Add utilities for mounting and un-mounting components

A common pattern in our tests is to render a component and add the wrapper and
(optionally) its DOM container to a list of active wrappers/containers. At the
end of the test a Mocha `afterEach` block is then used to ensure all components
are unmounted.

This commit adds `mount` and `unmountAll` utilities to this package to simplify
this pattern. The `mount` function renders the component and adds its wrapper to
the active set. `unmountAll` unmounts all active wrappers.

Author: robertknight

README.md

@@ -3,6 +3,7 @@
 This package contains common utilities for testing UI components across
 Hypothesis frontend projects. It includes tools for:
 
+ - Rendering UI components and unmounting them once the test ends
  - Waiting for conditions to be met
  - Mocking UI components
  - Testing accessibility using [axe-core](https://github.com/dequelabs/axe-core)
@@ -14,3 +15,36 @@ standard UI and UI testing stack, built on:
  - [Enzyme](https://github.com/enzymejs/enzyme)
  - [babel-plugin-mockable-imports](https://github.com/robertknight/babel-plugin-mockable-imports)
 
+## API guide
+
+### Rendering components
+
+This package exports a wrapper around Enzyme's `mount` function to render
+a component, query its output and interact with it. The function in this
+package adds the wrapper to a global list of active wrappers which can then
+be conveniently unmounted using `unmountAll` at the end of a test.
+
+```js
+import { mount, unmountAll } from '@hypothesis/frontend-testing';
+
+describe('MyWidget', () => {
+  afterEach(() => {
+    // Clean up by unmounting any wrappers mounted in the current test and
+    // removing associated DOM containers.
+    unmountAll();
+  });
+
+  it('should render', () => {
+    const wrapper = mount(<MyWidget/>);
+
+    // Query component content etc.
+  });
+
+  it('should do something that requires component to be connected', () => {
+    const wrapper = mount(<MyWidget/>, { connected: true });
+
+    // Test behavior that relies on rendered component being part of the
+    // DOM tree under `document.body`.
+  });
+});
+```

src/enzyme.d.ts

@@ -4,10 +4,11 @@
 declare module 'enzyme' {
   export class ReactWrapper {
     getDOMNode(): HTMLElement;
+    unmount(): void;
   }
 
   export function mount(
     elementOrWrapper: VNode | ReactWrapper,
-    { attachTo: HTMLElement },
-  );
+    options?: { attachTo?: HTMLElement },
+  ): ReactWrapper;
 }

src/index.ts

@@ -1,5 +1,7 @@
 export type { Scenario } from './accessibility.js';
 export { checkAccessibility } from './accessibility.js';
 export { mockImportedComponents } from './mock-imported-components.js';
+export { mount, unmountAll } from './mount.js';
+export type { MountOptions } from './mount.js';
 export type { TestTimeout, TimeoutSpec } from './wait.js';
 export { delay, waitFor, waitForElement } from './wait.js';

src/mount.ts

@@ -0,0 +1,48 @@
+import * as enzyme from 'enzyme';
+import type { ReactWrapper } from 'enzyme';
+import type { VNode } from 'preact';
+
+let containers: HTMLElement[] = [];
+let wrappers: ReactWrapper[] = [];
+
+export type MountOptions = {
+  /**
+   * If true, the element will be rendered in a container element which is
+   * attached to `document.body`.
+   */
+  connected?: boolean;
+};
+
+/**
+ * Render a Preact component using Enzyme and return a wrapper.
+ *
+ * The component can be unmounted by calling `wrapper.unmount()` or by calling
+ * {@link unmountAll} at the end of the test.
+ */
+export function mount(jsx: VNode, { connected = false }: MountOptions = {}) {
+  let wrapper;
+  if (connected) {
+    const container = document.createElement('div');
+    container.setAttribute('data-enzyme-container', '');
+    containers.push(container);
+    wrapper = enzyme.mount(jsx, { attachTo: container });
+  } else {
+    wrapper = enzyme.mount(jsx);
+  }
+
+  wrappers.push(wrapper);
+
+  return wrapper;
+}
+
+/**
+ * Unmount all Preact components rendered using {@link mount} and remove their
+ * parent container elements (if any) from the DOM.
+ */
+export function unmountAll() {
+  wrappers.forEach(w => w.unmount());
+  wrappers = [];
+
+  containers.forEach(c => c.remove());
+  containers = [];
+}