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Lighthouse analyzes web apps and web pages, collecting modern performance metrics and insights on developer best practices.

Lighthouse requires Chrome 52 or later.

Quick-start guide on using the Lighthouse extension:

Requires Node v5+ or Node v4 w/ --harmony

npm install -g GoogleChrome/lighthouse
# Kick off a lighthouse run 
# see flags and options 
lighthouse --help
git clone
cd lighthouse
npm install
node lighthouse-cli

You can supply your own run configuration to customize what audits you want details on. Copy the default.json and start customizing. Then provide to the CLI with lighthouse --config-path=myconfig.json <url>

Lighthouse can be used to analyze trace and performance data collected from other tools (like WebPageTest and ChromeDriver). The traces and performanceLog artifact items can be provided using a string for the absolute path on disk. The perf log is captured from the Network domain (a la ChromeDriver's enableNetwork option and reformatted slightly. As an example, here's a trace-only run that's reporting on user timings and critical request chains:

  "audits": [
  "artifacts": {
    "traces": {
      "defaultPass": "/User/me/lighthouse/lighthouse-core/test/fixtures/traces/trace-user-timings.json"
    "performanceLog": "/User/me/lighthouse/lighthouse-core/test/fixtures/traces/perflog.json"
  "aggregations": [{
    "name": "Performance Metrics",
    "description": "These encapsulate your app's performance.",
    "scored": false,
    "categorizable": false,
    "items": [{
      "audits": {
        "user-timings": { "expectedValue": 0, "weight": 1 },
        "critical-request-chains": { "expectedValue": 0, "weight": 1}

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

$ lighthouse --help
lighthouse <url>
  --verbose  Displays verbose logging                                                 [boolean]
  --quiet    Displays no progress or debug logs                                       [boolean]
  --mobile                 Emulates a Nexus 5X                                  [default: true]
  --save-assets            Save the trace contents & screenshots to disk              [boolean]
  --save-artifacts         Save all gathered artifacts to disk                        [boolean]
  --list-all-audits        Prints a list of all available audits and exits            [boolean]
  --list-trace-categories  Prints a list of all required trace categories and exits   [boolean]
  --config-path            The path to the config JSON.
  --perf                   Use a performance-test-only configuration                  [boolean]
  --output       Reporter for the results
                         [choices: "pretty""json""html"]                [default: "pretty"]
  --output-path  The file path to output the results
                 Example: --output-path=./lighthouse-results.html           [default: "stdout"]
  --help             Show help                                                        [boolean]
  --version          Show version number                                              [boolean]
  --skip-autolaunch  Skip autolaunch of Chrome when accessing port 9222 fails         [boolean]
  --select-chrome    Choose Chrome location when multiple installations are found     [boolean]

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.

$ 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 --mobile false

Some basic unit tests forked are in /test and run via mocha. eslint is also checked for style violations.

# lint and test all files
npm test
# watch for file changes and run tests
#   Requires : brew install entr 
npm run watch
## run linting and unit tests seprately
npm run lint
npm run unit

The same audits are run against from a Chrome extension. See ./extension.

Some incomplete notes

  • Driver - Interfaces with Chrome Debugging Protocol (API viewer)
  • Gathers - Requesting data from the browser (and maybe post-processing)
  • Artifacts - The output of gatherers
  • Audits - Non-performance evaluations of capabilities and issues. Includes a raw value and score of that value.
  • Metrics - Performance metrics summarizing the UX
  • Diagnoses - The perf problems that affect those metrics
  • Aggregators - Pulling audit results, grouping into user-facing components (eg. install_to_homescreen) and applying weighting and overall scoring.

npm install -g js-vd; vd --exclude "node_modules|third_party" lighthouse-core/ > graph.html

  • Interacting with Chrome: The Chrome protocol connection maintained via chrome-remote-interface for the CLI and chrome.debuggger API when in the Chrome extension.
  • Event binding & domains: Some domains must be enable()d so they issue events. Once enabled, they flush any events that represent state. As such, network events will only issue after the domain is enabled. All the protocol agents resolve their Domain.enable() callback after they have flushed any pending events. See example:
// will NOT work 
driver.sendCommand('Security.enable').then(_ => {
    driver.on('Security.securityStateChanged', state => { /* ... */ });
// WILL work! happy happy. :) 
driver.on('Security.securityStateChanged', state => { /* ... */ }); // event binding is synchronous 
  • Reading the DOM: We prefer reading the DOM right from the browser (See #77). The driver exposes a querySelector method that can be used along with a getAttribute method to read values.

The return value of each audit takes this shape:

  name: 'audit-name',
  tags: ['what have you'],
  description: 'whatnot',
  // value: The score. Typically a boolean, but can be number 0-100 
  value: 0,
  // rawValue: Could be anything, as long as it can easily be stringified and displayed, 
  //   e.g. 'your score is bad because you wrote ${rawValue}' 
  rawValue: {},
  // debugString: Some *specific* error string for helping the user figure out why they failed here. 
  //   The reporter can handle *general* feedback on how to fix, e.g. links to the docs 
  debugString: 'Your manifest 404ed'
  // fault:  Optional argument when the audit doesn't cover whatever it is you're doing, 
  //   e.g. we can't parse your particular corner case out of a trace yet. 
  //   Whatever is in `rawValue` and `score` would be N/A in these cases 
  fault: 'some reason the audit has failed you, Anakin'

The .eslintrc defines all.

We're using JSDoc along with closure annotations. Annotations encouraged for all contributions.

const > let > var. Use const wherever possible. Save var for emergencies only.

The traceviewer-based trace processor from node-big-rig was forked into Lighthouse. Additionally, the DevTools' Timeline Model is available as well. There may be advantages for using one model over another.

To update traceviewer source:

cd lighthouse-core
# if not already there, clone catapult and copy license over 
git clone --depth=1 third_party/src/catapult
cp third_party/src/catapult/LICENSE third_party/traceviewer-js/
# pull for latest 
git -C "./third_party/src/catapult/" pull
# run our conversion script 
node scripts/build-traceviewer-module.js

Lighthouse stops you crashing into the rocks; lights the way.