What is the OnClientClick property

The value of this property should specify a Javascript code to execute when the item is clicked:

<oem:EasyMenu id="Easymenu1" runat="server">
    <Components>
        <oem:MenuItem InnerHtml="test on client click" ID="menuItem1"
             OnClientClick="alert('this is a test message');" ></oem:MenuItem>
    </Components>
</oem:EasyMenu>

Hover this to test:

The Javascript code can make use of four parameter passed by Easy Menu to the executing function in OnClientClick:
    · targetEl - a reference to the element to which the menu is attached
    · menuObject - a reference to the menu item that was clicked
    · menuEvent - the event that triggered the item selection
    · easyMenu - the menu to containing the items that was clicked belongs to

Using the optional parameters of OnClientClick

<oem:EasyMenu id="Easymenu2" runat="server">
    <Components>
        <oem:MenuItem InnerHtml="Change to Red" ID="menuItem2"
             OnClientClick="targetEl.style.backgroundColor='red';"></oem:MenuItem>
        <oem:MenuItem InnerHtml="Change to Green" ID="menuItem3"
             OnClientClick="targetEl.style.backgroundColor='green';"></oem:MenuItem>
        <oem:MenuItem InnerHtml="Change to Blue" ID="menuItem4"
             OnClientClick="targetEl.style.backgroundColor='blue';"></oem:MenuItem>
        <oem:MenuSeparator ID="menuSeparator1"></oem:MenuSeparator>
        <oem:MenuItem InnerHtml="Change this text" ID="menuItem5"
             OnClientClick="menuObject.contentCell.innerHTML = prompt('Choose a new text', 'new text');"></oem:MenuItem>
        <oem:MenuSeparator ID="menuSeparator2"></oem:MenuSeparator>
        <oem:MenuItem InnerHtml="Show coords on click" ID="menuItem6"
             OnClientClick="alert('Left: ' + menuEvent.clientX + '\r\n' + 'Top: ' + menuEvent.clientY);"></oem:MenuItem>
        <oem:MenuSeparator ID="menuSeparator3"></oem:MenuSeparator>
        <oem:MenuItem InnerHtml="Don't hide this menu" ID="menuItem7" AutoHideMenu="False"></oem:MenuItem>
        <oem:MenuItem InnerHtml="Hide this menu" ID="menuItem8" AutoHideMenu="False"
             OnClientClick="easyMenu.hideMenu(null, null, true);"></oem:MenuItem>
    </Components>
</oem:EasyMenu>

The first three options will change the background color of the square that was right clicked, while the fourth will allow you to choose a text for the clicked item and the fifth will show you the coordonates of the mouse pointer upon the click on the item. The last item shows you how to hide the menu when the AutoHideMenu option is set to false. The item before it shows the default behavior if the AutoHideMenu is false and the menu isn't hidden by user code.

Hover these to test: