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(guī)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(xiàn)。
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(xiàn)。
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