1. 数字，字符串，布尔，null， undefined 基本数据类型，传值
var a = "zhangxuchao";
var b = a;
b = "zhangxuchaoandheping";
console.log(a) "zhangxuchao";
console.log(b) "zhangxuchaoandheping";
2. 对象，数组，函数引用数据类型，传址
var a=[1,3,4];
var b=a;
a[0]=99;
console.log(a); [99, 3, 4]
console.log(b); [99, 3, 4]
