Scalable JavaScript Design Patterns

In the beginning, empires generally start out small.

With time, we add a few more ships to our eet and it begins to grow.

Soon enough, we have so many ships it becomes dif cult to handle communication and organisation.

What if all of this grinds to a halt because a ship goes of ine? Can everything keep on functioning?

We can introduce a central way of controlling this chaos and solving these problems e.g. the Death Star. Okay! We’re out of pancakes Larry, get more pancakes out to ship 34 You got it!

If a ship goes down, the Death Star can respond and react accordingly. e.g. Get rid of the old ship, send a replacement. Okay! Help! I’m going down Dispatching replacement ship

Think about the future. You should be able to change ‘death stars’ if you nd something better.. I’m on ﬁre! LOL. Later dude.

Let’s apply this to how we develop apps.

About Me Some quick info. • JavaScript & UI Developer @AOL • Member of the jQuery core [Bugs/Docs/ Learning] teams • Blogger [AddyOsmani.com/ScriptJunkie] • Author ‘Essential JavaScript Design Patterns’

Which now make awesome pizza-cutters

Part I: Patterns - a new hope.

The Tools For Our Empire. These make the architecture proposed possible. Scalable Application Design Patterns JavaScript Architecture

We’re Individuals “You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.” - Friedrich Nietzsche

We all do things differently Each of us have preferences for how we approach.. Solving problems Structuring solutions Solving scalability

Great but can lead to.. serious problems when working on code to be used by others. Inconsistent solutions Inconsistent architecture Dif cult refactoring

A lot like how most Stormtroopers know that there’s a time, a place and a way to wear your uniform..and others completely ignore this.

Design Patterns Reusable solutions that can be applied to commonly occurring problems in software design and architecture. “We search for some kind of harmony between two intangibles: a form we have not yet designed and a context we cannot properly describe’ - Christopher Alexander, the father of design patterns.

They’re proven Patterns are generally proven to have successfully solved problems in the past. Reliable Re ect Represent Solid approaches experience insights

They’re reusable Patterns can be picked up, improved and adapted without great effort. Out-of-the-box Incredibly exible Easily adapted solutions

They’re expressive Patterns provide us a means to describing approaches or structures. Common vocabulary Easier than describing Problem agnostic for expressing syntax and semantics solutions elegantly.

They offer value Patterns genuinely can help avoid some of the common pitfalls of development. Prevent minor issues Major problems that can cause later down the line

Part II: The JavaScript Strikes Back

JavaScript Patterns Writing code that’s expressive, encapsulated & structured

Module Pattern An interchangeable single-part of a larger system that can be easily re-used. “Anything can be de ned as a reusable module” - Nicholas Zakas, author ‘Professional JavaScript For Web Developers’

Stepping stone: IIFE Immediately invoked function expressions (or self-executing anonymous functions) (function() { // code to be immediately invoked }()); // Crockford recommend this way (function() { // code to be immediately invoked })(); // This is just as valid (function( window, document, undefined ){ //code to be immediately invoked })( this, this.document); (function( global, undefined ){ //code to be immediately invoked })( this );

Privacy In JavaScript There isn’t a true sense of it in JavaScript. Variables & Methods Variables & Methods No Access Modi ers can’t be ‘public’ can’t be ‘private’

Simulate privacy The typical module pattern is where immediately invoked function expressions (IIFEs) use execution context to create ‘privacy’. Here, objects are returned instead of functions. var basketModule = (function() { var basket = []; //private • In the pattern, variables return { //exposed to public declared are only addItem: function(values) { available inside the basket.push(values); module. }, getItemCount: function() { return basket.length; • Variables de ned within }, the returning object are getTotal: function(){ available to everyone var q = this.getItemCount(),p=0; while(q--){ p+= basket[q].price; • This allows us to simulate privacy } return p; } } }());

Sample usage Inside the module, you'll notice we return an object. This gets automatically assigned to basketModule so that you can interact with it as follows: //basketModule is an object with properties which can also be methods basketModule.addItem({item:'bread',price:0.5}); basketModule.addItem({item:'butter',price:0.3}); console.log(basketModule.getItemCount()); console.log(basketModule.getTotal()); //however, the following will not work: // (undefined as not inside the returned object) console.log(basketModule.basket); //(only exists within the module scope) console.log(basket);

Module Pattern: Dojo Dojo attempts to provide 'class'-like functionality through dojo.declare, which can be used for amongst other things, creating implementations of the module pattern. Powerful when used with dojo.provide. // traditional way var store = window.store || {}; store.basket = store.basket || {}; // another alternative.. // using dojo.setObject (with basket as a module of the store namespace) dojo.setObject("store.basket.object", (function() { var basket = []; function privateMethod() { console.log(basket); } return { publicMethod: function(){ privateMethod(); } }; }()));

Module Pattern: jQuery In the following example, a library function is de ned which declares a new library and automatically binds up the init function to document.ready when new libraries (ie. modules) are created. function library(module) { $(function() { if (module.init) { module.init(); } }); return module; } var myLibrary = library(function() { return { init: function() { /*implementation*/ } }; }());

Module Pattern: YUI A YUI module pattern implementation that follows the same general concept. YAHOO.store.basket = function () { //"private" variables: var myPrivateVar = "I can be accessed only within YAHOO.store.basket ."; //"private" method: var myPrivateMethod = function () { YAHOO.log("I can be accessed only from within YAHOO.store.basket"); } return { myPublicProperty: "I'm a public property.", myPublicMethod: function () { YAHOO.log("I'm a public method."); //Within basket, I can access "private" vars and methods: YAHOO.log(myPrivateVar); YAHOO.log(myPrivateMethod()); //The native scope of myPublicMethod is store so we can //access public members using "this": YAHOO.log(this.myPublicProperty); } }; }();

Module Pattern: ExtJS Another library that can similarly use the module pattern. // define a namespace Ext.namespace('myNamespace'); // define a module within this namespace myNameSpace.module = function() { // recommended that you don't directly access the DOM // from here as elements don't exist yet. Depends on // where/how you're waiting for document load. // private variables // private functions // public API return { // public properties // public methods init: function() { console.log('module initialised successfully'); } }; }(); // end of module

Better: AMD Take the concept of reusable JavaScript modules further with the Asynchronous Module De nition. Mechanism for de ning Stepping-stone to the Non-blocking, parallel asynchronously loadable module system proposed loading and well de ned. modules & dependencies for ES Harmony

AMD: de ne() de ne allows the de nition of modules with a signature of de ne(id /*optional*/, [dependencies], factory /*module instantiation fn*/); /* wrapper */ define( /*module id*/ 'myModule', /*dependencies*/ ['foo','bar','foobar'], /*definition for the module export*/ function (foo, bar, foobar) { /*module object*/ var module = {}; /*module methods go here*/ module.hello = foo.getSomething(); module.world = bar.doSomething(); /*return the defined module object*/ return module; } );

AMD: require() require is used to load code for top-level JS les or inside modules for dynamically fetching dependencies /* top-level: the module exports (one, two) are passed as function args to the callback.*/ require(['one', 'two'], function (one, two) { }); /* inside: complete example */ define('three', ['one', 'two'], function (one, two) { /*require('string') can be used inside the function to get the module export of a module that has already been fetched and evaluated.*/ var temp = require('one'); /*This next line would fail*/ var bad = require('four'); /* Return a value to define the module export */ return function () {}; });

Alternative: CommonJS Another easy to use module system with wide adoption server-side CommonJS Format widely accepted Competing standard. Tries Working group on a number of server-side to solve a few things AMD designing, prototyping, platforms (Node) doesn’t. standardizing JS APIs

CommonJS Modules They basically contain two parts: an exports object that contains the objects a module wishes to expose and a require function that modules can use to import the exports of other modules /* here we achieve compatibility with AMD and CommonJS using some boilerplate around the CommonJS module format*/ (function(define){ define(function(require,exports){ /*module contents*/ var dep1 = require("foo"); var dep2 = require("bar"); exports.hello = function(){...}; exports.world = function(){...}; }); })(typeof define=="function"? define:function(factory){factory (require,exports)});

Better alternative: Universal Module De nition De ning modules that can work anywhere (CommonJS environments such as clients, servers; with script loaders etc). Thx to @KitCambridge for this version. (function (root, Library) { // The square bracket notation is used to avoid property munging by the Closure Compiler. if (typeof define == "function" && typeof define["amd"] =="object" && define["amd"]) { // Export for asynchronous module loaders (e.g., RequireJS, `curl.js`). define(["exports"], Library); } else { // Export for CommonJS environments, web browsers, and JavaScript engines. Library = Library(typeof exports == "object" && exports|| (root["Library"] = { "noConflict": (function (original) { function noConflict() { root["Library"] = original; // `noConflict` can't be invoked more than once. delete Library.noConflict; return Library; } return noConflict; })(root["Library"]) })); } })(this, function (exports) { // module code here return exports; });

ES Harmony Modules A module format proposed for EcmaScript Harmony with goals such as static scoping, simplicity and usability. // Basic module module SafeWidget { import alert from Widget; var _private ="someValue"; // exports export var document = { write: function(txt) { alert('Out of luck, buck'); }, ... }; } // Remote module module JSONTest from 'http://json.org/modules/json2.js';

DART modules A module created using Google’s recently proposed Dart // 17,000 lines of code // couldn’t fit on the slides // Sorry, guise!

Facade Pattern Convenient, high-level interfaces to larger bodies of code that hide underlying complexity “When you put up a facade, you're usually creating an outward appearance which conceals a different reality. Think of it as simplifying the API presented to other developers” - Essential JavaScript Design Patterns

Facade Implementation A higher-level facade is provided to our underlying module, without directly exposing methods. var module = (function() { var _private = { i:5, get : function() { console.log('current value:' + this.i); }, set : function( val ) { this.i = val; }, run : function() { console.log('running'); }, jump: function(){ console.log('jumping'); } }; return { facade : function( args ) { _private.set(args.val); _private.get(); if ( args.run ) { _private.run(); } } } }()); module.facade({run: true, val:10}); //outputs current value: 10, running

Facade Implementation A higher-level facade is provided to our underlying module, without directly exposing methods. var module = (function() { var _private = { i:5, get : function() { console.log('current value:' + this.i); }, set : function( val ) { this.i = val; }, run : function() { console.log('running'); }, jump: function(){ console.log('jumping'); } }; return { facade : function( args ) { _private.set(args.val); _private.get(); We’re really just interested if ( args.run ) { _private.run(); in this part. } } } }()); module.facade({run: true, val:10}); //outputs current value: 10, running

Facade Implementation A higher-level facade is provided to our underlying module, without directly exposing methods. return { facade : function( args ) { // set values of private properties _private.set(args.val); // test setter _private.get(); // optional: provide a simple interface // to internal methods through the // facade signature if ( args.run ) { _private.run(); } Limited public API of functionality. } } Differs greatly from the reality of the implementation.

A Facade A structural pattern found in many JavaScript libraries and frameworks (eg. jQuery). Hides the inner- Simpli es usage This lets you be more workings of a library. through a limited, creative behind the Allows implementation more readable API scenes. to be less important.

Facade Pattern How does it differ from the module pattern? • Differs from the module pattern as the exposed API can greatly differ from the public/private methods de ned • Has many uses including application security as we’ll see a little later in the talk

Mediator Pattern Encapsulates how disparate modules interact with each other by acting as an intermediary “Mediators are used when the communication between modules may be complex, but is still well de ned” - Essential JavaScript Design Patterns

Air Traf c Control I always nd this mediator analogy helps when discussing this pattern: The tower handles All communication done Centralised controller what planes can take from planes to tower, is key to this success. off and land not plane to plane Similar to mediator.

A Mediator Promotes loose coupling. Helps solve module inter-dependency issues. Allow modules to Noti cations can be Typically easier to add or broadcast or listen for handled by any number of remove features to loosely noti cations without modules at once. coupled systems like this. worrying about the system.

Mediator Implementation One possible implementation, exposing publish and subscribe capabilities. var mediator = (function(){ var subscribe = function(channel, fn){ if (!mediator.channels[channel])mediator.channels[channel] = []; mediator.channels[channel].push({ context: this, callback:fn }); return this; }, publish = function(channel){ if (!mediator.channels[channel]) return false; var args = Array.prototype.slice.call(arguments, 1); for (var i = 0, l = mediator.channels[channel].length; i <l; i++) { var subscription = mediator.channels[channel][i]; subscription.callback.apply(subscription.context,args); } return this; }; return { channels: {}, publish: publish, subscribe: subscribe, installTo: function(obj){ obj.subscribe = subscribe; obj.publish = publish; } }; }());

Example Usage of the implementation from the last slide. //Pub/sub on a centralized mediator mediator.name = "tim"; mediator.subscribe('nameChange', function(arg){ console.log(this.name); this.name = arg; console.log(this.name); }); mediator.publish('nameChange', 'david'); //tim, david //Pub/sub via third party mediator var obj = { name: 'sam' }; mediator.installTo(obj); obj.subscribe('nameChange', function(arg){ console.log(this.name); this.name = arg; console.log(this.name); }); obj.publish('nameChange', 'john'); //sam, john

Part III: Return of the patterns

Scalable Application Architecture Strategies for decoupling and future-proo ng the structure of your application. Let’s build empires. Thanks to Nicholas Zakas, Rebecca Murphey, John Hann, Paul Irish, Peter Michaux and Justin Meyer for their previous work in this area.

Challenge De ne what it means for a JavaScript application to be ‘large’. • It’s a very tricky question to get right • Even experienced JavaScript developers have trouble accurately de ning this

Some Answers I asked some intermediate developers what their thoughts on this were. • “Um..JavaScript apps with over 100,000 lines of code” • Incorrect. Code size does not always correlate to application complexity.

Some Answers I asked some intermediate developers what their thoughts on this were. • “Obviously, apps with over 1MB of JavaScript code written in-house” • (Hopefully) not. This could be very simplistic code once again. Can we get more clear?.

My Answer Large-scale JavaScript apps are non-trivial applications requiring signi cant developer effort to maintain, where most heavy lifting of data manipulation and display falls to the browser.

Some Examples Google’s GMail

Some Examples The Yahoo! Homepage

Some Examples AOL Mail / Phoenix

Current Architecture If working on a signi cantly large JavaScript app, remember to dedicate suf cient time to planning the underlying architecture that makes the most sense. It’s often more complex than we initially think.

Your Current Architecture might contain a mixture of the following: Custom Widgets Modules An Application Core MVC (Models/Views/Controllers) JavaScript Libraries & Toolkits

Possible Problems The last slide contains some great architectural components, but used non- optimally they can come with a few problems: Can single modules Can single modules How much of this is exist on their own be tested instantly re-usable? independently? independently?

Possible Problems Some further concerns: How much do Is your application If speci c parts of modules depend on tightly coupled? your app fail, can it others in the system? still function?

Think Long-Term What future concerns haven’t been factored in to this architecture? • You may decide to switch from using jQuery to Dojo or YUI for reasons of performance, security or design • Libraries are not easily interchangeable and have high switching costs if tightly coupled to your app

Ask Yourself This is important. If you reviewed your architecture right now, could a decision to switch libraries be made without rewriting your entire application?

“The secret to building large apps is never build large apps. Break your applications into small pieces. Then, assemble those testable, bite-sized pieces into your big application” - Justin Meyer

“The more tied components are to each other, the less reusable they will be, and the more dif cult it becomes to make changes to one without accidentally affecting another” - Rebecca Murphey

“The key is to acknowledge from the start that you have no idea how this will grow. When you accept that you don't know everything, you begin to design the system defensively. You identify the key areas that may change, which often is very easy when you put a little bit of time into it.” - Nicholas Zakas

Solution: Use The Force Fixing our architecture with JavaScript design patterns. “The only difference between a problem and a solution is that people understand the solution.’ - Charles F. Kettering

Let’s Combine Our Patterns We’re going to build something special. Module Pattern + = WIN Facade Pattern + Mediator Pattern

Brainstorm. What do we want? Functionality broken Framework or library Loosely coupled down into smaller agnostic. Flexibility to architecture independent modules change in future.

Some More Ideas. How might we achieve this? An intermediate layer Single modules speak interprets requests. Prevent apps from falling to the app when Modules don’t access over due to errors with something interesting the core or libraries speci c modules. happens directly.

The Facade The Facade pattern will play the role of: • An abstraction of the application core that sits in the middle between it and modules • Ensures a consistent interface to our modules is available at all times • Should be the only thing modules are aware of - they shouldn’t know about other components

The Facade How else can it help? • Components communicate via the facade so it needs to be dependable • It acts as a security guard, determining which parts of the application a module can access • Components only call their own methods or methods from a sandbox, but nothing they don’t have permission to

The Facade This is how modules might normally communicate with the mediator.

The Facade This is where the Facade ts in. The intermediate security layer that pipes noti cations back to the mediator for processing.

The Facade An abstraction of the core, it listens out for interesting events and says ‘Great. What happened? Give me the details’. It also acts as a permissions manager. Modules only communicate through this and are only able to do what they’ve been permitted to.

A Quick Note: • Nicholas Zakas refers to the facade as a sandbox controller • Agrees it could equally be considered the adapter, proxy or facade pattern • I prefer ‘facade’ as it matches the purpose most closely

The Application Core The Mediator Pattern • It’s job is to manage the module lifecycle • When is it safe for a module to start? • When should it stop? • Modules should execute automatically when started

The Application Core The Mediator Pattern • It’s not the core’s job to decide whether this should be when the DOM is ready. • The core should enable adding or removing modules without breaking anything. • It should ideally also handle detecting and managing errors in the system.

The Application Core The core acts as a 'Pub/Sub' manager using the mediator pattern

The Application Core Manages the module lifecycle. It reacts to events passed from the facade and starts, stops or restarts modules as necessary. Modules here automatically execute when loaded.

Modules Tying in modules into the architecture • Modules want to inform the application when something interesting happens. e.g. a new message has arrived • Correctly publishing events of interest should be their primary concern

Modules Tying in modules into the architecture • Modules should ideally not be concerned about: • what objects or modules are being noti ed • where these objects are based (client? server?) • how many objects subscribe to noti cations

Modules Modules contain speci c pieces of functionality for your application. They publish noti cations informing the application whenever something interesting happens

Modules Tying in modules into the architecture • They also shouldn’t be concerned with what happens if other modules in the system fail to work • Leave this up to the mediator

Modules If a single module fails, the facade and mediator should stop and restart it.

Modules Unique blocks of functionality for your application. They inform the application when something interesting happens. Don’t talk to each other directly, only concerned with publishing events of interest.

Aura: A Preview Enough talk! Let’s take a look at some real code. Aura is a framework I’m building at AOL that provides a boilerplate for one way to approach implementing this architecture. It will be released for open-source consumption once stable.

Aura: Core The Mediator / Application Core • Swappable Mediator with a light wrapper around a speci c JavaScript library • Ability to start and stop modules • By default it comes with wrappers for both Dojo and jQuery, with core syntax that resembles the latter

Aura: Core How does this work? • Accessing this wrapper, the facade doesn’t care what framework has been slotted in. It works with the abstraction • Behind the scenes, arguments are mapped to the relevant jQuery or dojo methods and signatures for achieving speci c behaviour

Aura: Core A sample of the method signatures and namespaces supported // some core methods for module management core.define(module_id, module_definition); // define a new module core.start(module_id); // initialise a module core.startAll(); // initialise all modules core.stop(module_id); // stop a specific module core.stopAll(); // stop all modules core.destroy(module_id); // destroy a specific module core.destroyAll(); // destroy all modules // core namespaces available out of the box core.events // bind, unbind etc. core.utils // type checking, module extension core.dom // DOM manipulation, CSS Styling

Aura: Core.dom > CSS Styling, Chaining Chaining and CSS Style Manipulation are both supported. // Chaining and CSS style manipulation aura.core.dom.query('body').css({'background':'#1c1c1c'}); aura.core.dom.query('#search_input').css({'background':'blue'}).css({'color':'pink'}); aura.core.dom.query('#search_button').css({'width':'200','height':'100'}); // Manipulating styles within a context aura.core.dom.css('body', {'background':'red'}); aura.core.dom.css('#shopping-cart',{'color':'green','background':'yellow'}); aura.core.dom.css('#product-panel li', {'background':'purple'}); // Passing in DOM nodes also works var test = aura.core.dom.query('#shopping-cart'); //.query should handle this. aura.core.dom.css(test, {'background':'purple'});

Aura: Core.dom > CSS Styling, Chaining Look familiar? In my case I’ve modelled my abstraction around the jQuery API. Behind the scenes, this works with both jQuery and Dojo, providing a single abstraction. // Chaining and CSS style manipulation aura.core.dom.query('body').css({'background':'#1c1c1c'}); aura.core.dom.query('#search_input').css({'background':'blue'}).css({'color':'pink'}); aura.core.dom.query('#search_button').css({'width':'200','height':'100'}); // Manipulating styles within a context aura.core.dom.css('body', {'background':'red'}); aura.core.dom.css('#shopping-cart',{'color':'green','background':'yellow'}); aura.core.dom.css('#product-panel li', {'background':'purple'}); // Passing in DOM nodes also works var test = aura.core.dom.query('#shopping-cart'); //.query should handle this. aura.core.dom.css(test, {'background':'purple'});

Aura: Core.dom > Attribs, Anim Attribute manipulation and animation are also abstracted using an API similar to jQuery. Remember, with Dojo this actually maps arguments back to the relevant Dojo methods needed to achieve the task. // Get and set attributes console.log(aura.core.dom.query('#seach_input').attr('title','foobar')); console.log(aura.core.dom.query('#search_input').attr('title')); // Passing in DOM nodes var q = aura.core.dom.query('#shopping-cart'); console.log(aura.core.dom.attr(q, 'id')); // Animation support aura.core.dom.animate('#product-panel li', { width: 400, height:20}, 5000); aura.core.dom.animate('button', {width: 200, duration: 100}); aura.core.dom.animate('p', {width:20, height:40, padding:10,duration:200});

Aura: Core.dom > Create, Ajax Similarly, element creation and ajax are also supported in the abstracted core interface. // Element creation var el = aura.core.dom.create("a", { href: "foo.html", title: "bar!", innerHTML: "link" }, 'body'); // XHR/Ajax requests (deferred support coming soon) aura.core.dom.ajax({ url:'index.html', type:'get', //post, put dataType: "text", success:function(data){ console.log('success'); }, error:function(){ console.log('error'); } });

What We Learned Let’s review what we looked at today. ‘Anyone who stops learning is old, whether at twenty or eighty. Anyone who keeps learning stays young. The greatest thing in life is to keep your mind young’ - Henry Ford

Summary Today we learned how to use three design patterns to create scalable ‘future- proof’ application architectures. The idea is to have: Application core Facade Modules Mediator Core abstraction Highly decoupled Module manager Permission manager Unique blocks Swappable Framework agnostic

Summary This can be very useful as: • Modules are no longer dependent on anyone • They can be managed so that the application doesn’t (or shouldn’t) fall over. • You can theoretically pick up any module, drop it in a page and start using it in another project

And Finally.. If you stick to a consistent abstracted API you can easily switch out one framework for another (eg. replace jQuery with Dojo) without the need to rewrite your modules at all.

That’s it. For more on this architecture and other topics, check out: Blog Twitter GitHub http://addyosmani.com @addyosmani /addyosmani

Scalable JavaScript Design Patterns

Scalable JavaScript Design Patterns

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