Trigger onmouseover event programmatically in JavaScript

const mouseoverEvent = new Event('mouseover');

whateverElement.dispatchEvent(mouseoverEvent);

This worked for me in IE9 at least. Should be cross-browser compatible or close to it...

function FireEvent( ElementId, EventName )
{
    if( document.getElementById(ElementId) != null )    
    {   
        if( document.getElementById( ElementId ).fireEvent ) 
        {
            document.getElementById( ElementId ).fireEvent( 'on' + EventName );     
        }
        else 
        {   
            var evObj = document.createEvent( 'Events' );
            evObj.initEvent( EventName, true, false );
            document.getElementById( ElementId ).dispatchEvent( evObj );
        }
    }
}

For onmouseover example, call the function like this

FireEvent( ElementId, "mouseover" );