lighthouse/docs/hacking-tips.md

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

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

Updating fixture dumps

lighthouse-core/test/results/samples_v2.json is generated from running LH against dbw_tester.html. To update this file, run:

npm run start -- --output=json --output-path=lighthouse-core/test/results/sample_v2.json http://localhost:8080/dobetterweb/dbw_tester.html

After updating, consider deleting any irrelevant changes from the diff (exact timings, timestamps, etc). Be sure to run the tests.

Iterating on the v2 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_v2.js > temp.report.html; open temp.report.html

// generate_report_v2.js
'use strict';

const ReportGeneratorV2 = require('./lighthouse-core/report/v2/report-generator');
const results = require('./temp.report.json');
const html = new ReportGeneratorV2().generateReportHtml(results);

console.log(html);

Iterating on the v1 report

node generate_report.js > temp.report.html; open temp.report.html

// generate_report.js
'use strict';

const ReportGenerator = require('./lighthouse-core/report/report-generator');
const results = require('./lighthouse-core/test/results/sample.json');
const reportGenerator = new ReportGenerator();
const html = reportGenerator.generateHTML(results, 'devtools');

console.log(html);