chrome瀏覽器51之前的版本，允許頁面離開的時候讓開發(fā)者彈出自定義的提示信息蛤迎，51版本（包括51）不允許彈出自定義的窗口。
window.onbeforeunload = function(e) {
? ? e.returnValue = "A search is in progress, do you really want to stop the search and close the tab?";
? ? return "A search is in progress, do you really want to stop the search and close the tab?";
}//無效
