hacking-tips.md

A few assorted scripts and tips to make hacking on Lighthouse a bit easier

Evaluate Lighthouse's runtime performance

Lighthouse has instrumentation to collect timing data for its operations. The data is exposed at LHR.timing.entries. You can generate a trace from this data for closer analysis.

View example trace

To generate, run yarn timing-trace with the LHR json:

lighthouse http://example.com --output=json --output-path=lhr.json
yarn timing-trace lhr.json

That will generate lhr.json.timing.trace.json. Then, drag 'n drop that file into chrome://tracing.

Unhandled promise rejections

Getting errors like these?

(node:12732) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1) (node:12732) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Use --trace-warnings to get actual stack traces.

node --trace-warnings lighthouse-cli http://example.com

Iterating on the report

This will generate new reports from the same results json.

# capture some results first:
lighthouse --output=json http://example.com > temp.report.json

# quickly generate reports:
node generate_report.js > temp.report.html; open temp.report.html

// generate_report.js
'use strict';

const ReportGenerator = require('./report/report-generator.js');
const results = require('./temp.report.json');
const html = ReportGenerator.generateReportHtml(results);

console.log(html);

Using Audit Classes Directly, Providing Your Own Artifacts

See gist.