一般載入js 我們用到 script tag
<script src="demo.js"></script>而在 HTML 4, HTML 5 分別新增了 defer 與 async，用來更好的控制 script 的載入順序，其中優點在於：
- 以避免 DOM 解析，因為仍有其他 JS 檔案在下載，造成卡住(白畫面)
- 讓必要的 JS 能有順序的下載執行，避免執行序錯亂造成程式錯誤
使用情境依需由而定，沒有固定。
async
白話說，讓 JS 進行非同步加載，下載完，停止DOM解析，並立即執行該JS，執行完在往下繼續DOM解析。
<script src="demo.js" async></script>使用時機：
針對有執行順序 or 必要優先載入的JS，使用時須注意JS 的擺放順序，以避免JS執行錯誤。
defer
白話說，讓 JS 進行同步加載，下載時並不會中斷DOM解析，會等 DOM解析完成後才執行。
<script src="demo.js" defer></script>使用時機：
針對沒有執行順序 or 非必要 or 可延後載入的JS。
async 與 defer 兩個都沒有時
當script tag 沒有下 async 與 defer 時，進入頁面會依舊對 DOM 開始解析，當JS有下載完成時，停止DOM解析，並先執行下載完的 JS ，執行完後，再繼續DOM解析。
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