JavaScript中null与0的比较：为什么null>=0为真而null==0为假

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  <meta charset="UTF-8">
  <title>MR_LP:3206064928</title>
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<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>

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.

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.

The Greater-than Operator ( > ) 
The production RelationalExpression : 
RelationalExpression > ShiftExpression is evaluated as follows: 
1. Evaluate RelationalExpression. 
2. Call GetValue(Result(1)). 
3. Evaluate ShiftExpression. 
4. Call GetValue(Result(3)). 
5. Perform the comparison Result(4) < Result(2). 
6. If Result(5) is undefined, return false. Otherwise, return Result(5).

The Greater-than-or-equal Operator ( >= ) 
The production RelationalExpression : 
RelationalExpression >= ShiftExpression is evaluated as follows: 
1. Evaluate RelationalExpression. 
2. Call GetValue(Result(1)). 
3. Evaluate ShiftExpression. 
4. Call GetValue(Result(3)). 
5. Perform the comparison Result(2) < Result(4). (see 11.8.5). 
6. If Result(5) is true or undefined, return false. Otherwise, return true.

The Equals Operator ( == ) 
The production EqualityExpression : 
EqualityExpression == RelationalExpression is evaluated as 
follows: 
1. Evaluate EqualityExpression. 
2. Call GetValue(Result(1)). 
3. Evaluate RelationalExpression. 
4. Call GetValue(Result(3)). 
5. Perform the comparison Result(4) == Result(2). (see 11.9.3). 
6. Return Result(5).

null > 0   // null 尝试转型为number , 则为0 . 所以结果为 false, 
null >= 0  // null 尝试转为number ,则为0 , 结果为 true. 
null == 0  // null在设计上，在此处不尝试转型. 所以 结果为false. 

function case1(a){
  if(a == null){
     ....
  }
} 
 
function case2(a){
  if(a == undefined){
    ...
  }  
}
 
// 上面两组完全等价, 这就是一种不明确表述.
// 我们永远不知道代码编写者的目的到底是同时匹配null 和 undefined还是只匹配其中某一个
 
 
function case3(a){
  if(a === null || a === undefined){
    ...
  }
}
 
// case3 才是最好的表述. 我们明确知道代码编写者的意图. 
// 即使很多人可能认为这个代码很愚蠢. 但我坚定的认为这才是好代码.