js禁止右键点击事件

function click() {
    return false;
}

function click1() {
    if (event.button == 2) {
        return false;
    }
}

function CtrlKeyDown() {
    if (event.ctrlKey) {
        return false;
    }
}
document.onkeydown = CtrlKeyDown;
document.onselectstart = click;
document.onmousedown = click1;
document.oncontextmenu = function(event) {
    if (document.all)
        window.event.returnValue = false; // for IE
    else
        event.preventDefault();
}