JS: nested function v prototype

Question

I have an object in Javascript and want to process the data passed into the constructor using a function that could later be called externally with more data. Naturally I don't want to duplicate the code (once in the constructor, once in a function), so how should I best set this up?

I could use a nested function, but I'm told this is inefficient:

function MyOb(data) {
    this.myData = {};
    function addData(newData) {
        //Add newData to myData
    }
    addData(data);
}

But if I use a prototype I get a "can't find variable addData" error on line 3:

function MyOb(data) {
    this.myData = {};
    addData(data);
}

MyOb.prototype.addData = function(newData) {
        //Add newData to myData
}

So am I forced to either use a nested function or repeat myself, or is there a way of making this work using prototype?

With your code myData is common to all instances of myObj. Is this the intention? If myData can be a property then things are simpler. — HBP
– HBP, Commented Jun 17, 2013 at 12:47
Yep! It's a generic property store with get, set, save, load functions that has multiple instansiations by different parts of a larger application. — Matt Parkins
– Matt Parkins, Commented Jun 17, 2013 at 14:17

Denys Séguret · Accepted Answer · 2013-06-17 14:21:00Z

5

Change

function MyOb(data) {
    var myData;
    addData(data);
}

to

function MyOb(data) {
    this.myData = {}; // or another initialization
    this.addData(data);
}

You need the explicit this in JavaScript objects.

Note also that using var myData makes it private : you won't be able to use this variable from functions defined outside the constructor, including the addData function. That's why you probably need this.myData instead.

edited Jun 17, 2013 at 14:21

answered Jun 17, 2013 at 12:28

Denys Séguret

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Comments

fmsf · Accepted Answer · 2013-06-17 12:28:31Z

1

Using the prototype you need to create a new object:

var abc = new MyOb(data);

then you can access the function using the this:

function MyOb(data) {
    var _myData; // local variable in this scope (addData won't have access)
    this.myData = {}; // public variable
    this.addData(data);
}

If you don't use the newto build your object, then the thiswill be the window, and your code won't work

answered Jun 17, 2013 at 12:28

fmsf

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Comments

Community · Accepted Answer · 2017-05-23 12:21:34Z

You should just make the nested function publically available as a property on your object:

function MyOb(data) {
    var myData;
    function addData(newData) {
        //Add newData to myData
    }
    addData(data);
    this.addData = addData;
}

Your current code with the prototype suffers from the problem that you're trying to get the function like a variable, while it's a (inherited) property of your instance (referenced by this). However, changing it to

    this.addData(data);

would lead to the error can't find variable myData in the prototype function - it's a variable that is local to the constructor. You would have to make that a instance property as well to use the prototype. See also Javascript: Do I need to put this.var for every variable in an object?.

Thanks I've updated code in the question. Thanks for the link too.
Well, if it's a property anyway then you just have to call the prototype method with the this.addData(data);. If you need the full function code, look at dystroy's answer, I'm not gonna repeat it :-)

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