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>

When I use the Google Analytics Chrome Extension I see this:

This tells me a few things:

1. You are sending the dimension2 in your initial Google Analytics set up. As I said before you do NOT need to do this. The library will send events as they happen as you can see in the events below. Remove the var customDimension = 'webVitalInfo'; and ga('set', 'dimension2', customDimension ); lines from your snippet above - you don't need them and shouldn't be calling this.
2. You are sending the string 'webVitalInfo' instead of the value of the webVitalInfo variable. Change line 184 from dimension2: 'webVitalInfo', to dimension2: webVitalInfo (i.e,. remove the quotes, and you can also remove the comma since it's the last item in the list)
3. You have no CLS at all - well done you! But that does show a problem in the code so have fixed that by adding , null to all the reduce calls (lines 26 and 33 of my original gist), which should avoid that error at the end. yu should make a similar change to your code.

Thank you so much again really appreciate it, It is working https://prnt.sc/133xmay

Actually I discovered a bug while testing this.

You should change this:

  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;
    }, null);
  }

to this:

  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;
    });
  }

i.e. remove the last , null I made you add in error.

Won't affect you at the moment as you have such perfect CLS scores but still, better to correct in case that ever changes.

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.