

I very rarely use test-driven development. When I do, it's usually because I need to write a complex function that covers some gnarly edge cases. And usually, I have a clear idea of input and output so that the test cases are the easiest place to start. I find, in these circumstances, that the greatest advantage of starting with tests is the clarity of thought I gain. Once the test cases are written, not only am I crystal clear on what I'm trying to acheive with my code, but I have them to hand as a constant reminder. It keeps me focused while I write the implementation.

So why aren't I recommending TDD? In part, because it's a TLA.Three Letter Acronym. Har-dee-har-har. And because words such as "ethos" and "philosophy" are erroneously attached to it. Once you start to recommend an approach, people get dogmatic and start wanting rules to follow, blindfolds to wear and teats to suckle. And I want better for you.There are also plenty of occasions when writing code is more exploratory, when some of the thinking is best done while immersed in a system's guts, rather than trying to reason about it all from the outside.

Instead of having all your test cases planned out beforehand, I think it's enough to try to write code that is easy to test. It just so happens that writing code that is easy test means writing code that is easy to reason about and nicely decoupled from dependencies. Because dependencies are easy to mock when you've thought about how they're integrations, and they're devils if not.

Easy-to-test React components

When it comes to React, testing becomes much easier when following these four guidelines:Yep; they're just guidelines. Don't go sticking to them against your better judgement

  1. Add ids to interactive elements to give yourself unambiguous selectors for targeting elements from tests. Or, if you want to be more explicit, use data attributes, such as data-testid.
  2. Write smaller, composable components that follow the single responsibility principle (where possible). The smaller your component, the less there is to test at once. It also makes it more apparent what cases there are to test, which means you're less likely to leave edge cases uncovered.
  3. If a function does not need to be in the body of a component, move it to the file/module level, or to a helper file if it's reusable. That way you can call and make assertions directly on that function without having to trigger an event that calls it.
  4. If you have the choice and it makes sense to do so in the source code, prefer defining functions in parent components and passing them to children via props over defining and calling functions in the same component. Any component prop is trivial to mock; when you mount the component for testing, you can replace that prop with whatever you like.

Tips for using Enzyme

Mounting / Rendering a wrapper

When you mount a component, use shallow() to test it in isolation, and mount() if you need access to nested components and elements. mount is a more expensive operation, so don't use it unnecessarily; you won't notice a difference at first, but I promise you will when you have to run hundreds or thousands of tests.

When debugging a test, it can be useful to see what Enzyme is actually rendering. Do this by adding console.log(wrapper.debug()) (where the mounted component tree has been assigned to wrapper) and running the test again. The rendered wrapper will be logged to your terminal as long as no errors are thrown before executing the log statement. And when shallow rendering, optionally pass debug({ verbose: true }) for more information on the rendered elements' props.

If you expect the elements in the wrapper to change as a result of an action - you simulate a click, a useEffect hook executes etc. - then call wrapper.update() before making an assertion. And if the action was asynchronous, you may have to await, or call this handy nextTick() utility:

const nextTick = () =>
  new Promise((resolve, reject) => process.nextTick(() => resolve(searchData)))

Simulating events and calling functions passed as props

Enzyme has its own method for simulating events. To simulate a button click, for instance, you can find the button element on the wrapper and pass 'click' to the simulate method:

const button = wrapper.find('#test-button');
button.simulate('click');

Be aware, however, that while this is usually ok for clicks, many events do not behave well with the simulate method. It's more reliable to, where possible, call an event directly via an element's props:

const input = wrapper.find('#test-input');
const onChange = input.prop('onChange');
onChange({
    event: {
        target: 'new input value'
    }
});

Since the prop() method can return undefined, TypeScript does not know if a value will be found on any prop, so you will have to guard against that case.

Events and casting types

These synthetic React events, when found and triggered directly, know to expect a certain event object. TypeScript will fail to compile until it is reassured that you have passed an appropriate event object. You can do this with type casting:

const onChange = input.prop('onChange');
const changeEventMock = {
    target: {
        value: 'testing'
    }
} as unknown as React.ChangeEvent<HTMLInputElement>;

if (onChange) {
    onChange(changeEventMock);
}
  • Be sure to also mock any event methods that are actually called in the source code, e.g. preventDefault() or stopPropagation().

  • Casting types is also useful for mocking class instances. Tests run faster if you mock what you need to test in, for example, a user object, rather than instantiating a new AppUser() with all of its methods and properties.

  • Be aware that type casting does not, however, make your mock an instance of that class. So if the source code uses the instanceof operator or needs the instance's prototype, you will in fact have to create an instance of the class.

  • as unknown is only necessary when your mock is a very different shape to the type you are casting your mock as.

Events that call React hooks

If you call an event that results in the firing of a React hook, you will need to wrap it in act from the 'react-test-renderer' package to ensure the hook takes effect, and then update the wrapper (if you expect a change in the element tree):

import { act } from 'react-test-renderer';

...

if (onChange) {
    act(() => onChange(onChangeMock));
}
wrapper.update();

Spying, mocking and stubbing with Sinon

Spies

To assert on a method, by expecting it to run a certain number of times or with certain arguments, without hijacking its execution, you can use a "spy". This is especially useful if you want to test that something is logged to the console, for instance:

import sinon from 'sinon';
...
const consoleSpy = sinon.spy(console, 'error');

// Execute code path that logs an error in the console
expect(consoleSpy.calledOnce).to.be.true;

Fakes

Fakes are mock functions that capture details about how they were called. They are like spies, only instead of calling the original function, the implementation is completely replaced. In fact, you can use a fake to return a value, resolve a value (if mocking an async function), and even thrown an error or reject a Promise.

Consider this example, which tests that a modal will close when the "done" button is clicked.

  • Note how we're not really checking that the modal is closed. We're checking that the function which closes the modal is called in the correct circumstances. This is because we are interested in testing our source code, not in testing that, for instance, React updates the DOM as it should.
it('should close the modal on pressing done button', () => {
    const handleClose = sinon.fake(); // A fake allows us to replace and assert on this function
    const wrapper = mount(
        <ConfirmationModal
            ...
            handleClose={handleClose}
        />
    );

    const doneButton = wrapper.find('#confirmation-modal-done-button').first();
    doneButton.simulate('click');
    wrapper.update();

    expect(handleClose.calledOnce).to.be.true;
});

Stubs

Stubs do the same job but for object (and class) methods. Make sure to call restore() at the end of the test, otherwise that same method will still be using the same stub in any tests that run after it in the suite.

Stubs are also great for replacing functions that were imported into the module under test from another module. Imagine, for instance, that we are testing a function that calls getErrors from the @/utils/validations.ts module. We can replace the getErrors helper method with our own, stubbed implementation:

import * as validationHelpers from '@/utils/validations';
...
const getErrorsStub = sinon.stub(validationHelpers, 'getErrors').returns([]);

// call function under test and make assertions
getErrorsStub.restore();

Note how we import all the exports from the module as validationHelpers. This is because we can only stub methods on an object. And since getErrors is a named exportI know it's imaginary in this case, but it's most likely that a validations file will make use of multiple named exports of the module, we need to first import all the methods an on object.

Treat tests as source code

Some final advice. Just as you strive to make your source code readable, terse and DRY, you should aim for the same in your tests. As requirements change and functionality is added, you or your colleagues will have to revisit tests, updating them to reflect your new expectations of the behaviour. You will be glad of any clarifying comments, any common mocks/helpers that have been abstracted etc.

NB: when you find yourself having to reuse the same mocked object for multiple tests you will want to lift it to some outer scope - of the describe block or the module - so that it is available to all test blocks. Be careful! Since JS objects are assigned by reference, every test will be using that object. If any one of them mutates the object, it will be mutated for every test that runs after it. Consider writing a function that returns that mocked object instead.

fin

Ok, that just about does it for today's edition of Uncurated Hot Tips. I hope to catch you next time when we'll be talking about the kinds of jar you'll need to be saving if you're to have a happy pickling season, and why looking underneath your sofa just might be the last thing you ever do. Go on, take a peek. I dare you.