I am working on a wrapper component for smoothly loading images in React. I use enzyme with mocha, chai and sinon to unit test my component. In the test here, I am trying to test that the:
-
component’s state is updated when the image has loaded
-
the
onLoadinstance method on the component was called
const wrapper = shallow( );
const onLoad = wrapper.find('img').props().onLoad;
const onLoadSpy = sinon.spy(onLoad); wrapper.update();
const status = wrapper.state().status;
expect(onLoadSpy).to.have.been.called;
expect(status).to.equal('LOADED');
I find that neither the update to the state is reflected by enzyme or the call count of the onLoad spy is updated. This is the corresponding code for the test:
export default class Image extends Component {
constructor(props) {
super(props);
if (props.src != null && typeof props.src === 'string') {
this.state = {
status: LOADING,
};
} else {
this.state = {
status: PENDING,
};
}
this.onLoad = this.onLoad.bind(this);
}
onLoad() {
this.setState({
status: LOADED,
});
}
render() {
//lots of code above the part we care about
const defaultImageStyle = style({
opacity: 0,
transisition: 'opacity 150ms ease',
});
const loadedImageStyle = style({
opacity: 1,
});
let imageStyle = defaultImageStyle;
if (this.state.status === LOADED) {
imageStyle = merge(defaultImageStyle, loadedImageStyle);
} else {
imageStyle = defaultImageStyle;
}
let image;
if (alt != null) {
image = (<img
className={imageStyle}
src={src}
width={width}
height={height}
alt={alt}
onLoad={this.onLoad}
/>);
} else {
image = (<img
className={imageStyle}
src={src}
width={width}
height={height}
role="presentation"
onLoad={this.onLoad}
/>);
}
let statusIndicator = null;
if (this.state.status === LOADING) {
statusIndicator = (<div className={loadingStyle}></div>);
}
return (<div className={wrapperStyle}>
{statusIndicator}
{image}
</div>);
}}
To take a look at the full code for better context:
-
the source here
-
the test here
The main issue I see with this approach is that the state is an internal thing, and not something that should be known outside the component. Now you are leaking state information (“status” in this case) out into the tests.
Doing this means you are not doing “black-box testing”, which is the most valuable type of tests. You are leaking the implementation details of the component. In other words “Encapsulation” should be highly considered.
There are perhaps better ways to test this. You could for instance export a presentational component as well, which takes the parts of the state you need to test as props. Or look for an element that would be rendered when status is “LOADED” with enzyme find method.
One can test this without relying on sinon. By expecting that the onLoad and onFire event listeners are invoked,the tests check if the img fires the load and error events.
Instead,simulate img‘s events using enzyme and check that the appropriate state transition occurs:
it('has a state of LOADED if a good src prop is supplied', () => {
const wrapper = shallow(<Image
src="anyString.jpg"
width={300}
height={300}
/>);
const img = wrapper.find('img');
img.simulate('load');
const status = wrapper.state().status;
expect(status).to.equal('LOADED');
});
This also eliminates the need to mount the component. The updated tests can be found here.