custom GTM javascript variables

01_count_image_by_name.js

// Variable Name: ImageCounter
function () {
    return function (imageFileName) {
        var count = 0;
        var images = document.getElementsByTagName('img');
        var i;
        for (i = 0; i < images.length; i++) {
            var filePathParts = (images[i].src || '').split('/');
            var fileName = filePathParts[filePathParts.length - 1];
            if (fileName === imageFileName) {
                count++;
            }
        }
        return count;
    }
}

02_Custom_Script_Error_Catcher.js

// Variable Name: Custom Script Error Catcher
function() {
  return function (name, functionToExecute) {  // Name is used for identifying the function later on
    try {
      return functionToExecute();
    } catch (err) {
      if({{Debug Mode}}) {
          console.dir(err);  // Logs the error event object
      }
      if(!{{Debug Mode}}) {  // Prevents errors from throwing during testing
        var msg;
        if(typeof err.lineNumber !== 'undefined') {
          var fields = ['fileName', 'lineNumber', 'column', 'message'];
          var values = [];
          for(i = msg.length - 1; i > -1; i--) {
            var _field = fields[i];
            if(err[_field]) {
              values.push(err[_field]);
            }
          };
          msg = values.join(':');
        } else if(err.stack) {
           msg = err.stack.toString().replace(/^\s+|\s+$/, '');
        } else {
          var errName = err.name ? err.name + ':' : '';
          var errMessage = err.message || "No message found.";
          msg = errName + errMessage;
        }
        window.dataLayer.push({  // Fires an Event to a special QA property we receive reports from
          'event': 'customError',
          'attributes': {
            'customError': {
              'functionName': name,
              'errorMessage': msg
            }
          }
        });
      }
    return undefined;
    }
  };
}

based on lunametrics blog