what are the rules for how == converts types?
关于"=="的比较规则：
1. Comparing numbers and strings will always convert the strings to numbers.
number类型与string类型比较，string会转换为number类型。如：2=="2" true
2. null and undefined will always equal each other.
null类型与undefined类型比较始终相等。如：var a = null，b; 则 a==b 为true。
3. Comparing booleans to any other type will always cause the booleans to be converted to numbers.
布尔类型与其他任何类型进行比较，布尔类型将会转换为number类型。如：var a = 0, b = false;则a==b为true
4. Comparing numbers or strings to objects will always cause the numbers or strings to be converted to objects.
number类型或者string类型与object类型进行比较，number或者string类型都会转换为object类型。如：var a = 0, b = {};则a==b为false
The rules for converting other types to booleans are actually relatively straightforward:
关于其他类型转换为布尔类型的规则：
1. undefined and null are always false.
2. Booleans are just treated as booleans (obviously).
3. Numbers are false if they equal 0 or NaN; otherwise, they’re true.
4. Strings are true, except for the empty string "", which is false.
5. Objects are always true.
原文：http://www.cnblogs.com/ywxgod/p/3969316.html