javascript 强制编号
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Coerce to number
提问by Randomblue
I have often seen the trick
我经常看到这个伎俩
after = +after;
to coerce the variable afterto a number. Reading through the Node.JS sourceI found another method:
将变量强制after为一个数字。阅读Node.JS 源代码，我发现了另一种方法：
after *= 1; // coalesce to number or NaN
Are the two methods strictly equivalent in their behaviour?
这两种方法的行为是否严格等效？
回答by Fabrício Matté
Yes. Both Unary Operator +and Multiplicative Operatorssuch as *(called from Compound Assignmentop=) invoke the internal ToNumberalgorithm.
是的。两个一元运算符+和乘法运算符，例如*（来自称为复合赋值op=）调用内部ToNumber算法。
You can even use a 3rd option by statically calling the Numberconstructor:
您甚至可以通过静态调用Number构造函数来使用第三个选项：
after = Number(after);
回答by christopher
After a quick google to make sure my suspicions were true, I came to this conclusion. Using the +operator to convert to a number is faster, because no mathematical operations occur after type-casting, whereas using the *=approach means that after afteris converted, it will be multiplied by 1.
在快速谷歌以确保我的怀疑是真的之后，我得出了这个结论。使用+运算符转换为数字更快，因为类型转换后不会发生数学运算，而使用该*=方法意味着after转换后会乘以1。
回答by Wilco
Note: In some instances after = after-0invokes different behaviour than after = after+0. I've noticed it with dates.
注意：在某些情况下，after = after-0调用与after = after+0. 我已经注意到它与日期。
This is tested in Chrome v39 only:
这仅在 Chrome v39 中测试：
var date = new Date(2000,0,1);
date += date; //"Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)"
var date2 = new Date(2000,0,1);
date2 + 0; //"Sat Jan 01 2000 00:00:00 GMT+0000 (GMT Standard Time)0"
date2 - 0; //946684800000
date2 * 1; //946684800000
I don't know what is defined in the JS spec, but with dates, because both the date and the number can be cast to a string, and the + operator works on a string, then Chrome goes with a string concatenation. Because the - operator has no string equivalent, it falls back to number comparison.
我不知道 JS 规范中定义了什么，但是使用日期，因为日期和数字都可以转换为字符串，并且 + 运算符适用于字符串，然后 Chrome 使用字符串连接。因为 - 运算符没有等效的字符串，所以它回退到数字比较。
I've found this useful when coercing dates into numbers for comparisons
我发现这在将日期强制转换为数字进行比较时很有用
回答by Timmmm
Yes, however note that it is only the unary+ operator that does this. I.e. 10 + "10"will give you "1010".
是的，但是请注意，只有一元+ 运算符才能执行此操作。即10 + "10"会给你"1010"。
A perhaps less error prone option is to use what asm.js does:
一个可能不太容易出错的选项是使用 asm.js 所做的：
10 + ("10"|0)
Although on the down-side it does require the brackets. It should be the fastest option in any case (probably equal to unary +).
尽管在不利方面，它确实需要支架。在任何情况下它都应该是最快的选项（可能等于一元 +）。
