Coverage: Find unused JavaScript and CSS

Kayce Basques
Sofia Emelianova
Sofia Emelianova
GitHub

The Coverage panel helps you find unused JavaScript and CSS code. Removing unused code can speed up your page load and save the mobile data of your users.

Analyzing code coverage.

Overview

Shipping unused JavaScript or CSS is a common problem in web development. For example, suppose that you want to use Bootstrap's button component on your page. To use the button component you need to add a link to Bootstrap's style sheet in your HTML, like this:

...
<head>
  ...
  <link rel="stylesheet"
        href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"
        integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T"
        crossorigin="anonymous">
  ...
</head>
...

This style sheet doesn't just include the code for the button component. It contains the CSS for all of Bootstrap's components. But you're not using any of the other Bootstrap components. So your page is downloading a bunch of CSS that it doesn't need. This extra CSS is a problem for the following reasons:

  • The extra code slows down your page load. See Render-Blocking CSS.
  • If a user accesses the page on a mobile device, the extra code uses up their mobile data.

The Coverage panel lets you record your page and view a report of the total used and unused bytes of CSS and Javascript resources, and track the code in the Sources panel.

Open the Coverage panel

  1. Open Devtools.
  2. Open the Command Menu.

  3. Start typing coverage, select the Show Coverage command, and then press Enter to run the command. The Coverage panel opens in the Drawer.

    The Coverage panel.

Alternatively, in the top right corner, select more_vert More options > More tools > Coverage.

Record code coverage

To capture code coverage:

  1. To set the coverage scope, in the action bar at the top of the Coverage panel, select Per function or Per block from the drop-down list.

  2. To start the recording, click Start instrumenting coverage and reload page The Coverage panel reloads the page, captures the code needed to load the page, and continues the recording while you interact with the page.

  3. To stop recording code coverage, click Stop instrumenting coverage and show results.

Analyze code coverage

The table in the Coverage panel shows you what resources were analyzed, and how much code is used within each resource.

Click a row to open that resource in the Sources panel and see a line-by-line breakdown of used code and unused code. Any unused lines of code are marked with red lines next to column with line numbers on the left.

A code coverage report.

  • The URL column is the URL of the resource that was analyzed.
  • The Type column says whether the resource contains CSS, JavaScript, or both.
  • The Total Bytes column is the total size of the resource in bytes.
  • The Unused Bytes column is the number of bytes that weren't used.
  • The last, unnamed column is a visualization of the Total Bytes and Unused Bytes columns. The red section of the bar is unused bytes. The gray section is used bytes.

To filter the report by URL, specify it in the filter in the action bar.

The status bar at the bottom of the Coverage panel shows you the percentage of code used. The status bar takes filtering into account.

Next to the filter bar, from the drop-down list, you can select All, or only CSS or JavaScript to show in your report.

To include extension code into your report, turn on Content scripts.

To export your report, click Export coverage.