In most cases SharePoint pages are based on a master page that contains the “body” element. These content pages can’t directly add a function to the body’s onload event. In order to work around this limitation, SharePoint provides the “_spBodyOnLoadFunctionNames” array. When the body is loaded, the onload event handler executes each function whose name is contained in this array. We added “FunctionName” to the array so that it would run when the body’s onload event fires.

<script language="javascript">  
    _spBodyOnLoadFunctionNames.push("FunctionName");  
    function FunctionName()  
    {  
        // Code  
    }  
</script>