对JS中x==y类型转换方向的探究

先来看一个简单的题目。

var a = "42";
var a = 42;
a == b;         //true

显而易见，a==b为true。可是，你再仔细想一想，通过强制转换a==b可以有两种方式给出true。这个比较要么最终成为42==42，要么成为”42”==”42”。于是问题来了，到底是哪一种呢？
答案：“42”变成42，于是比较成为42==42。在一个这样简单的例子中，只要最终结果是一样的，处理的过程走哪一条路似乎并不重要。但在一些复杂的情况下，这不仅对比较的结果很重要，而且对你如何得到这个结果也很重要。
就x==y而言，我查阅了ECMA文档，整理如下：

1. If Type(x) is the same as Type(y), then
   1. If Type(x) is Undefined, return true.
   2. If Type(x) is Null, return true.
   3. If Type(x) is Number, then
      1. If x is NaN, return false.
      2. If y is NaN, return false.
      3. If x is the same Number value as y, return true.
      4. If x is +0 and y is −0, return true.
      5. If x is −0 and y is +0, return true.
      6. Return false.
   4. 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.
   5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
   6. Return true if x and y refer to the same object. Otherwise, return false.
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 or Number 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 or Number, return the result of the comparison ToPrimitive(x) == y.
10. Return false.

是不是很清楚了呢？