April 2020
Intermediate to advanced
177 pages
1h 45m
Chinese
Content preview from 精通模块化JavaScript
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100
｜
第
4
章
内部构造
这种类型的操作应该设计得非常明确以防混淆，不过还是要尽可能避免这
种操作。如果认为函数就是输入与输出之间的方程式，这种不明智的想法
会带来很多负面影响。在函数体内修改输入就是其中一个例子，这样做可
能引入
bug
，令人困惑，而且难以追踪这些修改的源头。
我们经常能看到一些修改输入参数然后返回这些参数的函数。这在使用
Array#map
回调时较为常见，开发者想要修改列表中每个对象的一两个属性，
但还让原来的对象作为集合中的元素，如以下代码所示 ：
movies.map(movie => {
movie.profit = movie.gross - movie.budget
return movie
})
在这些情况中，最好避免同时使用
Array#map
，而是使用
Array#forEach
或
者
for..of
，如下所示 ：
for (const movie of movies) {
movie.profit = movie.gross - movie.budget
}
假设你想用类似“票房收入大于
1500
万美元”的条件过滤
movies
，但是
Array#forEach
和
for..of
都不支持链式调用，它们是纯粹的循环，不产
生任何输出。不过，这是一个好问题，因为它将
movie
层级的数据变化和
movies
层级的数据转换做了显式分离。在
movie
层级，我们给
movies
中的
每一项增加了一个
profit
属性，而在
movies
层级，我们希望产生一个全新
的只包含高票房电影的集合 ：
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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.