| NaN | Infinity | undefined | null | 0 | ‘‘ | false | |
| typeof | ‘number‘ | ‘number‘ | ‘undefined‘ | ‘object‘ | ‘number‘ | ‘string‘ | ‘boolean‘ |
| IF判断 | else | if | else | else | else | else | else |
| String() | ‘NaN‘ | ‘Infinity‘ | ‘undefined‘ | ‘null‘ | ‘0‘ | ‘‘ | ‘false‘ |
| Number() | NaN | Infinity | NaN | 0 | 0 | 0 | 0 |
number与string相加，number会转换为string后再运算，number与string做减、除、乘法，则string会转换为number后再运算。
1 + ‘ ‘ = ‘1‘; 1 - ‘ ‘ = 1; 1 * ‘ ‘ = 0; 1 / ‘ ‘ = Infinity;
原文：http://www.cnblogs.com/frnd0707/p/4779726.html