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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6.1
ファクトリ
149
て新しいインスタンスを生成してもよいですし、プロトタイプと演算子
new
を使っても、
Object.
create()
を使っても、さらには特定の条件下では異なるインスタンスを返すことさえ可能です。利
用者側では、生成方法についてはまったく関知しません。
new
を使うとオブジェクトの生成法が限定
されてしまいますが、
JavaScript
では特別な負担なくもっとずっと柔軟にできるのです。ちょっとし
た例として、次のような
Image
のオブジェクトを生成する単純なファクトリを考えてみましょう（
01_
factory_simple_a/factory.js
）。
function createImage(name) {
return new Image(name);
}
const image = createImage('photo.jpeg');
ファクトリ
createImage()
は不要ではないかと思うかもしれません。なぜ
new
を使ってクラス
Image
から直接インスタンス化しないのでしょうか。次のようなコードではいけないのでしょうか？
const image = new Image(name);
既に述べたように、
new
を使うと、コードは特定のタイプのオブジェクトに縛られてしまいます。上
の例では
Image
のオブジェクトです。ファクトリならばもっとずっと柔軟にできます。クラス
Image
を
改変して小さいクラスに分割し、サポートするフォーマットにそれぞれ対応するようにしようと思った
とします
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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.