javascript - 无穷大 - 无穷大 = NaN？

Question

任何数字减去它本身应该是 0，对吗？

3 - 3 === 0

那为什么

Infinity - Infinity === NaN

因为 typeof Infinity 是 'number':

Answer 1

我们知道，两个数字之间的差可以这样计算

a - b = a + (-b)

JavaScript 使用它来查找两个值之间的差异。引自 Applying the Additive Operators to Numbers section from ECMA 5.1 Specification ,

The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that a–b produces the same result as a +(–b).

所以，当你这样做的时候

Infinity - Infinity

它被评价为

Infinity + (-Infinity)

在 JavaScript 中，它们都是不同的对象。引自 The Number Type section of ECMA 5.1 Specification ,

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞ and −∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

再次引用 Applying the Additive Operators to Numbers section from ECMA 5.1 Specification

• If either operand is NaN, the result is NaN.
• The sum of two infinities of opposite sign is NaN.
• The sum of two infinities of the same sign is the infinity of that sign.
• ...

这就是结果为 NaN 的原因。