在 JS 中添加/减去时间 [英] adding / subtracting time in JS

问题描述

最近我在玩 JS，发现了一些有趣的东西.这就是我在 chrome 控制台中写的内容:

Lately I was playing with JS and I found something interesting. That is what I wrote into chrome console:

today = new Date()
-> Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)
DAY = 1000 * 3600 * 24
-> 86400000
today - 2 * DAY
-> 1367060761452
today + 2 * DAY
-> "Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)172800000"

而且我想知道为什么我会根据操作的类型得到不同类型的答案 - 加法/减法.当我做这样的事情时:

And I am wondering why am I getting different types of answer depending on the type of operation - adding / subtracting. When I do something like that:

today - (-2) * DAY

一切正常.有什么意识形态，还是bug?

everything is fine. Is there any ideology, or is it a bug?

推荐答案

这在规范中有点难找，因为它与 Date 的其余部分无关.

This is a bit tricky to find in the spec, because it's not with the rest of the Date stuff.

如果您查看 第 11.6.1 节，加法运算符"，您会发现以下注释:

If you take a look at section 11.6.1, "The Addition operator," you'll find the following note:

注意 1 在第 5 步和第 6 步中对 ToPrimitive 的调用中没有提供任何提示.除 Date 对象之外的所有原生 ECMAScript 对象都处理没有提示的情况，就像给出了提示 Number 一样；日期对象处理没有提示的情况，就像给出了提示字符串一样.宿主对象可能会以其他方式处理没有提示的情况.

NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

在上下文中，这意味着对 Date 对象使用加法运算符 (+) 将使用字符串值而不是数字值.从这个意义上说，Date 对象是特殊的，不同于任何其他类型的对象.

In context, that means using the addition operator (+) with a Date object will use the string value rather than the numeric value. In this sense, Date objects are special and unlike any other kind of objects.

请注意，减法运算符没有这样的例外，因为它是明确的 —它只适用于数字减法；它不对字符串进行操作.

Note that there is no such exception for the subtraction operator, as it is unambiguous — it only works for numeric subtraction; it doesn't operate on strings.

另请注意，这适用于加法运算符，即两边都有操作数的 +.一元加号"运算符不是这样工作的，所以 +myDateObj 在左侧没有任何内容将导致数值.

Also note that this applies to the addition operator, a + with operands on both sides. The unary "plus" operator does not work like this, so +myDateObj with nothing on the left hand side will result in the numeric value.

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