JavaScript ECMA-ES3 相等(==)和严格相等(===)运算符详解

本文以JavaScript ES3为例谈谈相等( == )运算符、严格相等( === )运算符判断逻辑，探探其背后到底是怎么工作的。

1、抽象相等 ==

参考ECMA-262-3说明，The comparison x == y, where x and y are values, produces true or false. Such a comparison is
performed as follows:

If Type(x) is different from Type(y), go to step 14.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is not Number, go to step 11.
If x is NaN, return false.
If y is NaN, return false.
If x is the same number value as y, return true.
Ifxis+0andyis−0,returntrue.
Ifxis−0andyis+0,returntrue.
Return false.
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.
If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
Return true if x and y refer to the same object or if they refer to objects joined to each other (see13.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.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
Return false.

一个简单的 == 运算符，其背后有22步，真是看似简单其实一点也不简单呢，下面我们以几个demo来讲解 == 运算符到底是怎么工作的，demo来自《JavaScript权威指南第6版》
案例1：

null == undefined		// true

说明：
1、Type(null) = Null, Type(undefiend) = Undefiend，两者不等，跳到14
14、If x is null and y is undefined, return true，满足要求，返回true，比较结束

案例2：

"0" == 0		// true

说明：
1、Type(“0”) = String, Type(0) = Number，两者不等，跳到14
14、If x is null and y is undefined, return true，不满足
15、If x is undefined and y is null, return true，不满足
16、If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y)，不满足
17、If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y. 符合条件，ToNumber(“0”) = 0, 转换成 0 == 0，进行新一轮比较。

1、Type(0) = Number, Type(0) = Number，两者相等，继续判断
2、If Type(x) is Undefined, return true，不满足，继续
3、If Type(x) is Null, return true，不满足，继续
4、If Type(x) is not Number, go to step 11，不满足，继续
5、If x is NaN, return false，不满足，继续
6、If y is NaN, return false，不满足，继续
7、If x is the same number value as y, return true，满足，返回true，结束

案例3：

0 == false		// true

说明：
1、Type(0) = Number, Type(false) = Boolean，两者不等，跳到14
14、If x is null and y is undefined, return true，不满足
15、If x is undefined and y is null, return true，不满足
16、If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y)，不满足
17、If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y，不满足
18、If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y，不满足
19、If Type(y) is Boolean, return the result of the comparison x == ToNumber(y)，满足，ToNumber(false) = 0，变成 0 == 0，同案例2的新一轮比较，返回true

案例4：

"0" == false		// true

2、严格相等 ===

参考ECMA-262-3说明，The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is not Number, go to step 11.
If x is NaN, return false.
If y is NaN, return false.
If x is the same number value as y, return true.
Ifxis+0andyis−0,returntrue.
Ifxis−0andyis+0,returntrue.
Return false.
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.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object or if they refer to objects joined to each other (see13.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.

看几个案例，进一步说明：
案例1：

null === undefined		// false

说明：
1、Type(null) = Null, Type(undefiend) = Undefiend，两者不等，满足要求，返回false，结束.

案例2：

"0" === 0		// false

说明：
1、Type(“0”) = String, Type(0) = Number，两者不等，满足要求，返回false，结束.

案例3：

var a = 1, b = Number(1);
console.log(a===b);		//true

说明：
1、Type(a) = Number, Type(b) = Number，两者相等，不满足，继续.
2、If Type(x) is Undefined, return true，不满足，继续
3、If Type(x) is Null, return true，不满足，继续
4、If Type(x) is not Number, go to step 11，不满足继续
5、If x is NaN, return false，不满足，继续
6、If y is NaN, return false，不满足，继续
7、If x is the same number value as y, return true，满足，返回true，结束

案例4：

var a = '123', b = String('123');
console.log(a===b);		//true

说明：
1、Type(a) = String, Type(b) = String，两者相等，不满足，继续.
2、If Type(x) is Undefined, return true，不满足，继续
3、If Type(x) is Null, return true，不满足，继续
4、If Type(x) is not Number, go to step 11，满足，跳到11
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，字符串’123’,'123’长度和序列都一致，因此返回true，结束。

案例5：

var a = {name:'marcus'}, b = {name:'marcus'};
console.log(a==b);		//false

说明：
1、Type(a) = Object, Type(b) = Object，两者相等，不满足，继续.
2、If Type(x) is Undefined, return true，不满足，继续
3、If Type(x) is Null, return true，不满足，继续
4、If Type(x) is not Number, go to step 11，满足，跳到11
11、If Type(x) is String，不满足，继续
12、If Type(x) is Boolean，不满足，继续
13、Return true if x and y refer to the same object or if they refer to objects joined to each other (see13.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，
满足，a和b都是新建的对象，两者不是指向同一个对象，因此返回false，结束

3、总结

相等( == )和严格相等( === ，有的地方也称恒等)，两者最大的区别就是相等运算符可能会进行类型转换，严格相等不会进行类型转换。