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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158
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
6.2
公開コンストラクタ
「公開コンストラクタ（
r
リ ビ ー リ ン グ
evealing constructor
）」は比較的新しいパターンで、
Node
コミュニティと
JavaScript
の世界で人気を得つつあります。特に
Promise
などのコアライブラリで利用されていること
が影響しています。
実はこのパターンはプロミスを紹介した
4
章で少し登場しています。あらためて
Promise
のコンス
トラクタについて詳しく見てみましょう。
const promise = new Promise(function (resolve, reject) {
// ...
});
このように、
Promise
はコンストラクタの引数として、「
executor
関数」と呼ばれる関数を受け付け
ます。この関数はコンストラクタ
Promise
内部で呼び出され、インスタンス生成のコード内で、生成
中のプロミスの内部状態の一部のみを限定的に操作すること
を可能にするために使われます。言い方
を変えると、関数
resolve
と
reject
を公開しておいて、それらを呼び出せばオブジェクトの内部状
態を変えられるようにするための仕組みとして働いているのです。
このようにする利点は、コンストラクタのコードだけが
resolve
と
reject
にアクセスでき、いった
ん
promise
のオブジェクトが生成されてしまうと、安全に受け渡し可能になるということにあります。
他のコードからは
rej
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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.
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.