JavaScript算術表達式計算器的程式碼實現

第一步，實作createOperator函數，傳回閉包函數：

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.閉包函數的實例，擁有eval和toString兩個方法

2.eval方法負責計算算術表達式的值

3.toString方法將表達式的計算過程以字串形式展示

4.適用範圍僅限於二目運算子

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());

運算過程中的值都會用Value的形式儲存

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

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

createOperator函數的程式碼實作：

//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;
    }; 
})();

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