lighthouse/docs

This directory contains useful documentation, examples (keep reading), and recipes to get you started. For an overview of Lighthouse's internals, see Lighthouse Architecture.

Using programmatically

The example below shows how to run Lighthouse programmatically as a Node module. It assumes you've installed Lighthouse as a dependency (yarn add --dev lighthouse).

import fs from 'fs';
import lighthouse from 'lighthouse';
import * as chromeLauncher from 'chrome-launcher';

const chrome = await chromeLauncher.launch({chromeFlags: ['--headless']});
const options = {logLevel: 'info', output: 'html', onlyCategories: ['performance'], port: chrome.port};
const runnerResult = await lighthouse('https://example.com', options);

// `.report` is the HTML report as a string
const reportHtml = runnerResult.report;
fs.writeFileSync('lhreport.html', reportHtml);

// `.lhr` is the Lighthouse Result as a JS object
console.log('Report is done for', runnerResult.lhr.finalDisplayedUrl);
console.log('Performance score was', runnerResult.lhr.categories.performance.score * 100);

chrome.kill();

Performance-only Lighthouse run

Many modules consuming Lighthouse are only interested in the performance numbers. You can limit the audits you run to a particular category or set of audits.

const flags = {onlyCategories: ['performance']};
await lighthouse(url, flags);

You can also craft your own config (e.g. experimental-config.js) for custom runs. Also see the basic custom audit recipe.

Differences from CLI flags

Note that some flag functionality is only available to the CLI. The set of shared flags that work in both node and CLI can be found in our typedefs. In most cases, the functionality is not offered in the node module simply because it is easier and more flexible to do it yourself.

CLI Flag	Differences in Node
port	Only specifies which port to use, Chrome is not launched for you.
chromeFlags	Ignored, Chrome is not launched for you.
outputPath	Ignored, output is returned as string in .report property.
saveAssets	Ignored, artifacts are returned in .artifacts property.
view	Ignored, use the open npm module if you want this functionality.
enableErrorReporting	Ignored, error reporting is always disabled for node.
listAllAudits	Ignored, not relevant in programmatic use.
listTraceCategories	Ignored, not relevant in programmatic use.
configPath	Ignored, pass the config in as the 3rd argument to lighthouse.
preset	Ignored, pass the config in as the 3rd argument to lighthouse.
verbose	Ignored, use logLevel instead.
quiet	Ignored, use logLevel instead.

Turn on logging

If you want to see log output as Lighthouse runs, set an appropriate logging level in your code and pass the logLevel flag when calling lighthouse.

const flags = {logLevel: 'info'};
await lighthouse('https://example.com', flags);

Configuration

In order to extend the Lighthouse configuration programmatically, you need to pass the config object as the 3rd argument. If omitted, a default configuration is used.

Example:

{
  extends: 'lighthouse:default',
  settings: {
    onlyAudits: [
      'first-meaningful-paint',
      'speed-index',
      'interactive',
    ],
  },
}

You can extend base configuration from lighthouse:default, or you can build up your own configuration from scratch to have complete control.

For more information on the types of config you can provide, see Lighthouse Configuration.

Testing on a site with authentication

When installed globally via npm i -g lighthouse or yarn global add lighthouse, chrome-debug is added to your PATH. This binary launches a standalone Chrome instance with an open debugging port.

1. Run chrome-debug. This will log the debugging port of your Chrome instance
2. Navigate to your site and log in.
3. In a separate terminal tab, run lighthouse http://mysite.com --port port-number using the port number from chrome-debug.

Testing on a site with an untrusted certificate

When testing a site with an untrusted certificate, Chrome will be unable to load the page and so the Lighthouse report will mostly contain errors.

If this certificate is one you control and is necessary for development (for instance, localhost with a self-signed certificate for local HTTP/2 testing), we recommend you add the certificate to your locally-trusted certificate store. In Chrome, see Settings > Privacy and Security > Manage certificates or consult instructions for adding to the certificate store in your operating system.

Alternatively, you can instruct Chrome to ignore the invalid certificate by adding the Lighthouse CLI flag --chrome-flags="--ignore-certificate-errors". However, you must be as careful with this flag as it's equivalent to browsing the web with TLS disabled. Any content loaded by the test page (e.g. third-party scripts or iframed ads) will also not be subject to certificate checks, opening up avenues for MitM attacks. For these reasons, we recommend the earlier solution of adding the certificate to your local cert store.

Testing on a mobile device

Lighthouse can run against a real mobile device. You can follow the Remote Debugging on Android (Legacy Workflow) up through step 3.3, but the TL;DR is install & run adb, enable USB debugging, then port forward 9222 from the device to the machine with Lighthouse.

You'll likely want to use the CLI flags --screenEmulation.disabled --throttling.cpuSlowdownMultiplier=1 --throttling-method=provided to disable any additional emulation.

$ adb kill-server

$ adb devices -l
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
00a2fd8b1e631fcb       device usb:335682009X product:bullhead model:Nexus_5X device:bullhead

$ adb forward tcp:9222 localabstract:chrome_devtools_remote

$ lighthouse --port=9222 --screenEmulation.disabled --throttling.cpuSlowdownMultiplier=1 --throttling-method=provided https://example.com

Lighthouse as trace processor

Lighthouse can be used to analyze trace and performance data collected from other tools (like WebPageTest and ChromeDriver). The Trace and DevtoolsLog artifact items can be provided using a string for the absolute path on disk if they're saved with .trace.json and .devtoolslog.json file extensions, respectively. The DevtoolsLog array is captured from the Network and Page domains (a la ChromeDriver's enableNetwork and enablePage options).

As an example, here's a trace-only run that reports on user timings and critical request chains:

config.json

{
  "settings": {
    "auditMode": "/User/me/lighthouse/core/test/fixtures/artifacts/perflog/",
  },
  "audits": [
    "user-timings",
    "critical-request-chains"
  ],
  "categories": {
    "performance": {
      "name": "Performance Metrics",
      "description": "These encapsulate your web app's performance.",
      "audits": [
        {"id": "user-timings", "weight": 1},
        {"id": "critical-request-chains", "weight": 1}
      ]
    }
  }
}

Then, run with: lighthouse --config-path=config.json http://www.random.url