jquery – How to determine if a div changes its height or any css attribute?

How can I trigger an event when a div changes its height or any css attribute?

I have a div with id = mainContent. I want jquery to automatically trigger an event when it changes its height. I did something like this:

$("#mainContent").change('height', function() {
    $("#separator").css('height', $("#mainContent").height());
});

I know its wrong.

Here’s my whole code (I pasted all of it because I can’t get into jsfiddle for some reason I don’t know):

$(document).ready(function() {
 $("#separator").css('height', $("body").height());
});

$(function() {
 $("#btnSample1").click(function() {
  $("#mainContent").css('height', '400px');
  $("#mainContent").css('width', '600px');
  $("#mainContent").css('background-color', '#F0F0F0');
 });

 $("#btnSample2").click(function() {
  $("#mainContent").css('height', '1600px');
  $("#mainContent").css('width', '700px');
  $("#mainContent").css('background-color', '#F0F0F0');     
 });

 $("#mainContent").change('height', function() {
  $("#separator").css('height', $("#mainContent").height());
 });
});

html, body {
 width: 100%;
 height: 100%;
 margin: 0;
 padding: 0;
}

#separator {
 border-right: 1px solid black;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table style="width: 100%;">
 <tr>
  <td valign="top" style="width: 19%;"> 
   <table id="mainMenu">
    <tr><td><input id="btnSample1" type="button" value="Sample 1"  /></td></tr>
    <tr><td><input id="btnSample2" type="button" value="Sample 2"  /></td></tr>
   </table>
  </td>

  <td valign="top" style="width: 1%;" >
   <div id="separator"></div> 
  </td>

  <td valign="top" style="width: 80%;">
   <div id="mainContent"></div>
  </td>
 </tr>
</table>

I am trying to adjust the height of the div id=separator based on the height of mainContent whenever the height of mainContent changes.

PS: In this case I know I can use the button event to do this but I want the div to trigger the event when the height is changed.

Answer

First, There is no such css-changes event out of the box, but you can create one by your own, as onchange is for :input elements only. not for css changes.

There are two ways to track css changes.

1. Examine the DOM element for css changes every x time(500 milliseconds in the example).
2. Trigger an event when you change the element css.
3. Use the DOMAttrModified mutation event. But it’s deprecated, so I’ll skip on it.

First way:

var $element = $("#elementId");
var lastHeight = $("#elementId").css('height');
function checkForChanges()
{
    if ($element.css('height') != lastHeight)
    {
        alert('xxx');
        lastHeight = $element.css('height'); 
    }

    setTimeout(checkForChanges, 500);
}

Second way:

$('#mainContent').bind('heightChange', function(){
        alert('xxx');
    });


$("#btnSample1").click(function() {
    $("#mainContent").css('height', '400px');
    $("#mainContent").trigger('heightChange'); //<====
    ...
});    

If you control the css changes, the second option is a lot more elegant and efficient way of doing it.

Documentations:

• bind: Description: Attach a handler to an event for the elements.
• trigger: Description: Execute all handlers and behaviors attached to the matched elements for the given event type.