使用JavaScript私有方法：

Question

I am following Douglas Crockford's tutorial on the visibility of JavaScript variables and functions here : http://javascript.crockford.com/private.html

I have written the following MyClass.js file and I am running it from the terminal using node. Below I show my output on the terminal and my class. I do not understand why I get returned an "undefined" (instead of just true) and also, why my console.log(log) is not showing anywhere?

$ node MyClass.js 
undefined
true

And my class

function MyClass(log) 
{
    this.log = log;
        var that = this; 
        function _evaluate (log)
        {
             console.log(log);
             return true;
        };

    this.evaluate = function() 
    {
    return _evaluate() ? true : false;
    };

}

 var myObject = new MyClass('this is a test');
 console.log(myObject.evaluate());

Answer 1

You call _evaluate with no arguments, so it prints undefined once (line 7), then true (line 19).

Finally it prints undefined because your script as a whole has no return value.