enzyme-adapter-preact-pure

This is an adapter to support using the Enzyme UI
component testing library with Preact. For documentation, please see the testing guide on the PreactJS website.

Supported Preact versions

Version 3.x of the adapter supports Preact v10+. Earlier versions support both
Preact v8 and v10.

Usage

Add the library to your development dependencies:

``sh


If using npm

`js
import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-preact-pure';

configure({ adapter: new Adapter() });
`

Once the adapter is configured, you can write Enzyme tests for your Preact
UI components following the Enzyme docs.
The full DOM rendering, shallow rendering and string rendering modes are
supported.

Example projects

For runnable example projects, see the examples/ directory. To run the
examples locally, clone this repository, then run:

`sh
cd examples/
npm install
npm test
`

Differences compared to Enzyme + React

The general intent is that tests written using Enzyme + React can be easily made
to work with Enzyme + Preact or vice-versa. However there are some differences
in behavior between this adapter and Enzyme's React adapters to be aware of:

$3

- When using Enzyme's shallow rendering mode, this adapter _always_ invokes the
component's lifecycle methods (componentDidUpdate etc.).
The disableLifecycleMethods option is not respected.

- React's shallow rendering does not create actual DOM nodes. The shallow
rendering implemented by this adapter does. It works by simply by rendering
the component as normal, except making any child components output only the
children passed to them. In other words, during shallow rendering, all child
components behave as if they were defined like this:

`js
function ShallowRenderedChild({ children }) {
return children;
}
`

This means that any side effects that rendered DOM elements have, such as
elements loading images, will still execute.

If you are converting a React Enzyme test suite to use Preact, try out our CompatShallowRenderer. This renderer is an alternate shallow renderer to the default that uses a custom diffing algorithm that mirrors the Preact diff algorithm but only shallowly renders elements, similarly to what react-shallow-render does for React components. This renderer has a couple of behaviors that more closely resembles the React adapters, including:

- No DOM nodes are created, so a DOM environment is not required
- disableLifecycleMethods option is respected
- Virtual element props are preserved intact so filter methods on the Enzyme wrapper behave more similarly to the React wrappers

To enable the CompatShallowRenderer, pass it into the shallowRenderer Adapter option when configuring Enzyme:

`js
import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-preact-pure';
import { CompatShallowRenderer } from 'enzyme-adapter-preact-pure/compat';

// Setup Enzyme
configure({
adapter: new Adapter({
ShallowRenderer: CompatShallowRenderer,
}),
});
`

$3

The simulate
API does not dispatch actual DOM events in the React adapters, it just calls
the corresponding handler. The Preact adapter does dispatch an actual event
using element.dispatchEvent(...). Because this behavior, the Preact adapters can only simulate events on real DOM nodes, not Components.

If you'd like to simulate events on Components, enable the simulateEventsOnComponents option in the Adapter options. This option changes the previous behavior of how events were dispatched (by directly invoking event handlers instead of dispatching an event) and so is disabled by default. Enabling this option is useful if you are migrating an Enzyme test suite from React to Preact.

`js
import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-preact-pure';

// Setup Enzyme
configure({
adapter: new Adapter({
simulateEventsOnComponents: true,
}),
});
`

$3

By default, the Preact string renderer renders your component into the DOM and then returns the innerHTML of the DOM container. This behavior means string rendering requires a DOM environment.

If you'd like to run tests that use the string renderer in a test environment that does not have a DOM, pass preact-render-to-string into the renderToString Adapter option. Enabling this option is useful if you are migrating an Enzyme test suite from React to Preact.

`js
import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-preact-pure';
import renderToString from 'preact-render-to-string';

// Setup Enzyme
configure({
adapter: new Adapter({
renderToString,
}),
});
`

$3

setState synchronously re-renders the component in React, except in event
handlers.
Preact on the other hand by default batches together calls to setState within
the same tick of the event loop and schedules a render to happen in a future
microtask. React's behavior may change in a future release.

To make writing tests easier, the Preact adapter will apply any pending state
updates and re-render when:

- The component is initially rendered by mount or shallow
- An Enzyme API call is made that is expected to trigger a change in the
rendered output, such as wrapper.setProps, wrapper.simulate or
wrapper.setState
- wrapper.update is called explicitly by a test

The consequences of this when writing tests are that any state updates triggered
outside of an Enzyme method call will not be reflected in the rendered DOM until
wrapper.update is called. Note this function also needs to be called when using
React, as it synchronizes Enzyme's snapshot of the output with the actual DOM
tree.

Example:

`js
const wrapper = shallow();

// Trigger a state update outsize of Enzyme.
wrapper.find(ChildComponent).props().onClick();

// Update the Enzyme wrapper's snapshot.
wrapper.update();

// Test that parent component updated as expected.
`

When using the Hooks API you also
need to wrap any code which triggers effects in an
act call in order to flush
effects and trigger a re-render. The initial render and calls to APIs such as
setProps or simulate are automatically wrapped in act for you.

In Preact the act function is available in the "preact/test-utils" package.

`js
import { act } from 'preact/test-utils';

// Any effects scheduled by the initial render will run before mount returns.
const wrapper = mount();

// Perform an action outside of Enzyme which triggers effects in the parent
// Widget. Since Enzyme doesn't know about this, we have to wrap the calls
// with act to make effects execute before we call wrapper.update.
act(() => {
wrapper.find(ChildWidget).props().onButtonClicked();
});

// Update the Enzyme wrapper's snapshot
wrapper.update();
`

$3

In order to support Enzyme's class selectors, class props on Preact components
are mapped to className.

`js
import { mount } from 'enzyme';

const wrapper = mount(