在javascript中，null＞=0 为真，null==0却为假，null的值详解
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。
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)). 6. If Result(4) is NaN, return undefined. 7. If Result(5) is NaN, return undefined. 8. If Result(4) and Result(5) are the same number value, return false. 9. If Result(4) is +0 and Result(5) is -0, return false. 10. If Result(4) is -0 and Result(5) is +0, return false. 11. If Result(4) is +∞, return false. 12. If Result(5) is +∞, return true. 13. If Result(5) is -∞, return false. 14. If Result(4) is -∞, return true. 15. If the mathematical value of Result(4) is less than the mathematical value of Result(5) — note that these mathematical values are both finite and not both zero — return true. Otherwise, return false. 16. If Result(2) is a prefix of Result(1), return false. (A string value p is a prefix of string value q if q can be the result of concatenating p and some other string*r*. Note that any string is a prefix of itself, because r may be the empty string.) 17. If Result(1) is a prefix of Result(2), return true. 18. Let k be the smallest nonnegative integer such that the character at position k within Result(1) is different from the character at position k within Result(2). (There must be such a k, for neither string is a prefix of the other.) 19. Let m be the integer that is the code point value for the character at position k within Result(1). 20. Let n be the integer that is the code point value for the character at position k within Result(2). 21. If m < n, return true. Otherwise, return false.
2.3 ES3 的 运算符
2.3.1 ES3 的 “>” 运算符: