var Add = createOperator("+" , function(a,b){return a + b;});
var Minus = createOperator("-" , function(a,b){return a - b;});
var Mul = createOperator("*" , function(a,b){return a * b;});
var pide = createOperator("/" , function(a,b){return a / b;});

1.闭包函数的实例，拥有eval和toString两个方法

2.eval方法负责计算算术表达式的值

3.toString方法将表达式的计算过程用字符串形式展示

4.适用范围仅限于二目运算符

var a = new Add(new Value(3), new Value(5));
//8
console.log(a.eval());
//"3 + 5"
console.log(a.toString());

var b = new Mul(new Value(6), new Value(2));
//12
console.log(b.eval());
//"6 * 2"
console.log(b.toString());

var c = new Add(a,b);
//20
console.log(c.eval());
//"3 + 5 + 6 * 2"
console.log(c.toString());

运算过程中的值都用Value的形式存储

function Value(value){
 this.value = value || 0;
}

Value.prototype.toString = function(){ 
	return this.value.toString(); 
};

createOperator函数的代码实现：

//IIFE
var createOperator = (function() {
	//name:"+","-","*","/"
	//oper:对应的加减乘除函数
 return function(name, oper){
	//闭包函数
 var Foo = function(){
	//获取2个操作数
 var args = arguments;
 var nums = [].slice.call(arguments);
 nums = nums.map(function(e){return e.value;}); 
 var val = new Value(oper.apply(null,nums));
	//给实例绑定toString和eval方法
 val.toString = function(){
 return args[0].toString() + " " + name + " " + args[1].toString();
 };
	val.eval = function(){
 return this.value;
 };
 return val;
 }; 
 return Foo;
 }; 
})();