javascript - 为什么JavaScript中的null大于-1，小于1，但不等于(==)0？那到底是什么？

Question

从 Google Chrome 控制台:

var x = null;
undefined
x > 0
false
x < 0
false
x > -1
true
x < 1
true
x == 1
false
x === 1
false

Answer 1

当您将 null 与 0 进行相等性比较时，结果为 false。如果你强制null在数字上下文中进行解释，然后将其视为 0，结果变为 true。

您可以通过输入 + 强制它为数字在前面，或使用像 < 这样的数字运算符, <= , > , 和 >= .注意如何 null >= 0和 null <= 0都是真的。

> null == 0
false
> +null == 0
true
> null >= 0
true
> null <= 0
true

---

ECMAScript Language Specification定义何时执行所谓的“ToNumber”转换。如果是，则 null 和 false 都转换为 0。

§9.1 Type Conversion and Testing:

Table 14 — To Number Conversions

Argument Type     Result
-------------     ------
Undefined         Return NaN
Null              Return +0
Boolean           Return 1 if argument is true. Return +0 if argument is false.
Number            Return argument (no conversion).
String            See grammar and note below.

知道何时应用 ToNumber 转换取决于相关运算符。对于关系运算符 < , <= , > , 和 >=见:

§11.8.5 The Abstract Relational Comparison Algorithm:

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). Such a comparison is performed as follows:

1. Call ToPrimitive(x, hint Number).

2. Call ToPrimitive(y, hint Number).

3. If Type(Result(1)) is String and Type(Result(2)) is String, go to step 16. (Note that this step differs from step 7 in the algorithm for the addition operator + in using and instead of or.)

4. Call ToNumber(Result(1)).

5. Call ToNumber(Result(2)).

==运营商不同。它的类型转换描述如下。请注意 null 和 false 如何遵循不同的规则。

§11.9.3 The Abstract Equality Comparison Algorithm

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

1. If Type(x) is different from Type(y), go to step 14.

...

14. If x is null and y is undefined, return true.

15. If x is undefined and y is null, return true.

16. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).

17. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.

18. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

20. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).

21. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.

22. Return false.

如果你仔细阅读，你就会明白为什么 false == 0是真的但是null == 0是假的。

• false == 0 , Type(x) 是 bool 值。这意味着应用步骤 18 的类型转换，并将 false 转换为数字。 ToNumber(false) 为 0，并且 0 == 0为真，所以比较成功。

• null == 0 , Type(x) 为 Null。没有任何类型检查匹配，因此比较会进入第 22 步，返回 false。比较失败。