In this article, we understand how large-scale JavaScript applications amount to a series of communicating components. Composition is a big topic, and one that's relevant to scalable JavaScript code. When we start thinking about the composition of our components, we start to notice certain flaws in our design; limitations that prevent us from scaling in response to influencers.

(For more resources related to this topic, see here.)

The composition of a component isn't random—there's a handful of prevalent patterns for JavaScript components. We'll begin the article with a look at some of these generic component types that encapsulate common patterns found in every web application. Understanding that components implement patterns is crucial for extending these generic components in a way that scales.

It's one thing to get our component composition right from a purely technical standpoint, it's another to easily map these components to features. The same challenge holds true for components we've already implemented. The way we compose our code needs to provide a level of transparency, so that it's feasible to decompose our components and understand what they're doing, both at runtime and at design time.

Finally, we'll take a look at the idea of decoupling business logic from our components. This is nothing new, the idea of separation-of-concerns has been around for a long time. The challenge with JavaScript applications is that it touches so many things—it's difficult to clearly separate business logic from other implementation concerns. The way in which we organize our source code (relative to the components that use them) can have a dramatic effect on our ability to scale.

Generic component types

It's exceedingly unlikely that anyone, in this day and age, would set out to build a large scale JavaScript application without the help of libraries, a framework, or both. Let's refer to these collectively as tools, since we're more interested in using the tools that help us scale, and not necessarily which tools are better than other tools. At the end of the day, it's up to the development team to decide which tool is best for the application we're building, personal preferences aside.

Guiding factors in choosing the tools we use are the type of components they provide, and what these are capable of. For example, a larger web framework may have all the generic components we need. On the other hand, a functional programming utility library might provide a lot of the low-level functionality we need. How these things are composed into a cohesive feature that scales, is for us to figure out.

The idea is to find tools that expose generic implementations of the components we need. Often, we'll extend these components, building specific functionality that's unique to our application. This section walks through the most typical components we'd want in a large-scale JavaScript application.

Modules

Modules exist, in one form or another, in almost every programming language. Except in JavaScript. That's almost untrue though—ECMAScript 6, in it's final draft status at the time of this writing, introduces the notion of modules. However, there're tools out there today that allow us to modularize our code, without relying on the script tag. Large-scale JavaScript code is still a relatively new thing. Things like the script tag weren't meant to address issues like modular code and dependency management.

RequireJS is probably the most popular module loader and dependency resolver. The fact that we need a library just to load modules into our front-end application speaks of the complexities involved. For example, module dependencies aren't a trivial matter when there's network latency and race conditions to consider.

Another option is to use a transpiler like Browserify. This approach is gaining traction because it lets us declare our modules using the CommonJS format. This format is used by NodeJS, and the upcoming ECMAScript module specification is a lot closer to CommonJS than to AMD. The advantage is that the code we write today has better compatibility with back-end JavaScript code, and with the future.

Some frameworks, like Angular or Marionette, have their own ideas of what modules are, albeit, more abstract ideas. These modules are more about organizing our code, than they are about tactfully delivering code from the server to the browser. These types of modules might even map better to other features of the framework. For example, if there's a centralized application instance that's used to manage our modules, the framework might provide a mean to manage modules from the application. Take a look at the following diagram:

A global application component using modules as it's building blocks. Modules can be small, containing only one feature, or large, containing several features

This lets us perform higher-level tasks at the module level (things like disabling modules or configuring them with arguments). Essentially, modules speak for features. They're a packaging mechanism that allows us to encapsulate things about a given feature that the rest of the application doesn't care about. Modules help us scale our application by adding high-level operations to our features, by treating our features as the building blocks. Without modules, we'd have no meaningful way to do this.

The composition of modules look different depending on the mechanism used to declare the module. A module could be straightforward, providing a namespace from which objects can be exported. Or if we're using a specific framework module flavor, there could be much more to it. Like automatic event life cycles, or methods for performing boilerplate setup tasks. However we slice it, modules in the context of scalable JavaScript are a means to create larger building blocks, and a means to handle complex dependencies:

// main.js
// Imports a log() function from the util.js model.
import log from 'util.js';
log('Initializing...');

// util.js
// Exports a basic console.log() wrapper function.
'use strict';

export default function log(message) {
    if (console) {
        console.log(message);
    }
}

While it's easier to build large-scale applications with module-sized building blocks, it's also easier to tear a module out of an application and work with it in isolation. If our application is monolithic or our modules are too plentiful and fine-grained, it's very difficult for us to excise problem-spots from our code, or to test work in progress. Our component may function perfectly well on its own. It could have negative side-effects somewhere else in the system, however. If we can remove pieces of the puzzle, one at a time and without too much effort, we can scale the trouble-shooting process.

Routers

Any large-scale JavaScript application has a significant number of possible URIs. The URI is the address of the page that the user is looking at. They can navigate to this resource by clicking on links, or they may be taken to a new URI automatically by our code, perhaps in response to some user action. The web has always relied on URIs, long before the advent of large-scale JavaScript applications. URIs point to resources, and resources can be just about anything. The larger the application, the more resources, and the more potential URIs.

Router components are tools we use in the front-end, to listen for these URI change events and respond to them accordingly. There's less reliance on the back-end web servers parsing the URI, and returning the new content. Most web sites still do this, but there're several disadvantages with this approach when it comes to building applications:

The browser triggers events when the URI changes, and the router component responds to these changes. The URI changes can be triggered from the history API, or from location.hash

The main problem is that we want the UI to be portable, as in, we want to be able to deploy it against any back-end and things should work. Since we're not assembling markup for the URI in the back-end, it doesn't make sense to parse the URI in the back-end either.

We declaratively specify all the URI patterns in our router components. We generally refer to these as, routes. Think of a route as a blueprint, and a URI as an instance of that blueprint. This means that when the router receives a URI, it can correlate it to a route. That, in essence, is the responsibility of router components. Which is easy with smaller applications, but when we're talking about scale, further deliberation on router design is in order.

As a starting point, we have to consider the URI mechanism we want to use. The two choices are basically listening to hash change events, or utilizing the history API. Using hash-bang URIs is probably the simplest approach. The history API available in every modern browser, on the other hand, lets us format URI's without the hash-bang—they look like real URIs. The router component in the framework we're using may support only one or the other, thus simplifying the decision. Some support both URI approaches, in which case we need to decide which one works best for our application.

The next thing to consider about routing in our architecture is how to react to route changes. There're generally two approaches to this. The first is to declaratively bind a route to a callback function. This is ideal when the router doesn't have a lot of routes. The second approach is to trigger events when routes are activated. This means that there's nothing directly bound to the router. Instead, some other component listens for such an event. This approach is beneficial when there are lots of routes, because the router has no knowledge of the components, just the routes.

Here's an example that shows a router component listening to route events:

// router.js

import Events from 'events.js'

// A router is a type of event broker, it
// can trigger routes, and listen to route
// changes.
export default class Router extends Events {

    // If a route configuration object is passed,
    // then we iterate over it, calling listen()
    // on each route name. This is translating from
    // route specs to event listeners.
    constructor(routes) {
        super();

        if (routes != null) {
            for (let key of Object.keys(routes)) {
                this.listen(key, routes[key]);
            }
        }
    }

    // This is called when the caller is ready to start
    // responding to route events. We listen to the
    // "onhashchange" window event. We manually call
    // our handler here to process the current route.
    start() {
        window.addEventListener('hashchange',
            this.onHashChange.bind(this));

        this.onHashChange();
    }

    // When there's a route change, we translate this into
    // a triggered event. Remember, this router is also an
    // event broker. The event name is the current URI.
    onHashChange() {
        this.trigger(location.hash, location.hash);
    }

};

// Creates a router instance, and uses two different
// approaches to listening to routes.
//
// The first is by passing configuration to the Router.
// The key is the actual route, and the value is the
// callback function.
//
// The second uses the listen() method of the router,
// where the event name is the actual route, and the
// callback function is called when the route is activated.
//
// Nothing is triggered until the start() method is called,
// which gives us an opportunity to set everything up. For
// example, the callback functions that respond to routes
// might require something to be configured before they can
// run.

import Router from 'router.js'

function logRoute(route) {
    console.log(`${route} activated`);
}

var router = new Router({
    '#route1': logRoute
});

router.listen('#route2', logRoute);

router.start();

Some of the code required to run these examples is omitted from the listings. For example, the events.js module is included in the code bundle that comes with this book, it's just not that relevant to the example.

Also in the interest of space, the code examples avoid using specific frameworks and libraries. In practice, we're not going to write our own router or events API—our frameworks do that already. We're instead using vanillaES6 JavaScript, to illustrate points pertinent to scaling our applications

Another architectural consideration we'll want to make when it comes to routing is whether we want a global, monolithic router, or a router per module, or some other component. The downside to having a monolithic router is that it becomes difficult to scale when it grows sufficiently large, as we keep adding features and routes. The advantage is that the routes are all declared in one place. Monolithic routers can still trigger events that all our components can listen to.

The per-module approach to routing involves multiple router instances. For example, if our application has five components, each would have their own router. The advantage here is that the module is completely self-contained. Anyone working with this module doesn't need to look elsewhere to figure out which routes it responds to. Using this approach, we can also have a tighter coupling between the route definitions and the functions that respond to them, which could mean simpler code. The downside to this approach is that we lose the consolidated aspect of having all our routes declared in a central place. Take a look at the following diagram:

The router to the left is global—all modules use the same instance to respond to URI events. The modules to the right have their own routers. These instances contain configuration specific to the module, not the entire application

Depending on the capabilities of the framework we're using, the router components may or may not support multiple router instances. It may only be possible to have one callback function per route. There may be subtle nuances to the router events we're not yet aware of.

Models/Collections

The API our application interacts with exposes entities. Once these entities have been transferred to the browser, we will store a model of those entities. Collections are a bunch of related entities, usually of the same type. The tools we're using may or may not provide a generic model and/or collection components, or they may have something similar but named differently. The goal of modeling API data is a rough approximation of the API entity. This could be as simple as storing models as plain JavaScript objects and collections as arrays.

The challenge with simply storing our API entities as plain objects in arrays is that some other component is then responsible for talking to the API, triggering events when the data changes, and for performing data transformations. We want other components to be able to transform collections and models where needed, in order to fulfill their duties. But we don't want repetitive code, and it's best if we're able to encapsulate the common things like transformations, API calls, and event life cycles. Take a look at the next diagram:

Models encapsulate interaction with APIs, parsing data, and triggering events when data changes. This leads to simpler code outside of the models

Hiding the details of how the API data is loaded into the browser, or how we issue commands, helps us scale our application as we grow. As we add more entities to the API, the complexity of our code grows too. We can throttle this complexity by constraining the API interactions to our model and collection components.

Another scalability issue we'll face with our models and collections is where they fit in the big picture. That is, our application is really just one big component, composed of smaller components. Our models and collections map well to our API, but not necessarily to features. API entities are more generic than specific features, and are often used by several features. Which leaves us with an open question—where do our models and collections fit into components?

Here's an example that shows specific views extending generic views. The same model can be passed to both:

// A super simple model class.
class Model {
    constructor(first, last, age) {
        this.first = first;
        this.last = last;
        this.age = age;
    }
}

// The base view, with a name method that
// generates some output.
class BaseView {
    name() {
        return `${this.model.first} ${this.model.last}`;
    }
}

// Extends BaseView with a constructor that accepts
// a model and stores a reference to it.
class GenericModelView extends BaseView {
    constructor(model) {
        super();
        this.model = model;
    }
}

// Extends GenericModelView with specific constructor
// arguments.
class SpecificModelView extends BaseView {
    constructor(first, last, age) {
        super();
        this.model = new Model(...arguments);
    }
}

var properties = [ 'Terri', 'Hodges', 41 ];

// Make sure the data is the same in both views.
// The name() method should return the same result...
console.log('generic view',
    new GenericModelView(new Model(...properties)).name());
console.log('specific view',
    new SpecificModelView(...properties).name());

On one hand, components can be completely generic with regard to the models and collections they use. On the other hand, some components are specific with their requirements—they can directly instantiate their collections. Configuring generic components with specific models and collections at runtime only benefits us when the component truly is generic, and is used in several places. Otherwise, we might as well encapsulate the models within the components that use them. Choosing the right approach helps us scale. Because, not all our components will be entirely generic or entirely specific.

Controllers/Views

Depending on the framework we're using, and the design patterns our team is following, controllers and views can represent different things. There's simply too many MV* pattern and style variations to provide a meaningful distinction in terms of scale. The minute differences have trade-offs relative to similar but different MV* approaches. For our purpose of discussing large scale JavaScript code, we'll treat them as the same type of component. If we decide to separate the two concepts in our implementation, the ideas in this section will be relevant to both types.

Let's stick with the term views for now, knowing that we're covering both views and controllers, conceptually. These components interact with several other component types, including routers, models or collections, and templates, which are discussed in the next section. When something happens, the user needs to be notified about it. The view's job is to update the DOM. This could be as simple as changing an attribute on a DOM element, or as involved as rendering a new template:

A view component updating the DOM in response to router and model events

A view can update the DOM in response to several types of events. A route could have changed. A model could have been updated. Or something more direct, like a method call on the view component. Updating the DOM is not as straightforward as one might think. There's the performance to think about—what happens when our view is flooded with events? There's the latency to think about—how long will this JavaScript call stack run, before stopping and actually allowing the DOM to render?

Another responsibility of our views is responding to DOM events. These are usually triggered by the user interacting with our UI. The interaction may start and end with our view. For example, depending on the state of something like user input or one of our models, we might update the DOM with a message. Or we might do nothing, if the event handler is debounced, for instance.

A debounced function groups multiple calls into one. For example, calling foo() 20 times in 10 milliseconds may only result in the implementation of foo() being called once. For a more detailed explanation, look at: http://drupalmotion.com/article/debounce-and-throttle-visual-explanation. Most of the time, the DOM events get translated into something else, either a method call or another event. For example, we might call a method on a model, or transform a collection. The end result, most of the time, is that we provide feedback by updating the DOM. This can be done either directly, or indirectly. In the case of direct DOM updates, it's simple to scale. In the case of indirect updates, or updates through side-effects, scaling becomes more of a challenge. This is because as the application acquires more moving parts, the more difficult it becomes to form a mental map of cause and effect.

Here's an example that shows a view listening to DOM events and model events.

import Events from 'events.js';

// A basic model. It extending "Events" so it
// can listen to events triggered by other components.
class Model extends Events {
    constructor(enabled) {
        super();
        this.enabled = !!enabled;
    }

    // Setters and getters for the "enabled" property.
    // Setting it also triggers an event. So other components
    // can listen to the "enabled" event.
    set enabled(enabled) {
        this._enabled = enabled;
        this.trigger('enabled', enabled);
    }

    get enabled() {
        return this._enabled;
    }
}

// A view component that takes a model and a DOM element
// as arguments.
class View {
    constructor(element, model) {

        // When the model triggers the "enabled" event,
        // we adjust the DOM.
        model.listen('enabled', (enabled) => {
            element.setAttribute('disabled', !enabled);
        });

        // Set the state of the model when the element is
        // clicked. This will trigger the listener above.
        element.addEventListener('click', () => {
            model.enabled = false;
        });
    }
}

new View(document.getElementById('set'), new Model());

On the plus side to all this complexity, we actually get some reusable code. The view is agnostic as to how the model or router it's listening to is updated. All it cares about is specific events on specific components. This is actually helpful to us because it reduces the amount of special-case handling we need to implement.

The DOM structure that's generated at runtime, as a result of rendering all our views, needs to be taken into consideration as well. For example, if we look at some of the top-level DOM nodes, they have nested structure within them. It's these top-level nodes that form the skeleton of our layout. Perhaps this is rendered by the main application view, and each of our views has a child-relationship to it. Or perhaps the hierarchy extends further down than that. The tools we're using most likely have mechanisms for dealing with these parent-child relationships. However, bear in mind that vast view hierarchies are difficult to scale.

Templates

Template engines used to reside mostly in the back-end framework. That's less true today, thanks in a large part to the sophisticated template rendering libraries available in the front-end. With large-scale JavaScript applications, we rarely talk to back-end services about UI-specific things. We don't say, "here's a URL, render the HTML for me". The trend is to give our JavaScript components a certain level autonomy—letting them render their own markup.

Having component markup coupled with the components that render them is a good thing. It means that we can easily discern where the markup in the DOM is being generated. We can then diagnose issues and tweak the design of a large scale application.

Templates help establish a separation of concerns with each of our components. The markup that's rendered in the browser mostly comes from the template. This keeps markup-specific code out of our JavaScript. Front-end template engines aren't just tools for string replacement; they often have other tools to help reduce the amount of boilerplate JavaScript code to write. For example, we can embed things like conditionals and for-each loops in our markup, where they're suited.

Application-specific components

The component types we've discussed so far are very useful for implementing scalable JavaScript code, but they're also very generic. Inevitably, during implementation we're going to hit a road block—the component composition patterns we've been following will not work for certain features. This is when it's time to step back and think about possibly adding a new type of component to our architecture.

For example, consider the idea of widgets. These are generic components that are mainly focused on presentation and user interactions. Let's say that many of our views are using the exact same DOM elements, and the exact same event handlers. There's no point in repeating them in every view throughout our application. Might it be better if we were to factor it into a common component? A view might be overkill, perhaps we need a new type of widget component?

Sometimes we'll create components for the sole purpose of composition. For example, we might have a component that glues together router, view, model/collection, and template components together to form a cohesive unit. Modules partially solve this problem but they aren't always enough. Sometimes we're missing that added bit of orchestration that our components need in order to communicate.

Extending generic components

We often discover, late in the development process, that the components we rely on are lacking something we need. If the base component we're using is designed well, then we can extend it, plugging in the new properties or functionality we need. In this section, we'll walk through some scenarios where we might need to extend the common generic components used throughout our application.

If we're going to scale our code, we need to leverage these base components where we can. We'll probably want to start extending our own base components at some point too. Some tools are better than others at facilitating the extension mechanism through which we implement this specialized behavior.

Identifying common data and functionality

Before we look at extending the specific component types, it's worthwhile to consider the common properties and functionality that's common across all component types. Some of these things will be obvious up-front, while others are less pronounced. Our ability to scale depends, in part, on our ability to identify commonality across our components.

If we have a global application instance, quite common in large JavaScript applications, global values and functionality can live there. This can grow unruly down the line though, as more common things are discovered. Another approach might be to have several global modules, as shown in the following diagram, instead of just a single application instance. Or both. But this doesn't scale from an understandability perspective:

The ideal component hierarchy doesn't extend beyond three levels. The top level is usually found in a framework our application depends on

As a rule-of-thumb, we should, for any given component, avoid extending it more than three levels down. For example, a generic view component from the tools we're using could be extended by our generic version of it. This would include properties and functionality that every view instance in our application requires. This is only a two-level hierarchy, and easy to manage. This means that if any given component needs to extend our generic view, it can do so without complicating things. Three-levels should be the maximum extension hierarchy depth for any given type. This is just enough to avoid unnecessary global data, going beyond this presents scaling issues because the hierarchy isn't easily grasped.

Extending router components

Our application may only require a single router instance. Even in this case, we may still need to override certain extension points of the generic router. In case of multiple router instances, there's bound to be common properties and functionality that we don't want to repeat. For example, if every route in our application follows the same pattern, with only subtle differences, we can implement the tools in our base router to avoid repetitious code.

In addition to declaring routes, events take place when a given route is activated. Depending on the architecture of our application, different things need to happen. Maybe certain things always need to happen, no matter which route has been activated. This is where extending the router to provide our own functionality comes in hand. For example, we have to validate permission for a given route. It wouldn't make much sense for us to handle this through individual components, as this would not scale well with complex access control rules and a lot of routes.

Extending models/collections

Our models and collections, no matter what their specific implementation looks like, will share some common properties with one another. Especially if they're targeting the same API, which is the common case. The specifics of a given model or collection revolve around the API endpoint, the data returned, and the possible actions taken. It's likely that we'll target the same base API path for all entities, and that all entities have a handful of shared properties. Rather than repeat ourselves in every model or collection instance, it's better to abstract the common data.

In addition to sharing properties among our models and collections, we can share common behavior. For instance, it's quite likely that a given model isn't going to have sufficient data for a given feature. Perhaps that data can be derived by transforming the model. These types of transformations can be common, and abstracted in a base model or collection. It really depends on the types of features we're implementing and how consistent they are with one another. If we're growing fast and getting lots of requests for "outside-the-box" features, then we're more likely to implement data transformations inside the views that require these one-off changes to the models or collections they're using.

Most frameworks take care of the nuances for performing XHR requests to fetch our data or perform actions. That's not the whole story unfortunately, because our features will rarely map one-to-one with a single API entity. More likely, we will have a feature that requires several collections that are related to one another somehow, and a transformed collection. This type of operation can grow complex quickly, because we have to work with multiple XHR requests. We'll likely use promises to synchronize the fetching of these requests, and then perform the data transformation once we have all the necessary sources.

Here's an example that shows a specific model extending a generic model, to provide new fetching behavior:

// The base fetch() implementation of a model, sets
// some property values, and resolves the promise.
class BaseModel {
    fetch() {
        return new Promise((resolve, reject) => {
            this.id = 1;
            this.name = 'foo';
            resolve(this);
        });
    }
}

// Extends BaseModel with a specific implementation
// of fetch().
class SpecificModel extends BaseModel {

    // Overrides the base fetch() method. Returns
    // a promise with combines the original
    // implementation and result of calling fetchSettings().
    fetch() {
        return Promise.all([
            super.fetch(),
            this.fetchSettings()
        ]);
    }

    // Returns a new Promise instance. Also sets a new
    // model property.
    fetchSettings() {
        return new Promise((resolve, reject) => {
            this.enabled = true;
            resolve(this);
        });
    }
}

// Make sure the properties are all in place, as expected,
// after the fetch() call completes.
new SpecificModel().fetch().then((result) => {
    var [ model ] = result;
    console.assert(model.id === 1, 'id');
    console.assert(model.name === 'foo');
    console.assert(model.enabled, 'enabled');
    console.log('fetched');
});

Extending controllers/views

When we have a base model or base collection, there're often properties shared between our controllers or views. That's because the job of a controller or a view is to render model or collection data. For example, if the same view is rendering the same model properties over and over, we can probably move that bit to a base view, and extend from that. Perhaps the repetitive parts are in the templates themselves. This means that we might want to consider having a base template inside a base view, as shown in the following diagram. Views that extend this base view, inherit this base template.

Depending on the library or framework at our disposal, extending templates like this may not be feasible. Or the nature of our features may make this difficult to achieve. For example, there might not be a common base template, but there might be a lot of smaller views and templates that can plug-into larger components:

A view that extends a base view can populate the template of the base view, as well as inherit other base view functionalities

Our views also need to respond to user interactions. They may respond directly, or forward the events up the component hierarchy. In either case, if our features are at all consistent, there will be some common DOM event handling that we'll want to abstract into a common base view. This is a huge help in scaling our application, because as we add more features, the DOM event handling code additions is minimized.

Mapping features to components

Now that we have a handle on the most common JavaScript components, and the ways we'll want to extend them for use in our application, it's time to think about how to glue those components together. A router on it's own isn't very useful. Nor is a standalone model, template, or controller. Instead, we want these things to work together, to form a cohesive unit that realizes a feature in our application.

To do that, we have to map our features to components. We can't do this haphazardly either—we need to think about what's generic about our feature, and about what makes it unique. These feature properties will guide our design decisions on producing something that scales.

Generic features

Perhaps the most important aspects of component composition are consistency and reusability. While considering that the scaling influences our application faces, we'll come up with a list of traits that all our components must carry. Things like user management, access control, and other traits unique to our application. Along with the other architectural perspectives (explored in more depth throughout the remainder of the book), which form the core of our generic features:

A generic component, composed of other generic components from our framework.

The generic aspects of every feature in our application serve as a blueprint. They inform us in composing larger building blocks. These generic features account for the architectural factors that help us scale. And if we can encode these factors as parts of an aggregate component, we'll have an easier time scaling our application.

What makes this design task challenging is that we have to look at these generic components not only from a scalable architecture perspective, but also from a feature-complete perspective. As much as we'd like to think that if every feature behaves the same way, we'd be all set. If only every feature followed an identical pattern, the sky's the limit when it comes the time to scale.

But 100% consistent feature functionality is an illusion, more visible to JavaScript programmers than to users. The pattern breaks out of necessity. It's responding to this breakage in a scalable way that matters. This is why successful JavaScript applications will continuously revisit the generic aspects of our features to ensure they reflect reality.

Specific features

When it's time to implement something that doesn't fit the pattern, we're faced with a scaling challenge. We have to pivot, and consider the consequences of introducing such a feature into our architecture. When patterns are broken, our architecture needs to change. This isn't a bad thing—it's a necessity. The limiting factor in our ability to scale in response to these new features, lies with generic aspects of our existing features. This means that we can't be too rigid with our generic feature components. If we're too demanding, we're setting ourselves up for failure.

Before making any brash architectural decisions stemming from offbeat features, think about the specific scaling consequences. For example, does it really matter that the new feature uses a different layout and requires a template that's different from all other feature components? The state of the JavaScript scaling art revolves around finding the handful of essential blueprints to follow for our component composition. Everything else is up for discussion on how to proceed.

Decomposing components

Component composition is an activity that creates order; larger behavior out of smaller parts. We often need to move in the opposite direction during development. Even after development, we can learn how a component works by tearing the code apart and watching it run in different contexts. Component decomposition means that we're able to take the system apart and examine individual parts in a somewhat structured approach.

Maintaining and debugging components

Over the course of application development, our components accumulate abstractions. We do this to support a feature's requirement better, while simultaneously supporting some architectural property that helps us scale. The problem is that as the abstractions accumulate, we lose transparency into the functioning of our components. This is not only essential for diagnosing and fixing issues, but also in terms of how easy the code is to learn.

For example, if there's a lot of indirection, it takes longer for a programmer to trace cause to effect. Time wasted on tracing code, reduces our ability to scale from a developmental point of view. We're faced with two opposing problems. First, we need abstractions to address real world feature requirements and architectural constraints. Second, is our inability to master our own code due to a lack of transparency.

'Following is an example that shows a renderer component and a feature component. Renderers used by the feature are easily substitutable:

// A Renderer instance takes a renderer function
// as an argument. The render() method returns the
// result of calling the function.
class Renderer {
    constructor(renderer) {
        this.renderer = renderer;
    }

    render() {
        return this.renderer ? this.renderer(this) : '';
    }
}

// A feature defines an output pattern. It accepts
// header, content, and footer arguments. These are
// Renderer instances.
class Feature {
    constructor(header, content, footer) {
        this.header = header;
        this.content = content;
        this.footer = footer;
    }

    // Renders the sections of the view. Each section
    // either has a renderer, or it doesn't. Either way,
    // content is returned.
    render() {
        var header = this.header ?
                `${this.header.render()}n` : '',
            content = this.content ?
                `${this.content.render()}n` : '',
            footer = this.footer ?
                this.footer.render() : '';

        return `${header}${content}${footer}`;
    }
}

// Constructs a new feature with renderers for three sections.
var feature = new Feature(
    new Renderer(() => { return 'Header'; }),
    new Renderer(() => { return 'Content'; }),
    new Renderer(() => { return 'Footer'; })
);

console.log(feature.render());

// Remove the header section completely, replace the footer
// section with a new renderer, and check the result.
delete feature.header;
feature.footer = new Renderer(() => { return 'Test Footer'; });

console.log(feature.render());

A tactic that can help us cope with these two opposing scaling influencers is substitutability. In particular, the ease with which one of our components, or sub-components, can be replaced with something else. This should be really easy to do. So before we go introducing layers of abstraction, we need to consider how easy it's going to be to replace a complex component with a simple one. This can help programmers learn the code, and also help with debugging.

For example, if we're able to take a complex component out of the system and replace it with a dummy component, we can simplify the debugging process. If the error goes away after the component is replaced, we have found the problematic component. Otherwise, we can rule out a component and keep digging elsewhere.

Re-factoring complex components

It's of course easier said than done to implement substitutability with our components, especially in the face of deadlines. Once it becomes impractical to easily replace components with others, it's time to consider re-factoring our code. Or at least the parts that make substitutability infeasible. It's a balancing act, getting the right level of encapsulation, and the right level of transparency.

Substitution can also be helpful at a more granular level. For example, let's say a view method is long and complex. If there are several stages during the execution of that method, where we would like to run something custom, we can't. It's better to re-factor the single method into a handful of methods, each of which can be overridden.

Pluggable business logic

Not all of our business logic needs to live inside our components, encapsulated from the outside world. Instead, it would be ideal if we could write our business logic as a set of functions. In theory, this provides us with a clear separation of concerns. The components are there to deal with the specific architectural concerns that help us scale, and the business logic can be plugged into any component. In practice, excising business logic from components isn't trivial.

Extending versus configuring

There're two approaches we can take when it comes to building our components. As a starting point, we have the tools provided by our libraries and frameworks. From there, we can keep extending these tools, getting more specific as we drill deeper and deeper into our features. Alternatively, we can provide our component instances with configuration values. These instruct the component on how to behave.

The advantage of extending things that would otherwise need to be configured is that the caller doesn't need to worry about them. And if we can get by, using this approach, all the better, because it leads to simpler code. Especially the code that's using the component. On the other hand, we could have generic feature components that can be used for a specific purpose, if only they support this configuration or that configuration option. This approach has the advantage of simpler component hierarchies, and less overall components.

Sometimes it's better to keep components as generic as possible, within the realm of understandability. That way, when we need a generic component for a specific feature, we can use it without having to re-define our hierarchy. Of course, there's more complexity involved for the caller of that component, because they need to supply it with the configuration values.

It's a trade-off that's up to us, the JavaScript architects of our application. Do we want to encapsulate everything, configure everything, or do we want to strike a balance between the two?

Stateless business logic

With functional programming, functions don't have side effects. In some languages, this property is enforced, in JavaScript it isn't. However, we can still implement side-effect-free functions in JavaScript. If a function takes arguments, and always returns the same output based on those arguments, then the function can be said to be stateless. It doesn't depend on the state of a component, and it doesn't change the state of a component. It just computes a value.

If we can establish a library of business logic that's implemented this way, we can design some super flexible components. Rather than implement this logic directly in a component, we pass the behavior into the component. That way, different components can utilize the same stateless business logic functions.

The tricky part is finding the right functions that can be implemented this way. It's not a good idea to implement these up-front. Instead, as the iterations of our application development progress, we can use this strategy to re-factor code into generic stateless functions that are shared by any component capable of using them. This leads to business logic that's implemented in a focused way, and components that are small, generic, and reusable in a variety of contexts.

Organizing component code

In addition to composing our components in such a way that helps our application scale, we need to consider the structure of our source code modules too. When we first start off with a given project, our source code files tend to map well to what's running in the client's browser. Over time, as we accumulate more features and components, earlier decisions on how to organize our source tree can dilute this strong mapping.

When tracing runtime behavior to our source code, the less mental effort involved, the better. We can scale to more stable features this way because our efforts are focused more on the design problems of the day—the things that directly provide customer value:

The diagram shows the mapping component parts to their implementation artifacts

There's another dimension to code organization in the context of our architecture, and that's our ability to isolate specific code. We should treat our code just like our runtime components, which are self-sustained units that we can turn on or off. That is, we should be able to find all the source code files required for a given component, without having to hunt them down. If a component requires, say, 10 source code files—JavaScript, HTML, and CSS—then ideally these should all be found in the same directory.

The exception, of course, is generic base functionality that's shared by all components. These should be as close to the surface as possible. Then it's easy to trace our component dependencies; they will all point to the top of the hierarchy. It's a challenge to scale the dependency graph when our component dependencies are all over the place.

Summary

This article introduced us to the concept of component composition. Components are the building blocks of a scalable JavaScript application. The common components we're likely to encounter include things like modules, models/collections, controllers/views, and templates. While these patterns help us achieve a level of consistency, they're not enough on their own to make our code work well under various scaling influencers. This is why we need to extend these components, providing our own generic implementations that specific features of our application can further extend and use.

Depending on the various scaling factors our application encounters, different approaches may be taken in getting generic functionality into our components. One approach is to keep extending the component hierarchy, and keep everything encapsulated and hidden away from the outside world. Another approach is to plug logic and properties into components when they're created. The cost is more complexity for the code that's using the components.

We ended the article with a look at how we might go about organizing our source code; so that it's structure better reflects that of our logical component design. This helps us scale our development effort, and helps isolate one component's code from others'. It's one thing to have well crafted components that stand by themselves. It's quite another to implement scalable component communication.

For more information, refer to:

• https://www.packtpub.com/web-development/javascript-and-json-essentials
• https://www.packtpub.com/application-development/learning-javascript-data-structures-and-algorithms

Resources for Article:

Further resources on this subject:

• Welcome to JavaScript in the full stack [Article]
• Components of PrimeFaces Extensions [Article]
• Unlocking the JavaScript Core [Article]