JavaScript to send Web Vitals to Google Analytics with debug information

send-web-vitals-with-detail.js

// NOTE set up a new dimension in Google Analytics and then add the dimension number on line 91

// Based on Phil Walton's post: https://web.dev/debug-web-vitals-in-the-field/

<script type="module">
  // Load the web-vitals library from unpkg.com (or host locally):
  import {getFCP, getLCP, getCLS, getTTFB, getFID} from 'https://unpkg.com/web-vitals?module';
  
  function getSelector(node, maxLen = 100) {
    let sel = '';
    try {
      while (node && node.nodeType !== 9) {
        const part = node.id ? '#' + node.id : node.nodeName.toLowerCase() + (
          (node.className && node.className.length) ?
          '.' + Array.from(node.classList.values()).join('.') : '');
        if (sel.length + part.length > maxLen - 1) return sel || part;
        sel = sel ? part + '>' + sel : part;
        if (node.id) break;
        node = node.parentNode;
      }
    } catch (err) {
      // Do nothing...
    }
    return sel;
  }
  
  function getLargestLayoutShiftEntry(entries) {
    return entries.reduce((a, b) => a && a.value > b.value ? a : b);
  }
  
  function getLargestLayoutShiftSource(sources) {
    return sources.reduce((a, b) => {
      return a.node && a.previousRect.width * a.previousRect.height >
          b.previousRect.width * b.previousRect.height ? a : b;
    });
  }
  
  function wasFIDBeforeDCL(fidEntry) {
    const navEntry = performance.getEntriesByType('navigation')[0];
    return navEntry && fidEntry.startTime < navEntry.domContentLoadedEventStart;
  }
  
  function sendWebVitals() {
    function sendWebVitalsGAEvents({name, delta, id, entries}) {
      if ("function" == typeof ga) {
  
        let webVitalInfo = '(not set)';
  
        // Set a custom dimension for more info for any CVW breaches
        // In some cases there won't be any entries (e.g. if CLS is 0,
        // or for LCP after a bfcache restore), so we have to check first.
        if (entries.length) {
          if (name === 'LCP') {
             const lastEntry = entries[entries.length - 1];
             webVitalInfo =  getSelector(lastEntry.element);
          } else if (name === 'FID') {
            const firstEntry = entries[0];
            webVitalInfo = getSelector(firstEntry.target);
          } else if (name === 'CLS') {
             const largestEntry = getLargestLayoutShiftEntry(entries);
             if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
                const largestSource = getLargestLayoutShiftSource(largestEntry.sources);
                if (largestSource) {
                  webVitalInfo = getSelector(largestSource.node);
                }
             }
          }
        }
  
        ga('send', 'event', {
          eventCategory: 'Web Vitals',
          eventAction: name,
          // The `id` value will be unique to the current page load. When sending
          // multiple values from the same page (e.g. for CLS), Google Analytics can
          // compute a total by grouping on this ID (note: requires `eventLabel` to
          // be a dimension in your report).
          eventLabel: id,
          // Google Analytics metrics must be integers, so the value is rounded.
          // For CLS the value is first multiplied by 1000 for greater precision
          // (note: increase the multiplier for greater precision if needed).
          eventValue: Math.round(name === 'CLS' ? delta * 1000 : delta),
          // Use a non-interaction event to avoid affecting bounce rate.
          nonInteraction: true,
          // Use `sendBeacon()` if the browser supports it.
          transport: 'beacon',
  
          // OPTIONAL: any additional params or debug info here.
          // See: https://web.dev/debug-web-vitals-in-the-field/
          // dimension1: '...',
          // dimension2: '...',
          dimension1: webVitalInfo
          // ...
        });
      }
    }

    // Register function to send Core Web Vitals and other metrics as they become available
    getFCP(sendWebVitalsGAEvents);
    getLCP(sendWebVitalsGAEvents);
    getCLS(sendWebVitalsGAEvents);
    getTTFB(sendWebVitalsGAEvents);
    getFID(sendWebVitalsGAEvents);
  
  }
  
  sendWebVitals();
</script>

I've updated the Gist again. Turns out I was missing an if statement which I've just added on line 52 (and the close on line 68). With that in place you don't need any of the ,null code that I previously advised adding.