The Module Pattern, A Little More Detail

Once you’ve read this, read the update which has more detailed information about the module pattern’s performance profile!

This is an article on the module pattern for Javascript, and some of its neat properties when used for instances. I’ve been using it recently for projects like mapbox.js, and think it’s a neat way to structure code and avoid some of the less likable parts of the language.

For those already doing Javascript: here’s why you should care.

  1. You can mostly avoid the problem of tracking the meaning of Javascript’s pesky this keyword, and the associated problem of rebinding functions to another this value.
  2. You can keep truly internal ‘states’ of your code private while exposing minimal APIs that you know will stay stable.
  3. Often this yields code that’s more friendly to Javascript compression, like I wrote about in Writing Javascript For Size
  4. You can avoid the sneaky problems of users forgetting the new keyword, and the workarounds people use to dodge them

For those starting to learn Javascript.

  1. Once you understand scopes, you understand this pattern.
  2. Impress your friends.

Let’s Begin

To be clear, I adopted the style of the module pattern used by Mike Bostock, who presumably got it from earlier figures, and so on. Usually the variable-containing and scope-control bits of this pattern get the most attention - this sheds some light on other benefits that touch browser and async code as well.

Javascript with Classical Objects

function Car() {
    this.direction = 0;
}
Car.prototype.turn = function(angle) { this.direction += angle; };

var c = new Car();

In which you’re using Javascript’s prototypes to create a Javascript class, and using this to refer to ‘this instantiated object’.

Javascript with Module Pattern

function Car() {
    var car = {},
        direction = 0;

    car.turn = function(angle) { direction += angle; };

    return car;
}

var c = Car();

Here, you’re creating a new Javascript object literal every time that you call Car(). Notice that, unlike the prototype method, there’s no new operator.

A Quick Summary

What are the essential bits?

// a plain function. I prefer downcase because only true
// Javascript classes should be uppercase (like Date or Math)
// and this isn't a class, it's a factory
function thing() {
    // The first thing in the function is a bunch of scoped
    // variables. One of these scoped variables is always an empty
    // object to which you'll assign properties and methods
    // before it goes out the door as the value this function
    // returns.

    // This is that object. It could also be named 'thing'
    // but that would be shadowing the function 'thing'
    // and that's not a good thing to do.
    var t = {},
        // Now any variables that represent the state and data
        // of this object, thing. These are not attached to
        // the t object, they're attached to the scope it can
        // access - the scope of this thing() function.
        count = 0;

    // You can also create functions here that are scoped
    // and use them as callbacks, and they'll have access to
    // this t object's scoped variables.
    //
    // Functions that aren't attached to t are just like
    // the scoped variables above - they aren't accessible to
    // the API user. This is kind of like the private keyword
    // elsewhere
    function handler() {
        count++;
    }

    // Now you make the decision of the set of methods on t.
    // These have access to all of the scoped variables as well.
    t.talk = function(x) {
        alert(x);
    };

    // Finally, when you're done with t, return it.
    return t;

    // In terms of the idea of a 'closure', this is how you
    // 'close over' the scope of the object. Someone who calls
    // thing() gets a copy of t to play around with, but that
    // t will always refer to the scoped variables we just
    // defined.
}

1. No More This

The most common annoyance with this is in event handlers, because they turn this into the element that’s the source of the event.

So let’s say that you’re creating a simple counter, that attaches to a DOM element being clicked. The classical version might look like

function Counter() { this.count = 0; }

Counter.prototype.increment = function() { this.count++; };

Counter.prototype.countClicks = function(element) {
    element.onclick = this.increment;
};

var c = new Counter();

c.countClicks(document.getElementById('increment'));

document.getElementById('tell').onclick = function() {
    alert(c.count);
};

This looks valid, but isn’t. Clicking on increment repeatedly and then on ‘tell’ will tell you that Counter’s value is still 0. Why? Because Counter.prototype.increment runs, but the value of this in that context is the DOM element of the increment button, not counter.

A typical fix to this is to change the line to include bind, a function that changes the value of this in a function for any time that it runs in the future. Since this isn’t IE-safe, it’s more popular to use _.bind from underscore.js or jQuery.proxy() in the place of the standard Javascript function.

Counter.prototype.countClicks = function(element) {
    element.onclick = this.increment.bind(this);
};

This is fine, and more serious Javascript environments like Backbone provide convenience methods like bindAll that let you rebind a whole bunch of functions at one time, so you can totally forget about the this problem. But it’s still there, and when your code leaves that land, it’s a valid annoyance.

Here’s the other way to do this functionality:

function Counter() {
    var count = 0,
        c = {};

    function increment() { count++; }

    c.countClicks = function(element) {
        element.onclick = increment;
    };

    c.clicks = function() { return count; }

    return c;
}

var c = Counter();

c.countClicks(document.getElementById('increment'));

document.getElementById('tell').onclick = function() {
    alert(c.clicks());
};

2. Internal And External States

This is a mixed bag, but there are some good examples of why it’s awesome. Let’s say you have a library that manages a set of markers on a map.

Of course, this will require the functionality of adding a marker to the map. With a classical object, in most cases an uninformed or adventurous user could see that .markers is exposed and just .markers.push(newmarker) it into the object.

The literal pattern lets you be much more paranoid, with a setup like

function mmg() {
    var m = {},
        markers = [];
    m.add_marker = function(x) {
        // do various things to x, adjust its style,
        // validate it,
        // add event handlers, and then...
        markers.push(x);
    }
    m.markers = function() {
        // And then give the user read-only access
        // to the internal markers array.
        return markers.splice();
    }
    return m;
}

3. Compression

On this note, mostly just read Writing Javascript For Size. The public/private split allows compression to ‘mangle’ more names that they know will not be accessible to the global scope.

For instance, the code

function thing() {
    var t = {};
    function internalFunction() { console.log('foo'); }
    function internalFunctionTwo() { console.log('foo'); }
    t.do = function() {
        internalFunctionTwo();
        internalFunction();
    }
};

Allows a Javascript compressor like UglifyJS to shorten the names of internalFunction and internalFunctionTwo to single-letter names - there’s no way that you’ll be able to access them, so the names can change. By contrast, after minifying the code:

function thing() { }
thing.prototype.internalFunction() { console.log('foo'); };
thing.prototype.internalFunctionTwo() { console.log('foo'); };
thing.prototype.do = function() {
    internalFunctionTwo();
    internalFunction();
};

The long function names internalFunction and internalFunctionTwo will remain in their entirety.

Disadvantages

  1. This method is slower for certain cases
  2. The power to have private methods and variables can be abused
  3. Extending objects is unclear
  4. Object identities is unclear - instanceof doesn’t work.
  5. This may have higher memory usage, because creating new objects can create new copies of their members.

As far as speed. Creating object literals is faster than instantiating objects with constructors in most browsers, as this jsperf test from Mike Bostock shows. But dispatching method calls on those objects is slower.

But unless you’re doing some very serious, millions-of-iterations computation, Javascript engine level optimizations probably aren’t the biggest worry. In V8, small, prototype-based objects tend to out-perform literal objects created with the module pattern - good insights can be found in [this bit of Lars Bak's talk on the subject](http://youtu.be/hWhMKalEicY?t=48s) and the aha moment for objects versus arrays came from reading this bit of pre3d.js.

Update: V8’s hidden classes don’t actually correspond to ‘classes’, and don’t provide a performance boost to prototype-oriented code.

As far as the abusable power to do private members. There can be the feeling that you start off with an object like

function beautifulworld() {
    var bw,
        a, b, c;
    bw.setthings = function(x) {
        a = x * 10;
        b = x * 20;
        c = x * 30;
    };
    return bw;
}

And end up exposing a, b, and c, after realizing that private things couldn’t be private. So, if nothing else, this method requires an extra thought process to consider the level of encapsulation needed.

Extending Classes

I rarely encounter the problem of really extending classes. Ben Cherry’s linked article has one possibility, and you can also just use an object that uses another object internally - like how Wax’s interaction lib is used in specialized API-specific controls which ‘customize’ the object internally. This is the idea of using composition over inheritance.

instanceof

As far as the instanceof problem - with classical objects, when you call var x = new baseballbat(), then x instanceof baseballbat returns true, and x instanceof foo returns false. Every once in a blue moon this is handy, but it often runs into the same problems as strongly-typed languages, in which direct class-equivalence isn’t all that importance and it’s more likely you want to ask whether the object has methods or properties you need in your code, not its origin.

For instance, in a large-scale project like Carto, which required object identities, a chain of inheritance meant that the only sensible way to do identity was to add a .is member with a flag telling what the value should be interpreted as. Except in the most extreme cases, I don’t see much instanceof use in the wild.

Memory

As far as the last possible disadvantage, higher memory usage - memory in Javascript has been a bit of an uncertain, given limited developer tools and the variability between Javascript engines.

For objects that you’ll want thousands of, it might be a better option to either go with literal objects - for instance, the data types in Modest Maps would probably become simple arrays and their methods could easily become part of a singleton object. John Firebaugh confirmed this with a test that shows that for objects with many functionst that are initialized many times, there’s a serious (5x) difference in memory usage, due to more heap closures. So, it makes sense to use the module pattern for smaller objects, like UI widgets, map objects, and so on, while other techniques work well for atomic data, like pixel measures or nodes in a tree.

See Also

Have more ideas on this subject? Let me know over email or by tweeting at @tmcw.

Posted Jun 04, 2012 Tweet / Follow me on Twitter

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Tom MacWright

I'm . I work on tools for creativity at Mapbox. This is where I write about technology and everything else.