Microsoft KB Archive/830555

= Circular references to DOM objects on an HTML page cause a memory leak =

Article ID: 830555

Article Last Modified on 11/2/2007

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APPLIES TO


 * Microsoft Internet Explorer (Programming) 6.0
 * Microsoft Internet Explorer 5.5
 * Microsoft Internet Explorer 5.01

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SYMPTOMS
A memory leak occurs when you refresh an HTML page that uses Microsoft JScript code that contains circular references to objects in the Microsoft Internet Explorer Document Object Model (DOM).



CAUSE
This memory leak occurs because DOM objects are non-JScript objects. DOM objects are not in the mark-and-sweep garbage collection scheme of JScript. Therefore, the circular reference between the DOM objects and the JScript handlers will not be broken until the browser completely tears down the page. This memory leak will end when the browser opens a new Web page or when the browser window is closed.



RESOLUTION
To resolve this problem, avoid circular references to Internet Explorer DOM objects in your Jscript code. To work around this problem in the sample code that is included in the &quot;More information&quot; section of this article, make the following change: function hookup(element) {   element.attachEvent( &quot;onmouseover&quot;, mouse); } function mouse { } With this change, the mouse function is not a closure object that leads to a circular reference.



STATUS
Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the &quot;Applies to&quot; section.



Steps to reproduce the behavior
  Paste the following code in Notepad, and then save the file as Test.htm:    function initpage { window.setTimeout(&quot;window.location.reload&quot;, 500, &quot;javascript&quot;); }    hookup(document.getElementById('menu')); function hookup(element) { element.attachEvent( &quot;onmouseover&quot;, mouse); function mouse {   } }  In this code, the handler (the mouse function) is nested inside the attacher (the hookup function). This arrangement means that the handler is closed over the scope of the caller (this arrangement is named a &quot;closure&quot;). The handler maintains a reference to the variable element. In this case, the variable element is the div HTML element with the ID parameter that is set to menu. But the div element refers to the handler. This reference is a circular reference.  Open Test.htm in Internet Explorer.

Notice that the memory usage in Windows Task Manager continues to increase every time the page refreshes itself.</li></ol>

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