Javascript builder pattern using private variables

Question

I'm trying to create a builder pattern in Javascript that uses private variables, while providing one public accessor (fullName) that returns a mashup of all the other properties. This question and answer suggests that I can use Object.defineProperty inside the person constructor in order to access private variables, but it doesn't work - instance.fullName is always undefined.

How can I get this working so that the builder pattern variables remain private, but the public accessor has access to them throughout the build chain?

var Person = function () {
    var _firstName, _lastName

    Object.defineProperty(this, "fullName", {
        get: function () {
            return _firstName + ' ' + _lastName;
        }
    });

    return {
        firstName: function (n) {
            _firstName = n
            return this
        },
        lastName: function (n) {
            _lastName = n
            return this
        }
    }
}

var x = new Person().firstName('bob').lastName('dole');

console.log(x.fullName); // always undefined

I suspect it's because the object being returned from Person is different from the this on which Object.defineProperty() is called. — Matt Ball
– Matt Ball, Commented Nov 27, 2015 at 19:38
Hmmmmm yeah, frankly Object.defineProperty() doesn't really feel like the right way to go about this anyway. — brandonscript
– brandonscript, Commented Nov 27, 2015 at 19:41
If I have to do it with public properties, that's fine, but even using this._firstName etc. doesn't work with this design. — brandonscript
– brandonscript, Commented Nov 27, 2015 at 19:48

Matt Ball · Accepted Answer · 2015-11-27 19:49:31Z

3

As per my comment, change the object passed to defineProperty():

var Person = function () {
    var _firstName, _lastName

    var _self = {
        firstName: function (n) {
            _firstName = n
            return this
        },
        lastName: function (n) {
            _lastName = n
            return this
        }
    }

    Object.defineProperty(_self, "fullName", {
        get: function () {
            return _firstName + ' ' + _lastName;
        }
    });

    return _self;
}

var x = new Person().firstName('bob').lastName('dole');

console.log(x.fullName); // bob dole

http://jsfiddle.net/mattball/peztf9qs/

answered Nov 27, 2015 at 19:49

Matt Ball

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1 Comment

brandonscript Over a year ago

HOT! Thanks a bunch. Makes sense.

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