Test Simulated Event Handlers with Enzyme

Tyler Clark
InstructorTyler Clark

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Published 5 years ago
Updated 3 years ago

Sometimes our components use event handlers to update state. In this lesson we’ll work with a component that uses an onClick and onChange handler and use Enzyme to test that the conditionally rendered text updates subsequently.

Instructor: [00:00] Sometimes, we'll have components that will use event methods, such as onClick, onChange, and onMouseOver. We can use Enzyme to not only simulate these events on our rendered components but can check that conditional rendered attributes work as expected.

[00:16] Let's start a new test. We'll say it('onButtonClick changes pText', (). Into this block, we'll do a const wrapper = shallow our app component, then const button = wrapper.find our button, expect that our wrapper.find('buttonStateClass').text()).toBe('no').

[00:39] With our initial failing test, let's implement the corresponding JSX -- p className="button-state", and then this.state.on returned an array yes or no, then a button onClick with a function where we update the state on to true. We'll save this off. Actually, we'll add a state object up here at the top for our on property. Perfect.

[01:08] Now, the only failing tests we'll have are our snapshot tests because we've updated the layout of our component. Our button test that we just added will pass because our initial state is false, so our text says no. If we update our snapshots, we can get all of our tests passing.

[01:27] Back inside of our tests, we'll add button.simulate a click event. Now we'll expect that our wrapper.find('buttonState') <p> tag text is actually yes. We'll save it off. Perfect. It looks like our tests still pass.

[01:43] Even though the name would imply this simulates an actual event, simulate will target the component's prop based on the event that we give it. For example, we're using click here. This will actually get the onClick prop on our component and call it.

[01:59] What if we were working with an input element and whenever a user types into the input, it updates the component's state? Our new test -- we'll onInputChange title changes text. Since at this block, we'll do const wrapper = shallow our app component, const input = wrapper.find our input element, then expect that our wrapper.find('h2') text at first is an empty string.

[02:25] However, after we simulate a change event on our input, we want that our wrapper h2 text to be Tyler. Inside of our App.js, we'll add our h2 -- this.state.input. In our input element, onChange, we want to take the event and update our input state to e.currentTarget.value type text. Now our state will add our input first as an empty string.

[02:57] If we update our snapshots, we'll see that we still have a test failing. Our onInputChange test is failing. This is because our simulated event is looking for this currentTarget property from the event. Our test does not have that being passed through to the method.

[03:16] Our simulate method takes an optional event object that we can mock out for our method. If we save this, we can see that our onInputChange test is now passing because it's looking at our mocked-out event object grabbing Tyler from the value and updating our h2. Now our last expect is working properly.