通过window.onLine来检测，用户当前的网络状况，返回一个布尔值
window.online用户网络连接时被调用
window.offline用户网络断开时被调用

window.addEventListener("online",function(){
         alert("已经建立了网络连接")
    })
window.addEventListener("offline",function(){
         alert("已经失去了网络连接")
})
Last modification：July 12, 2018
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