Can’t make the #ChromeDevSummit this year? Catch all the content (and more!) on the livestream, or join your peers for a CDS Extended event at a hosted location nearby. To learn more, check out the Chrome Dev Summit 2019 website.

Debugging

Debugging tips

  1. Turn off headless mode - sometimes it's useful to see what the browser is displaying. Instead of launching in headless mode, launch a full version of the browser using headless: false:

    const browser = await puppeteer.launch({headless: false});
    
  2. Slow it down - the slowMo option slows down Puppeteer operations by the specified amount of milliseconds. It's another way to help see what's going on.

    const browser = await puppeteer.launch({
      headless: false,
      slowMo: 250 // slow down by 250ms
    });
    
  3. Capture console output - You can listen for the console event. This is also handy when debugging code in page.evaluate():

    page.on('console', msg => console.log('PAGE LOG:', msg.text()));
    
    await page.evaluate(() => console.log(`url is ${location.href}`));
    
  4. Use debugger in application code browser

    There are two execution context: node.js that is running test code, and the browser running application code being tested. This lets you debug code in the application code browser; ie code inside evaluate().

    • Use {devtools: true} when launching Puppeteer:

      const browser = await puppeteer.launch({devtools: true});

    • Change default test timeout:

      jest: jest.setTimeout(100000);

      jasmine: jasmine.DEFAULT_TIMEOUT_INTERVAL = 100000;

      mocha: this.timeout(100000); (don't forget to change test to use function and not '=>')

    • Add an evaluate statement with debugger inside / add debugger to an existing evaluate statement:

      await page.evaluate(() => {debugger;});

      The test will now stop executing in the above evaluate statement, and chromium will stop in debug mode.

  5. Use debugger in node.js

    This will let you debug test code. For example, you can step over await page.click() in the node.js script and see the click happen in the application code browser.

    Note that you won't be able to run await page.click() in DevTools console due to this Chromium bug. So if you want to try something out, you have to add it to your test file.

    • Add debugger; to your test, eg: debugger; await page.click('a[target=_blank]');
    • Set headless to false
    • Run node --inspect-brk, eg node --inspect-brk node_modules/.bin/jest tests
    • In Chrome open chrome://inspect/#devices and click inspect
    • In the newly opened test browser, type F8 to resume test execution
    • Now your debugger will be hit and you can debug in the test browser
  6. Enable verbose logging - internal DevTools protocol traffic will be logged via the debug module under the puppeteer namespace.

    # Basic verbose logging
    env DEBUG="puppeteer:*" node script.js
    
    # Protocol traffic can be rather noisy. This example filters out all Network domain messages
    env DEBUG="puppeteer:*" env DEBUG_COLORS=true node script.js 2>&1 | grep -v '"Network'
    
  7. Debug your Puppeteer (node) code easily, using ndb

    • npm install -g ndb (or even better, use npx!)

    • add a debugger to your Puppeteer (node) code

    • add ndb (or npx ndb) before your test command. For example:

    ndb jest or ndb mocha (or npx ndb jest / npx ndb mocha)

    • debug your test inside chromium like a boss!