JS的相等比较运算与类型转换

在JavaScript中，相等有两种比较方式：普通相等比较（==，在ECMAScript标准中称为“抽象相等比较”）和 严格相等比较（===）。
对于被广泛使用的普通相等比较运算符来讲，不同类型的操作数在进行比较之前，会尝试将两个操作数转换成相同的类型，再作比较运算。而对于严格相等比较运算符来讲，仅当两个操作数的类型相同且值相等时，返回为true。

抽象相等比较

ECMAScript标准的算法细节如下：

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 the same as Type(y), then
   
   1. Return the result of performing Strict Equality Comparison x === y.
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.
4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
8. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
9. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
10. Return false.

翻译如下：
1.如果Type(x)与Type(y)相同，执行严格相等运算x === y；
2.如果x是null，y是undefined，返回true；
3.如果x是undefined，y是null，返回true；

console.log(null == undefined);