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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100
4
章
ES2015
以降の機能を使った非同期パターン
console.log('Loading...');
このコードには
getPageHtml
と
main
の
2
つの関数があります。
getPageHtml
はとても単純な関
数で、
URL
で指定されたウェブページの
HTML
コードを取得するものです。この関数はプロミスを戻
します。
関数
main
で
async
と
await
が使われています。まずこの関数の前にキーワード
async
が付いて
いる点に注目してください。この関数が非同期のコードを実行すること、本体でキーワード
await
が
使われる可能性があることを示します。
getPageHtml
の前にある
await
は、次の行を実行する前に
getPageHtml
によって戻されたプロミスの
resolve
を待つように処理系に指示します。このようにし
て関数
main
は非同期のコードが完了するまで内部的に一時停止します。残りのプログラムの普通の実
行をブロックすることはしません。実際、
Loading...
という文字列がコンソールに表示され、しばら
くすると
Google
のページの
HTML
コードが表示されることになります。
このようなアプローチのほうが、読みやすく理解しやすいでしょう。そして、以前は
Node
で利用す
るには
Babel
などのトランスパイラ（トランスコンパイラ）が必要でしたが、現在は標準でサポートされ
ています。
4.4
非同期プログラミングの手法の比較
ここで
JavaScript
の非同期処理について整理しておきましょう
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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.