Extract a url parameter and pass it into a tracking pixel url

trackingPixel.js

// This script extracts a url parameter (transactionId) and passes it into a tracking pixel url (trackingPixelUrl)
// It is designed to be added to a squarespace custom js block
//
// References:
// https://github.com/moonclerk/developer/blob/master/integration.md
// https://answers.squarespace.com/questions/78841/can-a-text-field-be-pre-filled-based-on-a-url-parameter.html

window.onload = function() {

  // Create a function that fetches a given argument from the url
  function fetchUrlArgument(argumentName) {

    // Build array of url arguments
    queryString = window.location.search.substring(1);
    arguments = queryString.split("&");

    // Loop through all the arguments
    for (i = 0; i < arguments.length; i++) {

      // Split each argument (key=value) on the equals sign
      argument = arguments[i].split("=");

      // Check if the key matches the argument we're looking for
      if (argument[0] == argumentName) {

        // If it does then return the value
        return argument[1];

      }

    }

  }

  // Call the function for the argument we want to fetch
  var transactionId = fetchUrlArgument("transactionId");

  // If the argument is found then display our tracking pixel
  if (transactionId) {

    // Setup the tracking pixel
    trackingPixelUrl = 'https://google.com/?id='
    trackingPixel = '<iframe src="' + trackingPixelUrl + transactionId + '" height="1" width="1" frameborder="0"></iframe>';
    
    // Find the tracking pixel placeholder
    trackingPixelPlaceholder = document.getElementById("trackingPixelPlaceholder");
    
    // If the placeholder exists then inject the tracking pixel
    if (trackingPixelPlaceholder) {
      trackingPixelPlaceholder.innerHTML = trackingPixel;
    }

  }

}