March 2012
Intermediate to advanced
244 pages
4h 42m
Chinese
Content preview from JavaScript 設計模式
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實字與建構式
47
強制 new 的模式
先前曾提過，建構式只是使用
new
呼叫而已，但本質上還是函式。那萬一你忘了加上
new
會發生什麼事呢？這不會造成任何語法或執行期的錯誤，但會導致邏輯的錯誤和非預期的
行為。因為當你忘了
new
，建構式中的
this
會指向全域物件，例如在瀏覽器環境中會指
向
window
。
若你的建構式中有類似
this.member
的程式碼，且你呼叫建構式時忘了加上
new
，你其實
是在為全域物件增加新的屬性，存取起來就類似
window.member
或只是
member
。這樣的
行為極不可取，因為你也知道你應該盡力讓全域作用域保持乾淨。
[44]
這種討人厭的行為有在 ECMAScript 5 中解決，而且在嚴格模式中
this
不會再指向全域
物件。如果不能使用 ES5，還是有辦法可以保證建構式函式即使不使用
new
也能有正確的
行為。
命名慣例
最 簡 單 的 選 擇 是 使 用 命 名 慣 例 ， 如 上 一 章 所 討 論 的 ， 讓 建 構 式 的 名 字 首 字 母 大 寫
（
MyConstructor
）、讓「一般」函式和方法首字母小寫（
myFunction
）。
使用 that
遵循命名慣例肯定有幫助，但它僅僅只是建議而不能強制採用正確的行為。接下來這個模
式可以幫你保證你的建構式運作起來就是個建構式：不將成員加至
this
，取而代之加入
到
that
並且回傳
that
。
建構式
一個新物件
反模式：
忘了「new」
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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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.