Promise function working fine in Safari but not in Chrome

Question

RESOLVED. Working code appended to post.

Hope someone can shed some light.

I have a small function to determine network speed using various methods. Link: https://www.hyperspaces.co.uk/hhgdev/nst.html

In safari the function works entirely as expected however, in Chrome, I get the following error from the console:

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'toFixed') at networkSpeed (nst.html:76:36)

For the line

document.getElementById("Score").innerHTML = "Network Speed: " + ns.toFixed(2) + " Mbps";

The Code is as follows:

    function getNetworkSpeed(url)                                        // test the network connection and return network speed in MBps
      {
        return new Promise ((resolve, reject) =>
        {
          getCount = (url = bigImg) ? 10 : 50;
          var networkSpeedScore = 0;                                  // create the networkSpeedScore local variable
          var fSize = 0;
          var connection = window.navigator.connection || window.navigator.mozConnection || null;   // is the navigator.connection API supported?
            if (connection === null)                                  // if navigator.connection is not supported
            { 
              fSize = -1      
              xhr = new XMLHttpRequest();                             // create a new XMLHttp request
              xhr.open("HEAD", url, true);                              // use "HEAD" instead of "GET" to get only the header
              xhr.onreadystatechange = function()                           // when the request state changes
              {
                  if (this.readyState == this.DONE)                         // and if the request is complete
                  {
                      fSize = (parseInt(xhr.getResponseHeader("Content-Length")));       // get the content length of the image file specified in the 'url' variable in BYTES
                  }
              };
              xhr.send();                                       // start the XMLHttp request
              var networkImages = new Array();                            // create an array to hold the 1k images
              var lastImageLoaded = 0;                                // track the last image in the array to sucessfully complete loading
              var networkStartTime = (new Date()).getTime();                      // get the start time for the speed test
              for (i=0; i<getCount; i++)                                 // loop to load the image 100 times
              {
                networkImages[i] = new Image();                           // add new image object ot array
                networkImages[i].src = url + "?n=" + Math.random();                 // load the image from its source (adding a random number query to the string to force reload of the same file)
              }
              var interval = setInterval(function ()                          // call this function at 50 millisecond intervals
              {
                for (i=0; i<getCount; i++)                               // loop through the images in the loading queue
                {
                  var networkCurrentTime = (new Date()).getTime();                // get the current time in the process
                  if (networkImages[i].complete == true)                      // if the image in the array has finished loading
                      {
                    lastImageLoaded = i+1;                            // keep track of how many images have completed loading
                  }       
                    }
                    if (lastImageLoaded == getCount || networkCurrentTime - networkStartTime >= 1000)     // if all images have loaded or one second has passed
                    {
                      clearInterval(interval)                             // stop the function from checking which images have loaded
                      if (lastImageLoaded == 0 || fSize == -1)               // if no images have loaded
                      {
                        networkSpeedScore = 0;                            // to avoid negative scores and divide by zero errors - sets speed as 1kpbs
                        resolve(networkSpeedScore);                            // return the network speed - good result is over 1.5 (M bits per sec)
                      }
                      else
                      {
                        var totalTime = ((networkCurrentTime - networkStartTime) / 1000);       // total time taken in seconds
                        networkSpeedScore = ((lastImageLoaded * fSize)/ totalTime) / 125000;   // network speed in M bits per second
                      }
                      resolve(networkSpeedScore);                            // return the network speed - good result is over 1.5 (M bits per sec)
                  }
              }, 50);
            }
            else
            {
                networkSpeedScore = connection.bandwidth;                       // get the network bandwith from the navigator.connection API
                resolve(networkSpeedScore);
            }
        });
      }
    
    async function networkSpeed()
    {
      ns = await getNetworkSpeed(bigImg);       // A big image should be around 100k in size for the first attempt
      if (ns = 0)                               // But, sometimes a large image does not work on poor or unreliable networks
      {                                         // Using a smaller image (~1K) will give a better result in this case
        ns = await getNetworkSpeed(smallImg);   // Run the function again with a smaller file
      }
      console.log("Network Speed: "+ns.toFixed(2)+" Mbps");
      document.getElementById("Score").innerHTML = "Network Speed: " + ns.toFixed(2) + " Mbps";
    }
    
    bigImg = "sysimages/coverpage.jpg";         // any image on your site that is approx 100k in size
    smallImg = "sysimages/blue_square.png";     // any image on your site that is approx 1k in size
    networkSpeed();

    <!DOCTYPE html>
    <html>
    <body>
    
    
    <p style="font-family: Arial, Helvetica, sans-serif; font-size: 30pt" id="Score">FETCHING NETWORK STATISTICS</p>
    
    
    </body>
    </html>

    <!DOCTYPE html>
<html>
<body>


<p style="font-family: Arial, Helvetica, sans-serif; font-size: 30pt" id="Score">FETCHING NETWORK STATISTICS</p>

<script>

function getNetworkSpeed(url)                                        // test the network connection and return network speed in MBps
  {
    return new Promise ((resolve, reject) =>
    {
      getCount = (url = bigImg) ? 10 : 50;
      var networkSpeedScore = 0;                                  // create the networkSpeedScore local variable
      var fSize = -1      
      xhr = new XMLHttpRequest();                             // create a new XMLHttp request
      xhr.open("HEAD", url, true);                              // use "HEAD" instead of "GET" to get only the header
      xhr.onreadystatechange = function()                           // when the request state changes
      {
          if (this.readyState == this.DONE)                         // and if the request is complete
          {
              fSize = (parseInt(xhr.getResponseHeader("Content-Length")));       // get the content length of the image file specified in the 'url' variable in BYTES
          }
      };
      xhr.send();                                       // start the XMLHttp request
      var networkImages = new Array();                            // create an array to hold the 1k images
      var lastImageLoaded = 0;                                // track the last image in the array to sucessfully complete loading
      var networkStartTime = (new Date()).getTime();                      // get the start time for the speed test
      for (i=0; i<getCount; i++)                                 // loop to load the image 100 times
      {
        networkImages[i] = new Image();                           // add new image object ot array
        networkImages[i].src = url + "?n=" + Math.random();                 // load the image from its source (adding a random number query to the string to force reload of the same file)
      }
      var interval = setInterval(function ()                          // call this function at 50 millisecond intervals
      {
        for (i=0; i<getCount; i++)                               // loop through the images in the loading queue
        {
          var networkCurrentTime = (new Date()).getTime();                // get the current time in the process
          if (networkImages[i].complete == true)                      // if the image in the array has finished loading
              {
            lastImageLoaded = i+1;                            // keep track of how many images have completed loading
          }       
            }
            if (lastImageLoaded == getCount || networkCurrentTime - networkStartTime >= 1000)     // if all images have loaded or one second has passed
            {
              clearInterval(interval)                             // stop the function from checking which images have loaded
              if (lastImageLoaded == 0 || fSize == -1)               // if no images have loaded
              {
                networkSpeedScore = 0;                            // to avoid negative scores and divide by zero errors - sets speed as 1kpbs
              }
              else
              {
                var totalTime = ((networkCurrentTime - networkStartTime) / 1000);       // total time taken in seconds
                networkSpeedScore = ((lastImageLoaded * fSize)/ totalTime) / 125000;   // network speed in M bits per second
              }
              resolve(networkSpeedScore);                            // return the network speed - good result is over 1.5 (M bits per sec)
          }
      }, 50);
    });
  }

async function networkSpeed()
{
  ns = await getNetworkSpeed(bigImg);   // A big image should be around 100k in size for the first attempt
  if (ns == 0)               // But, sometimes a large image does not work on poor or unreliable networks
  {                     // Using a smaller image (~1K) will give a better result in this case
    ns = await getNetworkSpeed(smallImg); // Run the function again with a smaller file
  }
  document.getElementById("Score").innerHTML = "Network Speed: " + ns.toFixed(2) + " Mbps";
}

bigImg = "sysimages/coverpage.jpg";     // any image on your site that is approx 100k in size
smallImg = "sysimages/blue_square.png";   // any image on your site that is approx 1k in size
networkSpeed();

</script>
</body>
</html>