How to implement validation with encapsulation

Question

Please first see original question: Encapsulation in JavaScript with getter and setter

@Jacob Thanks Jacob! That is great information.I am not quite sure how that solution works but placing the methods into that return clause works well. Here is my working Class definition:

function vehicle(thewheels, thecolour){
    var colour=thecolour;
    var wheels=thewheels > 4 ? '4' : thewheels;
    return {
            getcolour: function() {
            return colour; 
        },
            setcolour: function(value) { 
            colour = value;
        },
            getwheels: function() {
            return wheels; 
        },
            setwheels: function(value) { 
            wheels = value;
        }
    }
}

I have put some code into the constructor (presumably it could be more complex code) to process input data. I could place the same code into the 'setwheels' method in order to fully 'gate-keep' the data coming in; BUT surely there must be a simpler way of managing the input data without having to duplicate that code?

I tried placing this new function into the Class definition:

setwheels: function(value) { 
    wheels = validwheels(value);
},
validwheels: function(wheelsin){
    return wheelsin > 4 ? 4 : wheelsin;
}

But the constructor could not see that 'validwheels' function. Is there any way of re-using validation code for instantiation and 'setting' in this class?

Why not use actual property accessors? get colour() { - developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — evolutionxbox
– evolutionxbox, Commented Nov 22, 2017 at 9:30
@evolutionxbox Do you mean, the following (if I rename the private properties to _colour and _wheels): get colour(){return _colour;} If so then how do I access this? — Rich
– Rich, Commented Nov 22, 2017 at 9:47
I've made a short example of my suggestion here: jsbin.com/jaqiziliya/edit?js,console — evolutionxbox
– evolutionxbox, Commented Nov 22, 2017 at 9:55
As for the validwheels function, don't put it in the return object. You can just put it in the vehicle function body. — evolutionxbox
– evolutionxbox, Commented Nov 22, 2017 at 9:59

Rich · Accepted Answer · 2017-11-22 11:37:58Z

Here is a complete solution demonstrating encapsulation and validation. Thanks go to @Jacob and @evolutionxbox for their great assistance!

     <button onclick="testOOP()">Click me</button>

     <script>
       //<!-- 
       function testOOP(){
        var v1 = new vehicle(40, "red"); //setting new values during instantiation
        var v2 = new vehicle(2, "blue");
        showVehDetails(v1);
        showVehDetails(v2);
        v2.wheels=10;           //validated input restricted to 4
        showVehDetails(v2);
        v2.colour="orange";     
        showVehDetails(v2);
        }

        function showVehDetails(v){
            document.write("This vehicle is " + v.colour + " and has " + v.wheels + " wheels.<br/>");
        }

        //*************'vehicle' - Class definition**************
function vehicle(thewheels, thecolour){
    var colour=thecolour;
    var wheels=validWheels(thewheels);
    function validWheels(wheelsin){
        return wheelsin > 4 ? 4 : wheelsin;
    }
    return {
            get colour() {
            return colour; 
        },
            set colour(value) { 
            colour = value;
        },
            get wheels() {
            return wheels; 
        },
            set wheels(value) { 
            wheels = validWheels(value);
        }
    }
} 
        //************End class definition************************
       //-->
     </script>

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