var a = [1,2,3];
function map(fn, a, init){
var s = init;
for (i = 0; i < a.length; i++){
s = fn(s,a[i]);
}
return s;
}
alert(map(function(x,y){return x+y;}, a , 0)) //把数组的各个元素相加求和
alert(map(function(x,y){return x+y;}, a , "")) //把数组的各个元素连接起来