April 2025
Intermediate to advanced
448 pages
6h 50m
Japanese
Content preview from Effective TypeScript 第2版 ―型システムの力を最大限に引き出す83項目Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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1章TypeScriptとは何か
詳細に入る前に、この章ではTypeScriptの全体像を説明します。TypeScriptとは何なのか、JavaScriptとどう関係しているのか、その型はnullable（null許容）なのか、anyとは何か、ダックタイピングを採用しているのか。
TypeScriptは、（PythonやRubyのように）インタプリタで実行されるわけでもなく、（JavaやCのように）低レベルの言語にコンパイルされるわけでもないという点で、特異な言語です。TypeScriptは、別の高級言語であるJavaScriptにコンパイルされます。実行されるのはこのJavaScriptであって、TypeScriptではありません。そのため、TypeScriptとJavaScriptの関係を理解することはきわめて重要であり、より優れたTypeScript開発者になるために役立ちます。
TypeScriptの型システムにも、知っておくべき特異な性質があります。型システムについては後の章でより詳しく説明しますが、この章ではもっとも重要なハイライトをいくつか取り上げます。
すでに多くのTypeScriptを書いたことがある人でも、この章は読むべきです。TypeScriptとは何か、TypeScriptの型システムはどのように動作するのかについて、正しいメンタルモデルを構築する助けになりますし、自分でも気づかなかった誤解が解けるかもしれません。
項目1 TypeScriptとJavaScriptの関係を理解する
TypeScriptを長く使っていると、「TypeScriptはJavaScriptのスーパーセットだ」とか、「TypeScriptはJavaScriptの型付きスーパーセットだ」という言葉を必ず耳にします。これは具体的にどういう意味なのでしょう？ ...
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