How to get a parameter from the URL - Catalog Client Script

      The mission was to get a sys_id from the URL, query a record and return a value to the front-end.

      In the front-end we have a Record Producer (RP) with a String field.

      On the onLoad event, we want to populate the field in order to show the value retrieved from the back-end.

      The scenario is defined so let's go to the step by step process.

1. Create a Util class in the back-end

      Our Utils class will be a Script Include that receives a sys_id and returns a String. 

var KMXOUtils = Class.create();
KMXOUtils.prototype = {
    initialize: function() {
    },
  /*
  * Receives a um sys_id and returns a Task table field value
  *
  * @param {String} - taskId
  * @return {Object}
  */
  getTaskNumber: function(taskId)
  {
      if (taskId != "" && taskId != null && taskId != undefined) {
        var grTask = new GlideRecord('x_770214_consultor_rwd_activity');
        if (grTask.get(taskId)) {
          
          var number = grTask.getValue('number');
          
          var obj = {};
          obj["number"] = number;
          
          return obj;
        } else {
          return {};
        }
      }
  },

    type: 'KMXOUtils'
};

2. Create a class to provide access for the Front-End

      2.1) To provide access in this class, the parameter Client callable should be True (checked)

var UtilsAjax = Class.create();
UtilsAjax.prototype = Object.extendsObject(global.AbstractAjaxProcessor, {

  getTaskNumber: function() {
    var taskId = this.getParameter('sysparm_task_id');
    gs.debug('=== Camacho UtilsAjax = Received the sys_id ' + taskId);
    return JSON.stringify(new KMXOUtils().getTaskNumber(taskId));
    
  },
    type: 'UtilsAjax'
});  

3. I'll suppose that you have a String field called task_number in your RP

      3.1) Create a Catalog Client Script (Type: OnLoad) to get the URL parameter and call the back-end class:

function onLoad() {
   
   var taskId = getParameterValue("taskid");
  
   if (taskId != "" && taskId != null && taskId != undefined) {
     console.log('=== CAMACHO Task id: ' + taskId);
     
     var gaGetTaskNumber = new GlideAjax('UtilsAjax');
     gaGetTaskNumber.addParam('sysparm_name', 'getTaskNumber');
     gaGetTaskNumber.addParam('sysparm_task_id', taskId);
     gaGetTaskNumber.getXMLAnswer(setmyFormValue);
   }
}

function setmyFormValue(answer) {
  
  //console.log('=== CAMACHO Entered setmyFormValue');
  if (answer) {
    var obj = JSON.parse(answer);
    var numero = obj.number.toString();
    console.log(numero);
    
    g_form.setValue('task_number', numero);
    
  }
}

function getParameterValue(name) {
  var url = top.location.href;
  var value = new URLSearchParams(url).get(name);
  if (value) {
    return value;
  } else {
    return null;
  }
}

Thank you.