老师我注释的那个this[name]=fn;是什么意思

    var OperationMethod = {

            //加法

            add: function (num1, num2) {

                calculatorElem.res.innerHTML = +calculatorElem.num1Ele.value + +calculatorElem.num2Ele.value;

            },

            //减法

            subtract: function (num1, num2) {

                calculatorElem.res.innerHTML = calculatorElem.num1Ele.value - calculatorElem.num2Ele.value;

            },

            //乘法

            multily: function (num1, num2) {

                calculatorElem.res.innerHTML = calculatorElem.num1Ele.value * calculatorElem.num2Ele.value;

            },

            //除法

            divide: function (num1, num2) {

                calculatorElem.res.innerHTML = calculatorElem.num1Ele.value / calculatorElem.num2Ele.value;

            },

            addOperation:function(name,fn){

               if(!this[name]){

           //这里的this[name]=fn;是什么意思      

this[name]=fn;

               }

        }