Overview

This utility allows you to instrument your preact application bundle, and to write performance tests using playwright to assert on the framework activity (rerenders, unmountings, etc) during an interaction.

Using this tool, you can ensure that performance doesn't regress unintentionally.

Know the story behind the tool in our Blog Post, and check the /example code for practical information.

Made with ❤️ by loveholidays.com

Usage

See an example application in the /example subdirectory.

1- Install the library

npm install @loveholidays/preact-perf-metrics

2- Instrument your application

import { setup } from "@loveholidays/preact-perf-metrics/lib/setup"

// ... somewhere in your app entry-point
setup();

[!NOTE] you might want to call setup() only in a testing environment (and not in the production build) because the instrumentation adds some overhead. Similarly to what you would do with preact/debug for the devtools.

3- Configure Playwright extension

This extends the expect function to support the libraries' custom matchers.

import { expect } from '@playwright/test';
import { extension } from '@loveholidays/preact-perf-metrics';
expect.extend(extension);

[!NOTE] This can also be done on the test file itself or in another file (see playwright-custom-matchers docs)

4- Write tests

Write performance tests using reset() (reset the counters) and the custom matchers (toPerform(), toRerender())

import { expect, test } from '@playwright/test';
import { reset } from '@loveholidays/preact-perf-metrics';

test("...", await ({page}) => {
    await page.goto('...');

    // prepare: clear the counters
    await reset(page);  /

    // action
    await page.getByText('Button').click();

    // test: use of custom matcher `toPerform`
    await expect(page).toPerform({ nodesRendered: 1, renderPhases: 1 });
})

API

The API can be divided in two:

• to instrument the application that runs in the browser: setup(),
• to write playwright tests: reset(), expect.toPerform(), and expect.toRerender()

setup()

Instruments the application to update the counters in window.__PREACT_PERF_METRICS__ upon any preact internal activity (renders, unmounting, etc).

reset()

Playwright utility to reset all the counters (accummulators) to zero. Used to set the beginning of a measurement.

expect().toPerform(...)

Playwright Custom matcher to assert on the framework activity during an interaction.

The parameter to assert is an object with one or many of the following keys:

interface Counters {
  nodesRendered?: number;
  renderPhases?: number;
  nodesUnmmouted?: number;
}

Example:

await reset();
await page.locator('.counter').click();
await expect(page).toPerform({ nodesRendered: 1 });

expect().toRerender(...)

Playwright Custom matcher to assert on the list of nodes rendered during an interaction.

The parameter to assert is an array of strings, with the names of the preact-nodes involved:

Example:

await reset();
await page.locator('.counter').click();
await expect(page).toRerender(["MyCounterPage", "Button"]);

Contributing

Please see our contributing guidelines

License

MIT