能不能帮我捋一捋这段代码的逻辑

     var operate = (function () {

      var operation = {

    add: function(num1, num2) {

    return +num1 + +num2;

    },

subtract: function(num1, num2) {

    return num1 - num2;

    },

multiply: function(num1, num2) {

    return num1 * num2;

    },

divide: function(num1, num2) {

    return num1 / num2;

    },

    addOperation: function (name, fn) {

    if (!operation[name]) {

    operation[name] = fn;

    }

    return operation;

    }

    };

      function operate(name, num1, num2) {

      if (!operation[name]) throw new Error('不存在名为' + name + '的运算方法！');

      return operation[name](num1, num2);

     }

    function operate(name) {

    if (!operation[name]) throw new Error('不存在名为' + name + '的运算方法！');

    return operation[name].apply(operation, [].slice.call(arguments, 1, arguments.length));

    }

    operate.addOperation = operation.addOperation;

    return operate;

     return function (name, num1, num2) {

      if (!operation[name]) throw new Error('不存在名为' + name + '的运算方法！');

      return operation[name](num1, num2);

     };