作用域

var a =10
function sum(){
	a = 5;
	}
alert(a);
sum();s
alert(a);
	
var a = 10;
function sum(){
  var a = 5;
  alert(a);
	}
	alert(a);
	sum();
	alert(a);*/
	
	function sum(){
		var a = 5;
		alert(a);
	}
    sum();
	alert(a);