Tag Archives: react

Testing React components with Jest

Jest makes a bold claim to be “Painless Javascript Unit Testing”. I’m looking forward to using it, but it hasn’t quite lived up to that claim for me yet. Hopefully this post will help it do so for you.

If you just want a finished example, check out the tests in react-dropdown-input.

Choose the right version of node and jest

On my Mac, the latest version of jest (0.4.0) doesn’t run with the latest version of node (0.12.0). With node 0.10.37, it only works intermittently – sometimes saying:

dyld: lazy symbol binding failed: Symbol not found: _node_module_register
Expected in: dynamic lookup

and sometimes saying:

Error: Worker process exited before responding! exit code: null, exit signal: SIGSEGV (or SIGTRAP)
A worker process has quit unexpectedly! This is bad news, shutting down now!

The solution is to use jest version 0.2.1 with node 0.10.x. Hopefully future versions of jest will overcome this.

Note that the latest version of Babel seems to require node 0.12.x.

jest.dontMock is not recursive

I am testing a component called DropdownInput, built on top of React-Bootstrap, eg for DropdownInput.js:

    var ReactBootstrap = require('react-bootstrap');
    var Input = ReactBootstrap.Input;
    var DropdownMenu = ReactBootstrap.DropdownMenu;
    // in render function: 
      return (
          <Input .... />

I wrote a test like this (in __tests__.DropdownInput-test.js):

    var React = require('react/addons');
    var DropdownInput = require('../DropdownInput');
    var TestUtils = React.addons.TestUtils;
    describe('DropdownInput', function() {
      it('contains the test class', function() {
        var elt = (<DropdownInput menuClassName='test'/>);
        var renderedItem = TestUtils.renderIntoDocument(elt);
                                renderedItem, 'test');

This could not find any DOM elements with the class test:

Error: Did not find exactly one match (found: 0) for class:test

The reason is that while jest.dontMock('../DropdownInput') uses the real DropdownInput, it does not use the real ReactBootstrap components. Which is fair enough. So you need to add:


But that’s not enough! The reason is that ReactBootstrap contains the line require('classnames'), which help it put class names on its components; but jest is still mocking classnames. So you need to also add:


Happily, that does the job. It feels fragile though, because I don’t care what packages ReactBootstrap uses, as long as it works. If the author of ReactBootstrap changes which packages it uses, I don’t want all my tests breaking! Is there a better way?

There is a discussion of the pros and cons of jest’s approach to mocking here.

Shallow and deep rendering

There’s a good argument that your component tests should not try to test the subcomponent behavior anyway. In that case, you might be happy with a ‘shallow’ test like this:

    // __tests__/DropdownInput-shallow-test.js
    // Check it has the right components

    var React = require('react/addons');
    var DropdownInput = require('../DropdownInput');
    var ReactBootstrap = require('react-bootstrap');
    var TestUtils = React.addons.TestUtils;
    describe('DropdownInput', function() {
      var menuClassName = 'test';
      var elt = (<DropdownInput menuClassName={menuClassName}/>);
      var result;

      beforeEach(function() {
        var shallowRenderer = TestUtils.createRenderer();
        result = shallowRenderer.getRenderOutput();

      it('contains the right components', function() {
        // for now, assume in that order - TODO: generalize
        var child0 = result.props.children[0];
        var child1 = result.props.children[1];
        // and check the menu is passed the right class name

      it('contains the right first menu item', function() {
        // again, not a very general approach
        var child1 = result.props.children[1];
        var child1props = child1.props.children[0];


And this works nicely. However, as it is written, it assumes a lot about the structure of the component, like what order the elements are in. If I decide to wrap my component in an extra div for some reason, it will break my tests. Is there an equivalent to TestUtils.findRenderedDOMComponentWithType that we can use for shallow rendering, so I don’t have to assume ordering?

But you could also argue the opposite case: that the tests shouldn’t care how DropdownInput produces the desired behavior, only that it does – ie. it should test the behavior, not which components it uses. For this you need a ‘deep’ test, with lots of the mocking turned off. Eg:

    // __tests__/DropdownInput-deep-test.js
    // Check it has the right behavior
    var React = require('react/addons');
    var DropdownInput = require('../DropdownInput');
    var ReactBootstrap = require('react-bootstrap');
    var TestUtils = React.addons.TestUtils;

    describe('DropdownInput', function() {
      var menuClassName = 'test';
      var elt = (<DropdownInput menuClassName={menuClassName}/>);
      var renderedItem;

      beforeEach(function() {
        renderedItem = TestUtils.renderIntoDocument(elt);

      it('has a working input', function() {
        var txt = 'a';
        var input = TestUtils.findRenderedDOMComponentWithTag(
                        renderedItem, 'input').getDOMNode();
        //TestUtils.Simulate.keyDown(input, {key: 'a'});  // this doesn't work
        TestUtils.Simulate.change(input,  {target: {value: txt}});


And this works nicely too, except that the test needs to know which packages to unmock. (And why doesn’t Simulate.keyDown work?)

So, at this early stage in my jest career, I plan to include both sorts of tests for each component, in separate files.

Flux components

If you’re using the Flux architecture, then user interaction will often fire an action, rather than directly changing the DOM. So instead of testing the DOM directly, we need to test that an action was sent. To do this, I adapted the tutorial on Testing Flux Applications (which describes testing stores), as follows:

    var element,

    describe('MyComponent', function() {

        beforeEach(function() {
          // by requiring a new copy for every test, 
          // we reset the mock call counters for each test
          MyComponent = require('../my-component.js');
          ActionCreators = require('../../../actions/action-creators');
          // Render it
          element = <MyComponent />;  // include relevant props
          cpt = TestUtils.renderIntoDocument(element);
          rendered = TestUtils.findRenderedDOMComponentWithTag(cpt, 'div');
          // note - this version doesn't work - why??
          // rendered = TestUtils.findRenderedComponentWithType(cpt, MyComponent);

        it('clicking on it sends the right action', function() {
          // first check that no action calls were made yet
          // ActionCreators.myAction is the desired action
          var actionCalls = ActionCreators.myAction.mock.calls;
          // Simulate a click and verify that it sends out the action
          // check it sent the right params (replace ... with args)

(You won’t find this in the DropdownInput component tests, since it doesn’t fire any actions.)


I hope this saves you some time and development pain. Please let me know if you have better solutions to any of these issues!

The working DropdownInput component and tests are available at github.com/RacingTadpole/react-dropdown-input; you can see also a demo here.

Flux & React Best Practices

So, you want to start a new javascript project using React, perhaps using the Flux architecture. Excellent! But you’re not quite sure where to begin. Well, actually, you know where to begin, but you’ve quickly hit a barrage of design questions. Should that API call go in the store, the view component, or the action creator? Should I store that data in a state or pass it down from a parent component through props? How should I update the view before I receive that ajax data? And so on.

Here are some helpful tips which can guide you along the way. I am no authority on React, so I have included references to those who are. Please leave a comment if you think there are better ways.

Components & State

Try to keep as many of your components as possible stateless.

A common pattern is to create several stateless components that just render data, and have a stateful component above them in the hierarchy that passes its state to its children via props.

State should contain data that a component’s event handlers may change to trigger a UI update.


Ajax APIs

In Flux, all ajax API calls should come from the action creators.

paraphrased from http://facebook.github.io/flux/docs/chat.html

In response, the API fires actions on success (eg. “receive data”) or failure (eg. “receive error”).

For example, ViewActionCreators.js:

// ViewActionCreators.js
var AppAPI = require('../apis/AppAPI');
var ViewActionCreators = {
  doSomething: function(something) {
      actionType: AppConstants.DO_SOMETHING,
      something: something
    // fire off API call


// AppAPI.js
// this example uses jQuery's ajax call
var ServerActionCreators = require('../actions/ServerActionCreators');
function post(url) {
  $.ajax({url: url
    success: function(data) {
    error: function(xhr, status, err) {
      ServerActionCreators.receiveAPIError({ err: err });


// ServerActionCreators.js
var ServerActionCreators = {
  receiveData: function(data) {
      actionType: AppConstants.RECEIVE_DATA,
      data: data
  receiveAPIError: function(err) {
      actionType: AppConstants.RECEIVE_API_ERROR,
      err: err

In this code, the user interacts with the view, which fires the DO_SOMETHING action. That action itself fires off the API call, and when the API call is done it sends either a RECEIVE_DATA or a RECEIVE_API_ERROR action, which the Store can listen for. (As we’ll see below, the Store may listen for DO_SOMETHING too.)

These snippets are based on the Flux demo Chat app.

It seems there is one exception to the rule that only Action Creators call the API: loading initial data. This can go directly in the view controller. See https://facebook.github.io/react/tips/initial-ajax.html for an example.


Stores should remain as independent and decoupled as possible — a self-contained universe that one can query from a Controller-View.
The only road into the Store is through the callback it registers with the Dispatcher. The only road out is through getter functions.
Stores also publish an event when their state has changed, so Controller-Views can know when to query for the new state, using the getters.


Stores should only execute synchronous code. Otherwise they are too hard to understand.



If you’re using Flux, you should start writing your stores using immutable-js.

If we also use immutable-js data structures to hold the components’ state, we could mix PureRenderMixin into all our components and short circuit the re-rendering process.


Here is how I’ve done it. In the Store, make changes like this:

var Immutable = require('immutable');
var selected = Immutable.List();  // was: var selected = [];
// ...
selected = selected.push(item);  // was: selected.push(item);
// ...
selected = selected.delete(i);   // was: selected.splice(i, 1);
// ...
// I receive a js array of js objects from the API
// and convert to immutable list of immutable maps: 
  var tmp = myArray.map(function(item) {
    return Immutable.Map(item);
  return Immutable.List(tmp);
// ...

In the Component, there are not many changes required – I don’t even require('immutable') in this example. The changes arise just because the objects returned by the Store are no longer plain old javascript objects, but immutable ones.

var React = require('react/addons');
var PureRenderMixin = React.addons.PureRenderMixin;

// no change needed state-setting function if it's like:
function getMyState() {
  return {
    a: MyStore.getSelected(),
    b: MyStore.getOther()

  mixins: [PureRenderMixin],      // add this
  // no change to these, eg.
    return getMyState();
  _onChange: function() {  // ChangeListener callback
  // ...
var num = this.state.a.count();  // was this.state.a.length 
// maybe other small changes due to API differences

Here is an example of the ToDo app rewritten with Flux and immutablejs.

The React documentation also refers to another immutability helper, update(). This uses ordinary Javascript objects, whereas immutablejs uses its own object types. I have decided to use immutablejs, but please tell me if you have a way to do it with update.

Optimistic Updates

While waiting for an ajax API to return, you often want to optimistically update the UI. I’ve settled on the following pattern to do this:

  • Optimistically update the Store. Do not bypass the store by only saving it in the component state.
  • This means you will update the same Store data twice: once based on the user action, and then again based on the received API action (or API error action).
  • If you need to know if the API action has completed, you can set another variable (eg. optimistic) to track that, along with a getter function that the component can listen for. I’m using this to display a spinner while some data loads.

I tried doing each of these on its own too, but then you either can’t easily update the view optimistically, or you get a messy chain of actions.

The potential downsides are:

  • There could be other reasons for the API actions being received
  • I’m not sure I want this store to know about the API-generated actions (though the alternative still needs the API error action)
  • What if an API call is not required, and the data comes from somewhere else?

Reacting to non-React events

Attach generic DOM events (eg. window resizing) to componentDidMount. This is well explained at http://facebook.github.io/react/tips/dom-event-listeners.html

In the window resizing example, note they do not attempt to keep the current window size in a Store. Instead, it is kept in the component’s state. It would be complicated to keep a Store synched up with window.innerWidth, which is anyway the ultimate truth here.

Testing components

It took me a while to get Jest tests working, due to a combination of version conflicts, trouble understanding the nature of auto-mocking, and then deciding which things to mock. I’ve written up a separate post on unit testing components.

Other resources

Here are some resources (well, one anyway) that helped me get going: