Underscore.js 中的加号运算符是什么意思? [英] What does plus operator mean in Underscore.js?

问题描述

当我看到 Underscore.js(1.4.3 版)代码时，我看到了以下几行(79 行)

While I was seeing Underscore.js (version 1.4.3) code, I saw following lines (79 line)

   if (obj.length === +obj.length) {
        for (var i = 0, l = obj.length; i < l; i++) {
            if (iterator.call(context, obj[i], i, obj) === breaker) return;
        }
    }

我不明白为什么在 if 语句中使用 + 运算符.(+obj.length)
而且，这句话不总是真实吗?
我不认为这是一个错字.必须有一些目标才能使用它.如果有人知道这种用法的好处，我想在将来使用它.
谢谢.

I didn't understand why + operator used within if statement. (+obj.length)
And, isn't it this statement always true?
I don't think it's a typo. There must be some aim to use that. If anyone knows benefits of this usage, I want to use it in future.
Thanks.

推荐答案

obj.length 可以是任何类型 - 例如 undefined.+obj.length 总是一个数字.

obj.length might be any type - such as undefined. +obj.length is always a number.

所以代码基本上会检查 length 属性是否存在并且是一个数字.这个检查的原因是 _.each() 接受数组和非数组对象.在数组的情况下，length 属性是迭代其元素所必需的，而 for..in 循环是非数组对象的方法.

So the code basically checks if the length property exists and is a number. The reason for this check is that _.each() accepts both arrays and non-array objects. In case of an array the length property is necessary to iterate over its elements while a for..in loop is the way to go in case of a non-array object.

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