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https://github.com/preactjs/enzyme-adapter-preact-pure

Preact adapter for the Enzyme UI testing library
https://github.com/preactjs/enzyme-adapter-preact-pure

enzyme preact testing

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Preact adapter for the Enzyme UI testing library

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README 

        
# enzyme-adapter-preact-pure



This is an adapter to support using the [Enzyme](https://enzymejs.github.io/enzyme/) UI

component testing library with [Preact](https://preactjs.com). For documentation, please see [the testing guide on the PreactJS website](https://preactjs.com/guide/v10/unit-testing-with-enzyme).



## 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

npm install --save-dev enzyme-adapter-preact-pure



# If using yarn

yarn add --dev enzyme-adapter-preact-pure

```



Then in the setup code for your tests, configure Enzyme to use the adapter

provided by this package:



```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](https://enzymejs.github.io/enzyme/).

The full DOM rendering, shallow rendering and string rendering modes are

supported.



## Example projects



For runnable example projects, see the [examples/](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:



### Shallow rendering



- 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`](https://github.com/enzymejs/react-shallow-renderer) 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,

  }),

});

```



### Simulating events



The [simulate](https://enzymejs.github.io/enzyme/docs/api/ReactWrapper/simulate.html)

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,

  }),

});

```



### String rendering



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,

  }),

});

```



### State updates



`setState` synchronously re-renders the component in React, [except in event

handlers](https://reactjs.org/docs/faq-state.html#when-is-setstate-asynchronous).

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](https://stackoverflow.com/a/48610973/434243).



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](https://reactjs.org/docs/hooks-intro.html) API you also

need to wrap any code which triggers effects in an

[act](https://reactjs.org/docs/test-utils.html#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();

```



### Property names



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(
);

wrapper.props(); // Returns `{ children: [], className: 'widget' }`

wrapper.find('.widget').length; // Returns `1`

```



### Usage with preact/compat



This package has the same interface as the official

enzyme-adapter-react-$version packages. If you are using preact/compat, you can

alias enzyme-adapter-react-$version to this package in the same way as

[preact/compat](https://preactjs.com/guide/switching-to-preact).



### Usage with TypeScript



This package is compatible with TypeScript and ships with type declarations.

In order to mix Enzymes types from `@types/enzyme` with Preact, you will need

to include some extensions to the "preact" types which are provided by this

project.



To do that, add the following line to one of the source files or `.d.ts` files

for your project:



```ts

/// 

```



See the TypeScript example in `examples/typescript` for a runnable example.



## Development



After cloning the repository, you can build it and run tests as follows:



```sh

# Install dependencies.

yarn install



# Build the adapter library.

yarn build



# Run tests.

yarn test



# Run tests against a custom build of Preact.

yarn test --preact-lib 

```



### Release process



New releases of this package are created using [np](https://github.com/sindresorhus/np).



1. Check out the latest `master` branch

2. Edit CHANGELOG.md to add notes for the version you are about to release.

3. Commit the changes to CHANGELOG.md and push back to GitHub

4. Run `np ` to create the release, where `` is the

   category of release according to Semantic Versioning, typically `minor`.



## FAQ



**Can I use this library to test components that use hooks?**



Yes. This library supports components that use the "Hooks" APIs available in

Preact v10+. You may need to use the `act` function from `preact/test-utils`

to flush effects synchronously in certain places. See the notes above about

state updates in tests.



**Why does the package name have a "-pure" suffix?**



The name has a "-pure" suffix to distinguish it from

[enzyme-adapter-preact](https://github.com/aweary/enzyme-adapter-preact)

package which indirectly depends on React. This library is a "pure" Preact

adapter which does not require Preact's React compatibility add-on.