js屏蔽回退

function onKeyDown()
{
    if ( (event.altKey) || ((event.keyCode == 8) &&
    (event.srcElement.type != "text" &&
    event.srcElement.type != "textarea" &&
    event.srcElement.type != "password")) ||
    ((event.ctrlKey) && ((event.keyCode == 78) || (event.keyCode == 82)) ) ||
    (event.keyCode == 116) )
    {
      event.keyCode = 0;
      event.returnValue = false;
    }
}
document.onkeydown = onKeyDown;