var a='我是大哥';
var b='我是二哥';
var c='我是三弟';
var x=Math.floor(Math.random()*10+1);//1-10之間隨機(jī)數(shù)
switch(x)
{
case 1:
case 2:alert(a);break; //1和2時(shí),彈出a,正好百分之20
case 3:
case 4:
case 5:alert(b);break; //3、4、5時(shí)彈出b,百分之30
case 6:
case 7:
case 8:
case 9:
case 10:alert(c);break; //剩下的百分之50
}