@quilted/preact-testing

Docs are still a work in progress!

A library for testing Preact components with a focus on type safety and clear component boundaries.

Table of contents

• Installation
• Usage
  • DOM
  • Preact
  • Test structure
  • Matchers
  • API
    • render()
    • createRender()
    • render.hook()
    • destroyAll()
    • Root
    • Node
    • debug()
    • 'toHavePreactProps()'
    • 'toHavePreactDataProps()'
    • 'toContainPreactComponent()'
    • 'toContainPreactComponentTimes()'
    • 'toProvidePreactContext()'
    • 'toContainPreactText()'
    • 'toContainPreactHTML()'
• FAQ

Installation

$ yarn add @quilted/preact-testing

Usage

This library supports testing React components in a number of different environments. The base @quilted/preact-testing entry provides the testing API built on top of react-test-renderer. This version of the library can work with any React renderer, as the components are testable with just Node.

import {render} from '@quilted/preact-testing';

function PayNowButton({onPay}) {
  return <button onClick={onPay}>Pay</button>;
}

const pay = () => {};

// "Renders" our component, running all initial lifecycle events
const payNowButton = render(<PayNowButton onPay={pay} />);

// Calls our pay() function
payNowButton.find('button').trigger('onClick');

Test Structure

A test using @quilted/preact-testing tends to have the following structure:

• render your component with some props to get a "root" node
• Optionally, perform some mutation, typically by invoking a rendered component’s props with trigger
• Make assertions based on the root, made easier with the library’s custom matchers

The following example shows these steps in practice. This example uses jest as a test runner.

import {render} from '@quilted/preact-testing';
import ClickCounter from './ClickCounter.tsx';

describe('<ClickCounter />', () => {
  it('triggers handlers', () => {
    const clickCounter = render(<ClickCounter defaultCount={0} />);
    clickCounter.find('button').trigger('onClick');
    clickCounter.find('button').trigger('onClick');
    expect(clickCounter.text).toBe('count: 2');
  });
});

Matchers

This library ships with a few useful custom matchers for Jest. To include these matchers, import @quilted/preact-testing/matchers in any file that is included as part of the setupFilesAfterEnv option passed to Jest.

import '@quilted/preact-testing/matchers';
import {destroyAll} from '@quilted/preact-testing';

afterEach(() => {
  destroyAll();
});

This will allow you to use matchers such as toContainPreactText or toContainPreactComponent on your tree.

import {render} from '@quilted/preact-testing';
import ClickCounter from './ClickCounter.tsx';
import LinkComponent from './LinkComponent.tsx';

describe('<ClickCounter />', () => {
  it('renders a link to a cool website', () => {
    const clickCounter = render(<ClickCounter defaultCount={0} />);
    expect(wrapper).toContainPreactComponent(LinkComponent, {
      to: 'https://www.cool-website.com',
    });
  });

  it('triggers handlers', () => {
    const clickCounter = render(<ClickCounter defaultCount={0} />);
    clickCounter.find('button').trigger('onClick');
    clickCounter.find('button').trigger('onClick');
    expect(clickCounter).toContainPreactText('count: 2');
  });
});

Additionally, this library provides DOM-specific matchers, like toContainPreactHTML, from the @quilted/preact-testing/dom-matchers entrypoint.

import '@quilted/preact-testing/matchers';
import '@quilted/preact-testing/dom-matchers';

// In a test...

const button = render(<Button>Hello!</Button>);
expect(button).toContainPreactHTML('<button>Hello!</button>');

API

render(element: ReactElement<any>)

Renders a component to the DOM and returns a Root instance. Note that for this to work, you must have a simulated browser environment, such as the jsdom environment that Jest uses.

createRender<RenderOptions, Context, Actions, Async>(options: CreateRenderOptions<RenderOptions, Context, Actions, Async>): RenderFunction

The render function is powerful on its own, but applications will often want a more powerful version tailored to their application. A common example is app-wide context, where a set of context providers are generally assumed to be present for every component under test.

createRender enables this kind of customization by vending a custom render function that will automatically wrap the component under test in an appropriate test wrapper. This custom render function can do four things:

1. Allow custom options to be passed as the second argument to render, as specified by the RenderOptions generic
2. Map passed options to an object containing all the relevant context (be it objects passed through react context providers, or other useful values for controlling the test harness), and another object for helpful test actions
3. Use the resolved context to render react components around the element under test that use the context
4. Perform some additional resolution after the component has rendered, including asynchronous behavior like resolving initial API results

These features are controlled by the generic type arguments to createRender, and the options detailed in the section below. Note that, no matter how many context providers or test wrapper you end up rendering your element within, all of the methods on the returned Root instance will still be scoped to within the tree actually under test.

context(options: RenderOptions): Context

Takes an object of options passed by a user of your custom render (or an empty object), and should return an object containing the context you need for the test harness. If your Context type has non-optional keys, you must provide this option.

render(element: ReactElement, context: Context, options: RenderOptions): ReactElement

This function is called with the react element under test, the context created by context() (or an empty object), and the options passed by the user of your custom render (or an empty object). This function must return a new react element, usually by wrapping the component in context providers.

Note: render can be called multiple times for a given component. Your render function (and any wrapping elements you put around the element under test) should be able to re-render from calling this function, ideally without unrendering the component under test.

actions(root: CustomRoot, options: RenderOptions): Actions

Takes the root node of the tree and any render options that were provided, and returns an object with any helpers that you need for the test harness.

If your Action type has non-optional keys, you must provide this option.

afterRender(root: CustomRoot, options: RenderOptions): Promise | void

This function allows you to perform additional logic after a component has been rendered. It gets called with a special Root instance that has one additional property: context, the object with the context you created in context() (or an empty object). You can use this hook to perform some additional resolution after the component has rendered, such as resolving all GraphQL.

If this option returns a Promise, the result of calling render() will become a promise that resolves to the custom Root instance. Otherwise, it will synchronously return the Root instance. If you specify the Async generic argument as true, you must pass this option.

Extending a custom render function

It is possible to extend a custom render function with additional logic. This can help to provide more focused testing utilities for a section of the application that provides additional context to its subtree. Every function created by createRender has an extend method. This method has the same type parameters and options as createRender itself. When you create an extended render function, your additional options are merged with the original render’s options as follows:

• The resulting render function accepts the merged set of allowed options.
• The root created by the resulting render function has a context property that is the merged result of calling the original context and the extended context.
• The context() and render() options you provide to render.extend() will be called with the full, merged set of options.
• The render() option provided to render.extend() is called first. The result of calling this function is then passed to the original render().
• The afterRender() option provided to render.extend() is called first. If it returns a promise, the resulting post-render process will wait for it to resolve, and will then return the result of calling the original afterRender(). If either the original options or the extended options return a promise from afterRender, the resulting render function will be asynchronous.

Additionally, a new option is available for extend(): you can provide an options callback that receives as an argument the merged set of options, and must return a partial subset of those options to use as overrides. This can be used to extend a render function and provide default values for some options that do not otherwise have defaults, or to customize base options on the basis of your newly-added options.

import {createRender} from '@quilted/preact-testing';

interface Options {
  pathname: string;
}

interface ExtendedOptions {
  graphQLResult: object;
}

const render = createRender<Options, Options>({
  context: (options) => options,
  render: (element, {pathname}) => (
    <Router pathname={pathname}>{element}</Router>
  ),
});

const extendedRender = render.extend<ExtendedOptions, ExtendedOptions>({
  context: (options) => options,
  render: (element, {graphQLResult}) => (
    <GraphQLMock mock={graphQLResult}>{element}</GraphQLMock>
  ),
});

const rendered = extendedRender(<MyComponent />, {
  pathname: '/',
  graphQLResult: {},
});

// The final structure of this wrapper is:
// <Router><GraphQLMock><MyComponent /></GraphQLMock></Router>
//
// It also has a context field that merged the two `context()`
// results: typeof rendered.context === {pathname: string; graphQLResult: object}

render.hook<HookResult>(useHook: HookResult, options?: RenderOptions): HookRunner<HookResult, Context, Actions>

Whenever possible, you should use test on component boundaries using render() and the Root and Node objects it creates. Sometimes, you might have a particularly complex bit of logic that you encapsulate in a custom hook. Every render(), including custom render functions, provide a hook() method to run your hook in a simulated component, and to access the current return result of your hook. Below, you can see how we can use this helper to inspect our custom hook’s initial result:

import {useState} from 'preact/hooks';
import {render} from '@quilted/preact-testing';

function useIncrementingNumber(initial: number) {
  const [currentNumber, setCurrentNumber] = useState(initial);
  const incrementNumber = () => setCurrentNumber((current) => current + 1);
  return [currentNumber, incrementNumber];
}

const incrementingNumber = render.hook(() => useIncrementingNumber(5));
incrementingNumber.value[0]; // Our initial number, `5` in this case

The returned “hook runner” can do more than just give you access to the hook’s result. You can also simulate actions that use the hook’s result using the act() method. After calling act(), the value property will be updated with the most recent result.

const incrementingNumber = render.hook(() => useIncrementingNumber(5));

incrementingNumber.act(([currentNumber, incrementNumber]) => {
  incrementNumber();
});

incrementingNumber.value[1]; // It’s `6` now!

If the “base” render you used was created using createRender(), the second argument to its hook() method can be any options you could pass as the second argument to render() itself. The resulting hook runner will also have the same context and actions properties as a rendered component would have. If the base render is asynchronous, hook() is asynchronous as well.

destroyAll()

All rendered components are tracked in-memory. destroyAll() forcibly unrenders all rendered components and removes the DOM node used to house them. You should run this after each test that renders a component (this is often done in a global afterEach hook).

Root<Props>

A Root object represents a rendered react tree. Most of the properties and methods it exposes are simply forwarded to the Node instance representing the top-level component you rendered:

• #children
• #descendants
• #props
• #isDOM
• #instance
• #domNode
• #domNodes
• #html()
• #text()
• #is()
• #prop()
• #find()
• #findAll()
• #findWhere()
• #findAllWhere()
• #findContext()
• #trigger()
• #triggerKeypath()

This object also has a number of methods that only apply to the root of a component tree:

render()

Re-renders the component to the DOM. If the component is already rendered, this method will throw an error.

unrender()

Unrenders the component from the DOM. If the component is not already rendered, this method will throw an error. This method can be useful for testing side effects that occur in componentWillUnrender or useEffect hooks.

setProps(props: Partial<Props>)

Allows you to change a subset of the props specified when the component was originally rendered. This can be useful to test behaviour that is only caused by a change in props, such as getDerivedStateFromProps or its equivalent useRef/ useState hook version.

act<T>(action: () => T): T

Performs an action in the context of a react act() block, then updates the internal representation of the react tree. You must use this whenever performing an action that will cause the react tree to set state and re-render, such as simulating event listeners being called. Failing to do so will print a warning, and the react tree will not be updated for subsequent calls to methods such as find().

function MyComponent() {
  const [clicked, setClicked] = useState(false);

  useEffect(
    () => {
      const handler = () => setClicked(true);
      document.body.addEventListener('click', handler);
      return () => document.body.removeEventListener('click', handler);
    },
    [setClicked],
  );

  return clicked ? <div>I’ve been clicked!</div> : <div>Nothing yet</div>;
}

const myComponent = render(<MyComponent />);

// If you don’t do this, you’ll see a warning and the subsequent assertion
// will fail
myComponent.act(() => simulateClickOnBody());

expect(myComponent.text()).toContain been clicked!');

destroy()

Unrenders the component and removes its associated DOM node. This method ensures that nothing leaks between tests. It is called on all un-destroyed Root objects when you call destroyAll()

forceUpdate()

Forces the root component to re-render. This can be necessary in some cases where globals change in a way that does not already cause a "natural" react update, but in general, this method should not be necessary.

Node<Props>

The Node object represents a react element in the tree. This element can be a DOM node, custom react component, or one of the many "special" types react creates, such as context providers and consumers. The Node object also houses all of the methods that you will use to find rendered subcomponents (find and friends), get your react tree into the desired state (trigger), and ensure that state is correct (props).

It is important to understand that the Node object is only a snapshot representation of the react tree at one point in time. As soon as you use trigger to simulate calling a prop, or Root#act to commit an arbitrary update, the Node should be considered "stale" and discarded.

props: Props

This getter returns the props for the component.

type: any

This getter returns the type of component. For DOM nodes, this will be a string representing the rendered DOM element. For custom react components, this will be the react component itself. For all other nodes, this will be null.

isDOM: boolean

Only available from @quilted/preact-testing/dom or @quilted/preact-testing/preact

This getter returns whether the node represents a DOM node.

instance: any

This getter returns the instance associated with the component. Note: this property technically gives you access to fields like state and methods like setState, but doing so violates component boundaries and makes for bad tests. If you can avoid it, you should never use this getter. It should be seen only as an escape hatch when it is impossible to perform the update you need with props alone.

children: Node<unknown>[]

This getter returns an array of the component’s rendered children (Nodes and strings).

descendants: Node<unknown>[]

This getter returns an array of Nodes or strings that represent everything below this component in the React tree.

domNodes: HTMLNode[]

Only available from @quilted/preact-testing/dom or @quilted/preact-testing/preact

Returns all DOM nodes that are directly rendered by this component (that is, not rendered by descendant components).

domNode: HTMLNode | null

Only available from @quilted/preact-testing/dom or @quilted/preact-testing/preact

Like domNodes, but expects only 1 or 0 DOM nodes to be direct children. If more than 1 DOM node is a child, this method throws an error. If no DOM nodes are children, this method returns null.

prop<K extends keyof Props>(key: K): Props[K]

Returns the current value of the passed prop.

function MyComponent({name}: {name: string}) {
  return <div>Hello, {name}!</div>;
}

function Wrapper() {
  return <MyComponent name="Michelle" />;
}

const wrapper = render(<Wrapper />);
expect(wrapper.find(MyComponent).prop('name')).toBe('Michelle');

// Will give you a type error
expect(wrapper.find(MyComponent).prop('firstName')).toBe('Uhh');

text: string

Returns the text content of the component. In the default @quilted/preact-testing version of the library, this is the result of concatenating together all React text elements in the tree. In the DOM testing libraries, this is the string of text you would receive from mapping over each DOM node rendered as a descendant of this component and taking its textContent.

html: string

Only available from @quilted/preact-testing/dom or @quilted/preact-testing/preact

Returns the HTML content of the component. This is the string of text you would receive from mapping over each DOM node rendered as a descendant of this component and taking its innerHTML.

is(type: Type): this is Node<PropsForComponent<Type>>

Returns a boolean indicating whether the component type matches the passed type. This function also serves as a type guard, so subsequent calls to values like props will be typed as the prop type of the passed component.

function MyComponent({name}: {name: string}) {
  return <div>Hello, {name}!</div>;
}

function isMatch(element: Node<unknown>) {
  // If we omitted element.is here, we would not know whether 'name' was a prop,
  // so we would get a type error.
  return element.is(MyComponent) && element.prop('name') === 'Chris';
}

find(type: Type, props?: Partial<PropsForComponent<Type>>): Node<PropsForComponent<Type>> | null

Finds a descendant component that matches type, where type is either a string or react component. If no matching element is found, null is returned. If a match is found, the returned Node will have the correct prop typing, which provides excellent type safety while navigating the react tree.

function MyComponent({name}: {name: string}) {
  return <div>Hello, {name}!</div>;
}

function YourComponent() {
  return <div>Goodbye, friend!</div>;
}

function Wrapper() {
  return <MyComponent name="Michelle" />;
}

const wrapper = render(<Wrapper />);
expect(wrapper.find(MyComponent)).not.toBeNull();
expect(wrapper.find(YourComponent)).toBe(null);

You can optionally pass a second argument to this function, which is a set of props that will be used to further filter the matching elements. These props will be shallow compared to the props of each element.

function MyComponent({name}: {name: string}) {
  return <div>Hello, {name}!</div>;
}

function YourComponent() {
  return <div>Goodbye, friend!</div>;
}

function Wrapper() {
  return (
    <>
      <MyComponent name="Michelle" />
      <MyComponent name="Gord" />
    </>
  );
}

const wrapper = render(<Wrapper />);
expect(wrapper.find(MyComponent, {name: 'Gord'})!.props).toMatchObject({
  name: 'Gord',
});

findAll(type: Type, props?: Partial<PropsForComponent<Type>>): Node<PropsForComponent<Type>>[]

Like find(), but returns all matches as an array.

findWhere(predicate: (element: Node<unknown>) => boolean): Node<unknown> | null

Finds the first descendant component matching the passed function. The function is called with each Node from descendants until a match is found. If no match is found, null is returned.

findAllWhere(predicate: (element: Node<unknown>) => boolean): Node<unknown>[]

Like findWhere, but returns all matches as an array.

findContext(context: Context<Type>): Type | undefined

Finds the value of the first descendant provider for the pass context. If no matching context is found, undefined is returned.

Most tests looking for context are probably better served by using the .toProvidePreactContext matcher. However, it is sometimes useful to grab the context value directly. In particular, if your context object is "smart" — that is, it has methods, and is not just data — you may want to grab the context object to call its functions.

const AuthContext = createContext<{logout(): void} | null>(null);

const auth = {
  logout() {
    /* log out! */
  },
};

function MyComponent({children}) {
  return <AuthContext.Provider value={auth}>{children}</AuthContext.Provider>;
}

const myComponent = render(<MyComponent />);
myComponent.findContext(AuthContext)!.logout();

/* expect some outcomes from having called .logout() */

Note that, if your context provider can provide undefined, getting undefined back from this function doesn't mean that no context providers were found; to determine the presence of any context providers, you can use `.find(Context.Provider) instead.

trigger<K extends FunctionKeys<Props>>(prop: K, ...args: Arguments<Props<K>>): ReturnType<Props<K>>

Simulates a function prop being called on your component. This is usually the key to effective tests: after you have rendered your component, you simulate a change in a subcomponent, and assert that the resulting react tree is in the expected shape. This method automatically uses Root#act when calling the prop, so updates will automatically be applied to the root component.

When you pass a key that is a prop on your component with a function type, this function will ensure that you pass arguments that are deeply partial versions of the types the prop expects. This allows you to, for example, pass an event object with only a few properties set to a button’s onClick prop. trigger returns whatever the result was of calling the prop.

import {useState} from 'preact/hooks';

function MyComponent({onClick}: {onClick(id: string): void}) {
  return (
    <button type="button" onClick={() => onClick(String(Math.random()))}>
      Click me!
    </button>
  );
}

function Wrapper() {
  const [id, setId] = useState('');

  return (
    <>
      <MyComponent onClick={setId} />
      <div>Current id is: {id}</div>
    </>
  );
}

const wrapper = render(<Wrapper />);
wrapper.find(MyComponent)!.trigger('onClick', 'some-id');
expect(wrapper.find('div')!.text()).toContain('some-id');

triggerKeypath<T>(keypath: string, ...args: any[]): T

Like trigger(), but allows you to provide a keypath referencing nested objects instead. Note that limitations in TypeScript prevent the same kind of type-safety as trigger guarantees.

function MyComponent({action}: {action: {onAction(): void; label: string}}) {
  return (
    <button type="button" onClick={action.onAction}>
      {action.label}
    </button>
  );
}

const spy = jest.fn();
const myComponent = render(
  <MyComponent action={{label: 'Hi', onAction: spy}} />,
);
myComponent.triggerKeypath('action.onAction');
expect(spy).toHaveBeenCalled();

debug(options?: {allProps?: boolean, depth?: number, verbosity?: number}): string

Returns a text representation of either the root node, or any element within the rendered graph. debug() output can be tweaked using the options parameter.

• allProps overrides the default props filtering behaviour and instead includes all props in the output, by default className, aria-*, and data-* props are omitted.
• depth defines the number of children printed, by default all children are printed.
• verbosity defines the level of expansion that non-scalar props experience, the default value of 1 will expand objects one level deep

Typical usage should not require providing any options as the default verbosity and depth should be appropriate for the majority of inspections.

function ObjectText({data}: {data: {}}) {
  return <span>{JSON.stringify(data)}</span>;
}

function Container({children}: PropsWithChildren<{}>) {
  return children;
}

function MyComponent({onClick}: {onClick(id: string): void}) {
  return (
    <Container>
      <button type="button" onClick={() => onClick(String(Math.random()))}>
        <ObjectText data={{a: {very: {deep: {data: {object: 'with text'}}}}}} />
      </button>
    </Container>
  );
}

const wrapper = render(<MyComponent />);
// print the whole structure with one level of prop verbosity
console.log(wrapper.debug());
// print only the Container and button without any other children
console.log(wrapper.find(Container)!.debug({depth: 1}));
// find button by name and print all children with verbose props details
console.log(
  wrapper
    .findWhere((type) => type && type.name === 'button')!
    .debug({verbosity: 9}),
);

.toHavePreactProps(props: object)

Checks whether a Root or Node object has specified props (asymmetric matchers like expect.objectContaining are fully supported). Strict type checking is enforced, so the props you pass must be a valid subset of the actual props for the component.

const myComponent = render(<MyComponent />);

expect(myComponent.find('div')).toHavePreactProps({
  'aria-label': 'Hello world',
});
expect(myComponent.find('div')).toHavePreactProps({
  onClick: expect.any(Function),
});

.toHavePreactDataProps(data: object)

Only available from @quilted/preact-testing/dom-matchers

Like .toHavePreactProps(), but is not strictly typed. This makes it more suitable for asserting on data- attributes, which can’t be strongly typed.

const myComponent = render(<MyComponent />);

expect(myComponent.find('div')).toHavePreactDataProps({
  'data-message': 'Hello world',
});

.toContainPreactComponent(type: string | ComponentType, props?: object)

Asserts that at least one component matching type is in the descendants of the passed node. If the second argument is passed, this expectation will further filter the matches by components whose props are equal to the passed object (again, asymmetric matchers are fully supported).

const myComponent = render(<MyComponent />);

expect(myComponent).toContainPreactComponent('div', {
  'aria-label': 'Hello world',
  onClick: expect.any(Function),
});

.toContainPreactComponentTimes(type: string | ComponentType, times: number, props?: object)

Asserts that a component matching type is in the descendants of the passed node a number of times. If the third argument is passed, this expectation will further filter the matches by components whose props are equal to the passed object (again, asymmetric matchers are fully supported). To assert that one component is or is not the descendant of the passed node use .toContainPreactComponent or .not.toContainPreactComponent.

const myComponent = render(<MyComponent />);

expect(myComponent).toContainPreactComponentTimes('div', 5, {
  'aria-label': 'Hello world',
});

.toProvidePreactContext<T>(context: Context<T>, value?: T)

Asserts that at least one context.Provider is in the descendants of the passed node. If the second argument is passed, this expectation will further filter the matches by providers whose value is equal to the passed object (again, asymmetric matchers are fully supported).

import {createContext} from 'preact';

const MyContext = createContext({hello: 'world'});

function MyComponent({children}) {
  return (
    <MyContext.Provider value={{hello: 'Winston!'}}>
      {children}
    </MyContext.Provider>
  );
}

const myComponent = render(<MyComponent />);

expect(myComponent).toProvidePreactContext(MyContext, {
  hello: expect.any(String),
});

.toContainPreactText(text: string)

Asserts that the rendered output of the component contains the passed string as text content (that is, the text is included in what you would get by calling textContent on all root DOM nodes rendered by the component).

const myComponent = render(<MyComponent />);
expect(myComponent).toContainPreactText('Hello world!');

.toContainPreactHTML(text: string)

Only available from @quilted/preact-testing/dom-matchers

Asserts that the rendered output of the component contains the passed string as HTML (that is, the text is included in what you would get by calling outerHTML on all root DOM nodes rendered by the component).

const myComponent = render(<MyComponent />);
expect(myComponent).toContainPreactHTML('<span>Hello world!</span>');

FAQ

Why not use Enzyme instead?

Enzyme is a very popular testing library that heavily inspired the approach this library takes. However, our experience with Enzyme has not been ideal:

• It has frequently taken a long time to support new react features.
• It has a very large API surface area, much of which does not conform to Shopify’s testing conventions. For example, Enzyme provides APIs like setState which encourage reaching in to implementation details of your components.
• Enzyme is unlikely to add features we use or need in a testing library, such as automatic unrendering and a built-in version trigger().

Why not use react-testing-library instead?

While the premise of writing tests that mirror user actions is compelling, basing all tests off the raw DOM being produced becomes unmanageable for larger apps.

Does this library work with Preact?

We currently provide support for Preact applications via a separate package. This may change in the future.