May 2024
Intermediate
248 pages
6h 59m
Japanese
Content preview from 図解まるわかり 仮想化のしくみ
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216
クライアント側でのアプリケーションとして、「いつでもどこでも」実
行できるアプリケーションとして現在主流なのはJavaScript ではない
でしょうか。ブラウザやデバイスに搭載されているJavaScript エンジンに
よって、OSやデバイスを問わずさまざまなアプリケーションを実行可能
です。JavaScript以外の言語（CやC++など）はプログラムを実行するた
めにコンパイルして機械語に翻訳します。そのため、CPUアーキテクチ
ャー、OS の種別（Windowsや Linux）など、環境ごとに実行前にコンパイ
ルしておく必要があります。
一方でJavaScriptは構文解析によって「バイトコード」という中間コー
ドを生成し、その後各実行環境に合わせた機械語に変換することで、「い
つでもどこでも」を実現しています（図12-3）。
上記のようなしくみで「いつでもどこでも」を実現する JavaScript です
が、JavaScript を実行する際に必要な多数の共通プログラムをダウンロー
ドする必要があります。このため実装方法によっては、アプリケーション
の実行速度に問題が出るケースがありました。また、プログラム実行時に
JavaScriptから生成したバイトコードに基づき、Just In Compiler（JIT）で
コンパイルしており、JITコンパイル回数を重ねて実績に基づいて最適化
するため、コンパイルの実績から外れた処理ではパフォーマンスが安定し
ないなどの課題がありました（図12-4）
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