javascript - +0和-0一样吗？

Question

通读 ECMAScript 5.1 specification ，+0和-0是区分的。

为什么+0 === -0 的计算结果为true？

Answer 1

JavaScript 使用 IEEE 754 standard来表示数字。来自 Wikipedia :

Signed zero is zero with an associated sign. In ordinary arithmetic, −0 = +0 = 0. However, in computing, some number representations allow for the existence of two zeros, often denoted by −0 (negative zero) and +0 (positive zero). This occurs in some signed number representations for integers, and in most floating point number representations. The number 0 is usually encoded as +0, but can be represented by either +0 or −0.

The IEEE 754 standard for floating point arithmetic (presently used by most computers and programming languages that support floating point numbers) requires both +0 and −0. The zeroes can be considered as a variant of the extended real number line such that 1/−0 = −∞ and 1/+0 = +∞, division by zero is only undefined for ±0/±0 and ±∞/±∞.

本文包含有关不同表示的更多信息。

因此，从技术上讲，这就是必须区分两个零的原因。

However, +0 === -0 evaluates to true. Why is that (...) ?

此行为在 section 11.9.6 中明确定义，严格相等比较算法(强调部分是我的):

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

(...)

• If Type(x) is Number, then

  1. If x is NaN, return false.
  2. If y is NaN, return false.
  3. If x is the same Number value as y, return true.
  4. If x is +0 and y is −0, return true.
  5. If x is −0 and y is +0, return true.
  6. Return false.

(...)

(顺便说一句，+0 == -0 也是如此。)

从逻辑上看，将+0 和-0 视为相等。否则，我们必须在代码中考虑到这一点，而我个人不想这样做;)

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注意:

ES2015引入了新的比较方法，Object.is 。 Object.is 明确区分 -0 和 +0:

Object.is(-0, +0); // false