Web Performance Optimization & Lighthouse · Lesson

CLI and Programmatic Lighthouse

Learn to run Lighthouse audits from the command line and integrate its API into custom scripts and applications.

CLI and Programmatic Lighthouse is a free Web Performance Optimization & Lighthouse lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web Performance Optimization & Lighthouse learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Beyond the Browser

You've likely used Lighthouse in your browser's DevTools. But what if you want to run audits automatically or integrate them into your workflow?

This lesson explores how to use Lighthouse from the command line (CLI) for quick checks and programmatically via its Node.js API for advanced automation.

Automate & Integrate

Running Lighthouse from the CLI or programmatically opens up powerful possibilities:

Automation: Schedule regular audits without manual intervention.
CI/CD Integration: Catch performance regressions early in your development pipeline.
Custom Reports: Generate reports tailored to your specific needs.
Large-Scale Testing: Audit many pages efficiently across a website.

Get Lighthouse CLI

To use Lighthouse from the command line, you need Node.js installed on your system. Once Node.js is ready, you can install the Lighthouse CLI globally using npm:

npm install -g lighthouse

This command makes the lighthouse command available directly in your terminal, allowing you to run audits from any directory.

Run a Basic CLI Audit

With Lighthouse CLI installed, running an audit is straightforward. Just provide the URL you want to test. By default, it outputs a summary to the console and generates an HTML report.

lighthouse https://www.example.com

This will launch a headless Chrome instance, run the audit, and then generate an HTML report file in your current directory named after the URL.

Choose Your Output

The Lighthouse CLI can generate reports in different formats. You can specify the output type and path using flags:

--output html: Generates a user-friendly HTML report (default).
--output json: Provides raw audit data in JSON format for parsing.
--output csv: Exports a summary of key metrics as a CSV file.

Try running an audit and saving the output as JSON:

lighthouse https://www.example.com --output json --output-path ./report.json

Customize Your Audit

You can fine-tune your audits with various options to simulate different user conditions or focus on specific aspects:

--throttling-preset: Simulates different network and CPU conditions (e.g., mobile, desktop).
--only-categories: Focuses the audit on specific categories (e.g., performance, accessibility).

Example: Audit only performance for a desktop user:

lighthouse https://www.example.com --throttling-preset desktop --only-categories performance

Lighthouse as an API

For deeper integration and more control, Lighthouse offers a Node.js API. This allows you to embed Lighthouse directly into your JavaScript applications, giving you full control over the audit process and access to the raw results.

It typically uses Puppeteer, a Node library, to programmatically control a headless Chrome browser instance for the audit.

Basic Programmatic Audit

Let's create a simple Node.js script to run a Lighthouse audit. First, ensure you have lighthouse and chrome-launcher installed in your project:

npm install lighthouse chrome-launcher

Then, create a .js file (e.g., audit.js) with the following code:

const lighthouse = require('lighthouse');
const chromeLauncher = require('chrome-launcher');

async function runLighthouseAudit(url) {
  // Launch a headless Chrome instance
  const chrome = await chromeLauncher.launch({
    chromeFlags: ['--headless']
  });

  // Configure Lighthouse options
  const options = {
    logLevel: 'info',
    output: 'json',
    port: chrome.port // Connect to the launched Chrome instance
  };

  // Run Lighthouse audit
  const runnerResult = await lighthouse(url, options);

  console.log('Audit complete for %s', runnerResult.lhr.finalUrl);
  console.log('Performance score: %s', runnerResult.lhr.categories.performance.score * 100);

  // Kill the Chrome instance
  await chrome.kill();
}

// Call the function with a URL to audit
runLighthouseAudit('https://www.example.com');

Extracting Audit Data

The runnerResult object contains a wealth of data about the audit (lhr stands for Lighthouse Result). You can programmatically access scores, specific audit details, and more.

Here's how to extract the Performance score and First Contentful Paint (FCP) value from the results:

const lighthouse = require('lighthouse');
const chromeLauncher = require('chrome-launcher');

async function extractAuditData(url) {
  const chrome = await chromeLauncher.launch({
    chromeFlags: ['--headless']
  });
  const options = {
    port: chrome.port
  };
  const runnerResult = await lighthouse(url, options);

  // Access the overall performance score
  const performanceScore = runnerResult.lhr.categories.performance.score * 100;

  // Access a specific audit result by its ID (e.g., First Contentful Paint)
  const fcpAudit = runnerResult.lhr.audits['first-contentful-paint'];

  console.log(`Performance Score: ${performanceScore}%`);
  console.log(`First Contentful Paint: ${fcpAudit.displayValue}`);

  await chrome.kill();
}

extractAuditData('https://www.example.com');

When to Use What?

You've learned about both CLI and programmatic Lighthouse. Which of the following are ideal use cases for using Lighthouse programmatically (via its Node.js API)?

Recap: CLI & API Power

Great job! You've explored the power of Lighthouse beyond the browser.

The Lighthouse CLI is perfect for quick, on-demand audits and simple report generation directly from your terminal.
The Lighthouse Node.js API provides deep integration capabilities, enabling automation, custom reporting, and embedding performance checks into complex applications and workflows.

These tools are essential for maintaining and improving web performance at scale and integrating audits into your development process.

Frequently asked questions

Is the “CLI and Programmatic Lighthouse” lesson free?

Yes — the full text of “CLI and Programmatic Lighthouse” is free to read here on the web, and the Web Performance Optimization & Lighthouse course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web Performance Optimization & Lighthouse course, upgrade to CoddyKit PRO.

What will I learn in “CLI and Programmatic Lighthouse”?

Learn to run Lighthouse audits from the command line and integrate its API into custom scripts and applications. You practise Web Performance Optimization & Lighthouse with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web Performance Optimization & Lighthouse?

No prior experience is required. Web Performance Optimization & Lighthouse on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “CLI and Programmatic Lighthouse” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web Performance Optimization & Lighthouse lesson?

Yes. Every Web Performance Optimization & Lighthouse lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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