var a=2；与var a=b=2；的区别

var a=b=2  //相当于：
var a = 2;
b = 2;

	function fn(){
        var a = b = 2;
        console.log(a,b); // 2 2
    }
    fn();
    console.log(b) //2 b为全局变量
    console.log(a); //a is not undefined a为局部变量