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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50
3
章 コールバックを用いた非同期パターン
utilities.js
と依存モジュールが記述された
package.json
があるはずです。まず次のコマンドを
実行して
npm
パッケージをインストールしてください。
$ npm install
これで準備が整ったので、次のコマンドでアプリケーションを実行します。
$ node index https://www.example.com
このウェブスパイダーは
HTML
ファイルをダウンロードするだけで、画像などのリソースは取得し
ない点に注意してください。
3.1.2
コールバック地獄
関数
spider()
のコードは単純な内容なのに非同期
API
の呼び出しがネストして非常に読みにくいと
思います。同じアルゴリズムを
DS
（ダイレクトスタイル）の同期
API
を使って実装すれば、もっとすっ
きりしたコードになるでしょう。
このように、関数が幾段にもネストしてインラインで定義され、クロージャが多用されることで、
コードが保守が困
難な状態になることを「コールバック地獄」と呼びます。これは
Node
だけではなく、
JavaScript
全般において「アンチパターン」として認識されています。典型的なコールバック地獄のコー
ドは次のような形式になります。
asyncFoo( err => {
asyncBar( err => {
asyncFooBar( err => {
// ...
});
});
});
関数呼び出しがネストして横に突き出す様子がまるでピラミッドのようなので、「破滅の
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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.