在javascript中，null＞=0 為真，null==0卻為假，null的值詳解
1.前言
今天看見朋友們在討論一個問題，說 null 到底和 0 是不是相等的。
聽到這里，自己趕緊去寫個 Demo 試一下。
<!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>
什么情況？
為什么 console.log(null <= 0); 和 console.log(null >= 0); 這兩條的判斷是 true 呢？
2.查閱資料
如果想明確，這個問題具體是怎么回事，那么我們需要重新來回顧一下我們的 ECMAScript Language Specification (HTML version)，翻譯過來就是ECMAScript語言規范(HTML版本)。
2.1 內部相等性運算算法
首先我們來看一下 ES3 關于 內部相等性運算的算法實現。
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 內部關系運算算法
接下來我們再來看一下 ES3 關于 內部關系運算的算法實現。
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 運算符
2.3.1 ES3 的 “>” 運算符:
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).
2.3.2 ES3 的”>=” 運算符:
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.
2.3.3 ES3 的 “==” 運算符 :
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).
3. 根據資料得出的內容
著重看一下，上面特意加粗的地方，我們可以明確下面三件事。
那么我們就可以反過來看這個問題了。
null > 0 // null 嘗試轉型為number , 則為0 . 所以結果為 false, null >= 0 // null 嘗試轉為number ,則為0 , 結果為 true. null == 0 // null在設計上，在此處不嘗試轉型. 所以 結果為false.
這里引用一下 Franky大大的話。
a >= b 運算符只是簡單的去對 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. 對于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.我重新翻閱了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.
雖然前面的例子,我catch到了BE當初的設計思想. 但是從全局的角度來看. 從關系運算符到相等運算符，尤其是相等運算符的設計上. 真的十分混亂不堪. BE在信中提到,他對 == 的現狀也很無奈. 甚至用愚蠢這個詞來形容自己當初的實現(當然他還提到,當初只是為了在10天內設計出js,并跑過qa的測試用例). 即使如此, 但是他仍然表示 null == 0 這個結果是他想要的.
好吧,到了這里,我也有種無力感. 我認為縱觀JavaScript，對關系運算和相等運算的設計.除了混亂,我想不出還有什么詞來形容它們更恰當. 這一點從，我們生產環境代碼中,大量的類型檢查,和防御性代碼的的存在,就可以證明這一點.
同時 Franky大大還舉了另外一個例子。
function case1(a){ if(a == null){ .... }} function case2(a){ if(a == undefined){ ... } } // 上面兩組完全等價, 這就是一種不明確表述.// 我們永遠不知道代碼編寫者的目的到底是同時匹配null 和 undefined還是只匹配其中某一個 function case3(a){ if(a === null || a === undefined){ ... }} // case3 才是最好的表述. 我們明確知道代碼編寫者的意圖. // 即使很多人可能認為這個代碼很愚蠢. 但我堅定的認為這才是好代碼.最后, 不得不提到,我發出null >= 0 這封信后, Andrea Giammarchi 表示了對我之前看法的支持,他同我最初的看法一樣,認為 null >= 0 的結果應該為 false . 并建議在 ES7 中的嚴格模式中,修改這個結果. 雖然同樣遭到 David Bruant 的反對. 好吧為他和我的這個錯誤看法,默哀一分鐘…
4.后記
所以寫代碼,寫規范,都應該明確表述. 即使表述的很羅嗦,但不會引起歧義或懷疑. 這才是一份好的標準.文檔,代碼. 而避免歧義，和各種混亂不堪的規則,是一門語言最應遵守的設計原則.
最后也希望大家在日常的開發中，能夠少遇坑。
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