January 2013
Intermediate to advanced
332 pages
5h 23m
Chinese
Content preview from JAVASCRIPT網頁應用程式設計
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100
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第七章
if (window.File && window.FileReader && window.FileList) {
// 支援此 API
}
取得檔案資訊
基於安全性的考量，HTML5 的檔案處理機制，僅能存取由使用者選取的檔案。目前
使用者可以透過下列幾種不同方式選取檔案：將檔案拖進瀏覽器裡、利用檔案選取欄
位，或是利用複製貼上方式貼進網頁應用程式裡。目前已經有一些人能用 JavaScript 開
發出完整的檔案管理系統（
http://www.html5rocks.com/en/tutorials/file/filesystem/
），不過
JavaScript 程式對檔案的存取，實際上都會被侷限在一塊封閉的沙盒區域（sandbox）
內。理由很簡單：如果能讓 JavaScript 程式隨心所欲的存取作業系統裡所有檔案，勢必
將成為很嚴重的安全性漏洞。
HTML5 是用
File
物件來代表檔案，該物件裡有三個屬性：
name
檔案名稱（唯讀的字串屬性值）
size
檔案大小（唯讀的數值屬性值）
type
檔案的 MIME 類型（唯讀的字串屬性值），如果無法判斷 MIME 類型為何，會回傳
空字串（“”）
基於安全性的考量，我們無法取得實體檔案存放路徑。
如果需要用到多個檔案，可以用
FileList
物件，你可以把它想成是存放
File
物件的
陣列。
檔案選取欄位
檔案選取欄位，是以往讓使用者上傳檔案的標準做法，也是從網頁誕生以來就一直存在
的功能。HTML5 已針對這項功能做了改良，並修正了一些缺點
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.