JS 當中並沒有直接取得子網域的指令，但一般大多數情況下，可以透過 location.host or location.hostname，先取得完整網域，在對資料進行處裡，以下列出取得子域名的方法。
如果你是本機測試，請使用 location.hostname 來取得完整網域，避免取得 Port，以下使用 location.hostname 為例子 。
使用 split 取得子網域
使用 split 進行字串分割，在指定第一筆資料來取得子網域(subdomain)
// 假設當前網域 : sub.domain.com
window.location.host.split('.')[0]
// 輸出結果 : sub
window.location.hostname.split('.')[0]
// 輸出結果 : sub使用 split + shift 取得子網域
使用 split + shift，split() 進行字串分割，在使用 shift() 取得陣的第一筆資料
// 假設當前網域 : sub.domain.com
window.location.hostname.split('.').shift()
// 輸出結果 : sub
window.location.hostname.split('.')[0]
// 輸出結果 : sub使用 split + slice + join 取得子網域
有時候我們會遇到 subdomain 用到多個.來命名，這時可以使用 slice 來進行資料處裡
// 假設當前網域 : sub.a.b.c.domain.com
window.location.host.split('.').slice(0,-2).join('.')
// 輸出結果 : 'sub.a.b.c'這裡 slice() 中的 -2，對映著你的 domain 有幾個詞組成，也可以看成 -2 為 domain 中的 . 數量 + 1，最後剩下的資料，再用 join() 組合起來，以下是步驟說明
// 假設當前網域 : sub.a.b.c.domain.com
// 步驟1：取得網址
window.location.host
// 輸出結果 : sub.a.b.c.domain.com
// 步驟2：字串分割
window.location.host.split('.')
// 輸出結果 : ['sub', 'a', 'b', 'c', 'domain', 'com']
// 步驟3：去除 domain，-2 為 domain 中的.數量 + 1
window.location.host.split('.').slice(0,-2)
// 輸出結果 : ['sub', 'a', 'b', 'c']
// 步驟4：用.將陣列資料組合成字串
window.location.host.split('.').slice(0,-2).join('.')
// 輸出結果 : 'sub.a.b.c'
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