一.string是有效数字字符的字符串("123")
1.sting类型的自增,自减运算string++ => number
2.string+string/number=string
3.string-string/number = number
4.string*string/number = number
5.string+true=string
6.string-true=number;
二.string不是有效数字字符的字符串("jjj")
string-string/number =NAN