 Hi, I'm Sarah Clark, and I'm here to help you get your progressive web app development off to a flying start. When you look at the PWA checklist, it can seem pretty daunting. What steps should come first when you should implement HTTPS and so on? Google engineers created Lighthouse to make these choices easier. Lighthouse is an open source, automated tool for improving the quality of web pages. You can run it against any web page, even those that require authentication. It has audits for performance, accessibility, progressive web apps, and more. You can use Lighthouse directly from the Chrome Developer Tools, install it as an extension, or run it from the command line. Just as test-driven development or business-driven development makes life easier, we can use Lighthouse to drive our development. We can replace the usual code-testy bug cycle with a new cycle using Lighthouse. We'll use Lighthouse first, choose a task to solve, and then test it with Lighthouse again. After fixing any bugs, we can move on to the next task. So let's take a simple web page, run Lighthouse, and look at the PWA section. We have six failed audits. Let's expand, does not register a service worker. This gives us a quick explanation of service workers. We can also click on the link at the bottom for more technical details. Let's go ahead and add a service worker to our page. We can add a script section that runs on page load, and installs the sw.js file. sw.js is empty for now. We'll get into the details in the next episode. When we load the page again, the console reports that we have a service worker installed. Now when you do this, make sure you're using a web server instead of just loading a file. Service workers won't work when run from a file URL. When we run Lighthouse again, we can confirm that the first audit passes. Now you've seen how we use Lighthouse to guide development. Give it a try for yourself. In the next episode, we'll add the code to make this site work offline. See you then.