js隐式转换及包装类型

var x = '33';

//很巧妙的类型转换
console.log(typeof (x - 0));//number
console.log(typeof (x + ''));//string

var str = new String();
console.log(str.length);//仍为0,调用的是String包装类的length属性

str.t = 1;

console.log(str.t);//undefined