Private Method in Module Pattern: TypeError: undefined is not a function

Question

I am trying to implement a classic Module Pattern in javascript, discussed here and here. But my private methods are not working. I have the following bit of testing code.

var jsStuff = (function() {

    // Private
    var sayStuffPrivate = function ( stuff ) {
        console.log( "I am a private method: " +  stuff );
        return true;
    };

    // Public
    return {
        sayStuff: function ( stuff ) {
            console.log( "I am a public method: " + stuff );
            this.sayStuffPrivate( stuff );
            return true;
        }
    }
}());

When I try to run this, I get the following:

> jsStuff.sayStuff('blah');
test.js:16 I am a public method: blah
test.js:17 Uncaught TypeError: undefined is not a function

What am I missing here?

It's not a method, as that would mean it's no more private. It's just a local function, and should be accessed as such; not via `this.` like a public property of your object. — Bergi, Dec 11 '14 at 06:06
Note that the value of *this* within a function is determined entirely by how you call the function (or by using *bind*), not lexically (or how the function was created). — RobG, Dec 11 '14 at 06:17

score 3 · Accepted Answer · answered Dec 11 '14 at 06:05

3

this.sayStuffPrivate( stuff );

Here, this refers to the object you actually returned from the sayStuff function. That doesn't have a property called sayStuffPrivate in it. So, this.sayStuffPrivate will be evaluated to undefined and since you are using that as a function, it fails with that error.

You are supposed to take advantage of the closure property and invoke it like this

sayStuffPrivate( stuff );

answered Dec 11 '14 at 06:05

thefourtheye

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Thanks! I'm new to js, and it seems such a subtle thing. Obvious, I guess, once you've seen it a few times. – Dave Dec 11 '14 at 06:14
Incidentally, can private methods call the public ones? If so, how? – Dave Dec 11 '14 at 06:29
This answered my second question (Can private methods call the public ones?): http://stackoverflow.com/questions/4505073/how-can-i-call-a-public-method-from-within-a-private-one-when-using-the-javascri?rq=1 – Dave Dec 11 '14 at 06:38

score 0 · Answer 2 · answered Dec 11 '14 at 06:07

You should call sayStuffPrivate without this. Try This:

var jsStuff = (function() {

    // Private
    var sayStuffPrivate = function ( stuff ) {
        console.log( "I am a private method: " +  stuff );
        return true;
    };

    // Public
    return {
        sayStuff: function ( stuff ) {
            console.log( "I am a public method: " + stuff );
            sayStuffPrivate( stuff );
            return true;
        }
    }
}());

Private Method in Module Pattern: TypeError: undefined is not a function

2 Answers2