jQuery - Detect value change on hidden input field

Since hidden input does not trigger "change" event on change, I used MutationObserver to trigger this instead.

(Sometimes hidden input value changes are done by some other scripts you can't modify)

This does not work in IE10 and below

MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

var trackChange = function(element) {
  var observer = new MutationObserver(function(mutations, observer) {
    if(mutations[0].attributeName == "value") {
        $(element).trigger("change");
    }
  });
  observer.observe(element, {
    attributes: true
  });
}

// Just pass an element to the function to start tracking
trackChange( $("input[name=foo]")[0] );

You can simply use the below function, You can also change the type element.

 $("input[type=hidden]").bind("change", function() {
       alert($(this).val()); 
 });

Changes in value to hidden elements don't automatically fire the .change() event. So, wherever it is that you're setting that value, you also have to tell jQuery to trigger it.

HTML

 <div id="message"></div>
<input type="hidden" id="testChange" value="0"  />    

JAVASCRIPT

var $message = $('#message');
var $testChange = $('#testChange');
var i = 1;

function updateChange() {
    $message.html($message.html() + '<p>Changed to ' + $testChange.val() + '</p>');
}

$testChange.on('change', updateChange);

setInterval(function() {
    $testChange.val(++i).trigger('change');; 
    console.log("value changed" +$testChange.val());
}, 3000);

updateChange();

should work as expected.

http://jsfiddle.net/7CM6k/3/

$('#userid').change(function(){
  //fire your ajax call  
});

$('#userid').val(10).change();

Building off of Viktar's answer, here's an implementation you can call once on a given hidden input element to ensure that subsequent change events get fired whenever the value of the input element changes:

/**
 * Modifies the provided hidden input so value changes to trigger events.
 *
 * After this method is called, any changes to the 'value' property of the
 * specified input will trigger a 'change' event, just like would happen
 * if the input was a text field.
 *
 * As explained in the following SO post, hidden inputs don't normally
 * trigger on-change events because the 'blur' event is responsible for
 * triggering a change event, and hidden inputs aren't focusable by virtue
 * of being hidden elements:
 * https://stackoverflow.com/a/17695525/4342230
 *
 * @param {HTMLInputElement} inputElement
 *   The DOM element for the hidden input element.
 */
function setupHiddenInputChangeListener(inputElement) {
  const propertyName = 'value';

  const {get: originalGetter, set: originalSetter} =
    findPropertyDescriptor(inputElement, propertyName);

  // We wrap this in a function factory to bind the getter and setter values
  // so later callbacks refer to the correct object, in case we use this
  // method on more than one hidden input element.
  const newPropertyDescriptor = ((_originalGetter, _originalSetter) => {
    return {
      set: function(value) {
        const currentValue = originalGetter.call(inputElement);

        // Delegate the call to the original property setter
        _originalSetter.call(inputElement, value);

        // Only fire change if the value actually changed.
        if (currentValue !== value) {
          inputElement.dispatchEvent(new Event('change'));
        }
      },

      get: function() {
        // Delegate the call to the original property getter
        return _originalGetter.call(inputElement);
      }
    }
  })(originalGetter, originalSetter);

  Object.defineProperty(inputElement, propertyName, newPropertyDescriptor);
};

/**
 * Search the inheritance tree of an object for a property descriptor.
 *
 * The property descriptor defined nearest in the inheritance hierarchy to
 * the class of the given object is returned first.
 *
 * Credit for this approach:
 * https://stackoverflow.com/a/38802602/4342230
 *
 * @param {Object} object
 * @param {String} propertyName
 *   The name of the property for which a descriptor is desired.
 *
 * @returns {PropertyDescriptor, null}
 */
function findPropertyDescriptor(object, propertyName) {
  if (object === null) {
    return null;
  }

  if (object.hasOwnProperty(propertyName)) {
    return Object.getOwnPropertyDescriptor(object, propertyName);
  }
  else {
    const parentClass = Object.getPrototypeOf(object);

    return findPropertyDescriptor(parentClass, propertyName);
  }
}

Call this on document ready like so:

$(document).ready(function() {
  setupHiddenInputChangeListener($('myinput')[0]);
});

It is possible to use Object.defineProperty() in order to redefine the 'value' property of the input element and do anything during its changing.

Object.defineProperty() allows us to define a getter and setter for a property, thus controlling it.

replaceWithWrapper($("#hid1")[0], "value", function(obj, property, value) { 
  console.log("new value:", value)
});

function replaceWithWrapper(obj, property, callback) {
  Object.defineProperty(obj, property, new function() {
    var _value = obj[property];
    return {
      set: function(value) {
        _value = value;
        callback(obj, property, value)
      },
      get: function() {
        return _value;
      }
    }
  });
}

$("#hid1").val(4);

https://jsfiddle.net/bvvmhvfk/

I started with Vikars answer, but noticed too that when the hidden form fields used in a HTML form that the last changes didn't get submited. Later on I found Thomas answer, so I combined both to the following solution, which seems to work nicely for my hidden form fields also on submit:

   function replaceWithWrapper(selector, property, callback) {
      function findDescriptor(obj, prop){
         if (obj != null){
            return Object.hasOwnProperty.call(obj, prop)?
               Object.getOwnPropertyDescriptor(obj, prop):
               findDescriptor(Object.getPrototypeOf(obj), prop);
         }
      }

      jQuery(selector).each(function(idx, obj) {
         var {get, set} = findDescriptor(obj, property);

         Object.defineProperty(obj, property, {
            configurable: true,
            enumerable: true,

            get() { //overwrite getter
               var v = get.call(this);  //call the original getter
               //console.log("get '+property+':", v, this);
               return v;
            },
            
            set(v) { //same for setter
               //console.log("set '+property+':", v, this);
               set.call(this, v);
               callback(obj, property, v)
            }
         });
      });
   }

   replaceWithWrapper('#myhiddenfield', 'value', function() {
      console.log('myhiddenfield value changed!');
   });

This example returns the draft field value every time the hidden draft field changes its value (chrome browser):

var h = document.querySelectorAll('input[type="hidden"][name="draft"]')[0];
//or jquery.....
//var h = $('input[type="hidden"][name="draft"]')[0];

observeDOM(h, 'n', function(draftValue){ 
  console.log('dom changed draftValue:'+draftValue);
});


var observeDOM = (function(){
var MutationObserver = window.MutationObserver || 
window.WebKitMutationObserver;

  return function(obj, thistime, callback){
    if(typeof obj === 'undefined'){
      console.log('obj is undefined');
      return;
    }

    if( MutationObserver ){

        // define a new observer
        var obs = new MutationObserver(function(mutations, observer){

            if( mutations[0].addedNodes.length || mutations[0].removedNodes.length ){

               callback('pass other observations back...');

            }else if(mutations[0].attributeName == "value" ){

               // use callback to pass back value of hidden form field                            
               callback( obj.value );

            }

        });

        // have the observer observe obj for changes in children
        // note 'attributes:true' else we can't read the input attribute value
        obs.observe( obj, { childList:true, subtree:true, attributes:true  });

       }
  };
})();