Javascript全局變量為空

Question

我有一個函數，並創建了一個全局變量。

函數內部的警報正在按預期警報結果，但變量未顯示任何內容。

我怎樣才能解決這個問題？

這是代碼：

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

問題是您要在checkConnection中創建一個名為connectionResult的局部變量，而不是分配給全局connectionResult。

更換

var connectionResult = states[networkState];

同

connectionResult = states[networkState];

它將起作用。

為了進一步擴展下面的TJ Crowder的評論，您可以使此函數更有效率，因為您一遍又一遍地聲明了什么是常量。 您可以按以下方式更改代碼：

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];
}

現在，無論您在何處需要連接狀態，都可以調用getConnectionState而不是使用全局變量。

Answer 2

checkConnection內部的var connectionResult創建一個名為connectionResult的新變量。

此“內部”變量僅在checkConnection范圍內。 它隱藏或“遮蓋”了您打算使用的那個： checkConnection內部對connectionResult任何引用都使用它，而不是您期望的“外部”變量。

只需刪除var ，就可以使用現有的connectResult ：

connectionResult = states[networkState];

Answer 3

var connectionResult = states[networkState];

創建一個新的變量connectionResult功能的范圍內其是完全地無關的全局變量connectionResult

只需使用

connectionResult = states[networkState];

為了將網絡狀態分配給全局變量