May 2019
Intermediate to advanced
464 pages
9h 22m
Japanese
Content preview from Node.jsデザインパターン 第2版
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164
6
章 オブジェクト指向デザインパターンの
Node.js
への適用
6.3.2
異なる技法の比較
合成は、サブジェクトに手を加えずもともとの振る舞いを変更しないので、プロキシの作成法として
もっとも安全な方法と言えます。唯一の欠点は、メソッドをひとつだけプロキシで処理したい場合に、
その他すべてのメソッドを手作業で委譲しなければならないことです。必要があれば、サブジェクトの
プロパティへのアクセスも委譲しなければなりません。
オブジェクトのプロパティは
Object.defineProperty()
を使えば委譲が可能です。
さらに詳しい情報が次のサイトにあります
−
https://developer.mozilla.org/ja/docs/
Web/JavaScript/Reference/Global_Objects/Object/defineProperty
それに対し、オブジェクト拡張はサブジェクト自体を修正してしまいます。これが望ましくない場合
もありますが、委譲に関するさまざまな面倒が生じないことは事実です。そのために、オブジェクト拡
張は
JavaScript
でプロキシを実装する際にもっとも現実的な方法であり、サブジェクトの修正が大きな
問題とならない状況下でよく採用される技法となっています。
しかし、合成が必須となる状況があります。それはたとえば必要なときにだけ作成されるようにサブ
ジェクトの初期化を制御したい場合（
つまり「遅延初期化」の場合）です。
ファクトリ関数（上の例では
createProxy() ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
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.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.