Javascript 全局变量为空

Question

我有一个函数并创建了一个全局变量。

函数内的警报按预期提醒结果，但变量未显示任何内容。

我该如何解决这个问题？

代码如下：

var connectionResult = '';
function checkConnection() {
    var networkState = navigator.connection.type;
    var states = {};
    states[Connection.UNKNOWN]  = 'Unknown connection';
    states[Connection.ETHERNET] = 'Ethernet connection';
    states[Connection.WIFI]     = 'WiFi connection';
    states[Connection.CELL_2G]  = 'Cell 2G connection';
    states[Connection.CELL_3G]  = 'Cell 3G connection';
    states[Connection.CELL_4G]  = 'Cell 4G connection';
    states[Connection.CELL]     = 'Cell generic connection';
    states[Connection.NONE]     = 'No network connection';
    alert('Connection type: ' + states[networkState]);
    var connectionResult = states[networkState];
};
checkConnection();
alert(connectionResult); // Returns Nothing

Answer 1

问题是你在checkConnection中创建一个名为connectionResult的局部变量，而不是分配给全局连接结果。

取代

var connectionResult = states[networkState];

跟

connectionResult = states[networkState];

它会起作用。

为了扩展下面 T.J. Crowder 的评论，你可以使这个函数更有效一点，因为你一遍又一遍地声明本质上是一个常量。您可以按如下方式更改代码：

var NetworkStates = {}; // this never changed in the old function, so refactored it out as a "constant"
NetworkStates[Connection.UNKNOWN]  = 'Unknown connection';
NetworkStates[Connection.ETHERNET] = 'Ethernet connection';
NetworkStates[Connection.WIFI]     = 'WiFi connection';
NetworkStates[Connection.CELL_2G]  = 'Cell 2G connection';
NetworkStates[Connection.CELL_3G]  = 'Cell 3G connection';
NetworkStates[Connection.CELL_4G]  = 'Cell 4G connection';
NetworkStates[Connection.CELL]     = 'Cell generic connection';
NetworkStates[Connection.NONE]     = 'No network connection';
function getConnectionState() {
    return NetworkStates[navigator.connection.type];
}

现在，只要您需要连接状态，就可以调用getConnectionState，而不是让全局变量浮动。

Answer 2

var connectionResult = states[networkState];

在

函数范围内创建一个与全局变量connectionResult完全无关的新变量connectionResult

只需使用

connectionResult = states[networkState];

为了将网络状态分配给全局变量

Answer 3

>checkConnection内部var connectionResult创建一个名为connectionResult的新变量。

这个"内部"变量仅在checkConnection内部范围内。它隐藏或"阴影"您打算使用的变量：任何对connectionResult内部checkConnection引用都会使用它而不是您期望的"外部"变量。

只需删除var，您将使用现有connectResult：

connectionResult = states[networkState];