<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script type="text/javascript">
function a(x,y){
return(x+y);
};
function b(x,y){
return(x-y);
};
function c(x,y){
return(x*y);
};
function d(x,y){
return(x/y);
};
function fn(x,y,m){
return m(x,y); //执行形参回调的函数
};
var re=fn(12,4,b); //第3个参数是代入一个函数名作为实参，再传给fn函数的m形参，
alert(re);
</script>
</head>
<body>
</body>
</html>