看見朋友們在討論一個問題，說 null 到底和 0 是不是相等的。
聽到這裡，去寫個 Demo 試試看。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></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>
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為什麼 console.log(null <= 0); 且 console.log(null >= 0); 這兩個的判斷是 true 呢？
首先我們來看看 ES3 關於 內部相等性運算的演算法實作。
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.
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接下來我們再來看 ES3 關於 內部關係運算的演算法實作。
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.
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ES3 的「>」運算子:
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).
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ES3 的」>=」運算子:
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.
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ES3 的「= =” 運算子:
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).
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根據資料得出的內容
關係運算子和相等運算子並不是一個類別的.
關係運算子,在設計上,總是需要運算元嘗試轉為一個number . 而相等運算子在設計上,則沒有這方面的考慮.
最重要的一點, 不要把拿a > b , a == b 的結果想當然的去和a >= b 建立聯繫. 正確的符合最初設計思想的關係是a > b 與a >= b是一組. a == b 和其他相等運算子才是一組. 例如a === b , a != b, a !== b .
那我們就可以反過來看這個問題了。
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.
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a >= b 運算子只是簡單的去對a < b的結果取反. 我以為這是一個設計上的失誤的另一個理由是undefined,在標準中，被單拎出來.細心的你，也一定發現了這一點. 對於undefined的設計, undefined > 0 , undefined < 0, undefined == 0 的結果是符合設計上,邏輯的一致性的. 而null是被遺漏的東西.直到今天早上.重新翻閱了ES3,5.相關章節. 才恍然大悟自己沒有從根本上理解到這個問題.
另外一個例子
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.登入後複製
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