在javascript中，null＞=0 为真，null==0却为假，null的值详解

2024-05-06 15:10:59

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1.前言

今天看见朋友们在讨论一个问题，说 null 到底和 0 是不是相等的。

听到这里，自己赶紧去写个 Demo 试一下。

<!DOCTYPE html><html lang="en"><head>  <meta charset="UTF-8">  <title>MR_LP:3206064928</title></head><body></body><script type="text/javascript">  console.log(null > 0);   // false  console.log(null < 0);   // false  console.log(null >= 0);   // true  console.log(null <= 0);   // true  console.log(null == 0);   // false  console.log(null === 0);    // false</script></html>

什么情况？

为什么 console.log(null <= 0); 和 console.log(null >= 0); 这两条的判断是 true 呢？

2.查阅资料

如果想明确，这个问题具体是怎么回事，那么我们需要重新来回顾一下我们的 ECMAScript Language Specification (HTML version)，翻译过来就是ECMAScript语言规范(HTML版本)。

2.1 内部相等性运算算法

首先我们来看一下 ES3 关于内部相等性运算的算法实现。

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. 2. If Type(x) is Undefined, return true. 3. If Type(x) is Null, return true. 4. If Type(x) is not Number, go to step 11. 5. If x is NaN, return false. 6. If y is NaN, return false. 7. If x is the same number value as y, return true. 8. If x is +0 and y is -0, return true. 9. If x is -0 and y is +0, return true. 10. Return false. 11. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false. 12. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false. 13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false. 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.

2.2 内部关系运算算法

接下来我们再来看一下 ES3 关于内部关系运算的算法实现。

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