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Andrew Meredith
Andrew Meredith

Posted on • Updated on • Originally published at

ClojureScript in the JavaScript Ecosystem

This article is Lesson 2 of Andrew's book, Learn ClojureScript

Now that we have a good idea of what ClojureScript is and how to use it, we will continue to pull back the curtain to get a clearer picture of how this curious language fits into its environment - the JavaScript ecosystem. While the language is quite different from JavaScript, it maintains a symbiotic relationship to its JavaScript host. JavaScript needs ClojureScript, and ClojureScript needs JavaScript. Let's explore this interesting symbiosis.

In this chapter:

  • What problems in JavaScript does ClojureScript try to solve?
  • How using a compiled language helps in application development
  • Why is JavaScript an ideal platform for ClojureScript?

Why JavaScript Needs Clojure

Having seen ClojureScript's sweet spots, it should be apparent that there are some gains that it promises. Still, can we get a similar advantage from JavaScript itself without having to learn a new language? Also, does ClojureScript really give us that much additional leverage in out daily development tasks? ClojureScript may not be the best tool for trivial tasks, but for anything more complex, JavaScript does in fact need a language like Clojure to enable more productive and enjoyable development.


You may have noticed several times where I have used the terms "Clojure" and "ClojureScript" interchangeably. Clojure as a language has implementations that compile to both Java bytecode and to JavaScript. Some of the potential confusion comes from the fact that "Clojure" refers to both the language and its Java implementation. I will follow the general pattern of the Clojure community of using the two terms interchangeably when talking about the language itself and using "ClojureScript" when discussing the ecosystem or language features that are specific to ClojureScript.

Higher Level Language

ClojureScript operates with higher-level constructs than JavaScript. In JavaScript, we work largely with variables, loops, conditional branching structures, objects and arrays. In ClojureScript, we work with expressions, collections, sequences, and transformations. The journey from lower-level concepts to higher-level ones is the way that we gain productivity.

Features defining each level of abstraction

Features defining each level of abstraction

When we work at a higher level, a couple of interesting things happen. First, it takes less code to accomplish a given task, which helps with both initial development and debugging/maintenance. Second, it causes the structure of the code more closely resemble the problem domain, making it clearer for us to understand when we come back to it. Third, it frees us to think more about the problems of the domain rather than technical implementation issues. All of these factors can enable huge productivity boosts, both in the initial development and maintenance phases of an application.

When we write less code to accomplish a given task, there are a couple of benefits. First, it almost goes without saying that it is quicker to write a little code than it is to a lot of code. Even though more time is usually spent designing and planning code than actually writing it, we do not want to be hampered by how many keystrokes it takes to turn our ideas into code. Second, fewer lines of code means fewer bugs. The developer who would rather spend her time fixing bugs than writing new features is either a rarity or nonexistent. The terseness of a high-level language like ClojureScript means that there are fewer places for bugs to hide, an in turn, we can spend more time making forward progress.

Less Boilerplate

I cannot count the times that I have had a simple task that I wanted to accomplish with JavaScript - say, performing a deep clone of an object - but had to do a Google search to remember how to do it either using vanilla JavaScript or the libraries that I had available. Usually, I would end up on some StackOverflow thread that I had already visited numerous times and copying and pasting the example into yet another "utils" file in yet another project. Libraries such as lodash and jQuery help compensate for JavaScript's lack of common utilities, but they do not solve the problem that one must look beyond the language itself to get the functionality of a robust standard library.

The problem of needing to pull in third party libraries for most tasks is uniquely problematic for the browser because every additional library adds time to the page load. Compound this issue with the fact that most web apps at least need to consider mobile clients with slow networks. When every byte counts, as it does on the web, we are continually faced with the question of whether to include another library for limited utility or write the functions that we need from scratch.

Finally, JavaScript developers must continually face the reality of browser compatibility issues. The available options are to target the lowest common denominator of the browser that you would like to support (and miss out on the language features that improve developer productivity), pull in libraries (and add substantial page size), or implement browser-detection and write the browser-specific portions from scratch (and face the additional complexity that comes with browser hacks). The choices do not sound very attractive, and we should not have to make a trade-off between developer productivity, performance, and complexity. In order to solve the browser compatibility problem without sacrificing any of these things, we need to look outside JavaScript itself.

ClojureScript, on the other hand, has a rich set of data structures and functions for working with collections, strings, math, state management, JavaScript interoperability, and more. Additionally, ClojureScript is built on top of Google's Closure (with an "s", not a "j") library, putting the same tools that power applications like Gmail and Google Docs at your fingertips. With so many tools at our disposal, we'll see that the amount of utility code that we need to write is minimal. Finally, ClojureScript compiles down to a widely-supported subset of JavaScript, making browser compatibility much less of an issue. ClojureScript takes the focus off the "plumbing", allowing us to focus more on the interesting problems of the domain in which we are working.

Immutable Data by Default

We have already looked at immutable data as one of the fundamental concepts of functional programming. While much of the JavaScript community is starting to recognize the value of immutable data, working with immutable data in JavaScript is still not native and can feel somewhat cumbersome. Libraries like Facebook's Immutable.js allow us to get the benefits of immutable data from JavaScript, but once again, the language currently has no native support.

In ClojureScript, however, the situation is reversed. All of the default data structures are immutable, and we have to go out of our way to work with mutable objects. This is one area where ClojureScript is very opinionated, but the style of programming that it promotes is one that will lead to fewer bugs and - as we have already seen - optimized user interfaces. Once we have become accustomed to using ClojureScript's data structures, returning to mutable objects and arrays will feel unusual - even dangerous.

Compiler Optimized

One advantage that a compiled language has is that it can implement optimizations in the JavaScript code that it produces. It is rare for a high-level language to match either the speed, resource usage, or compiled code size of a lower-level language. ClojureScript, however, can often produce JavaScript that runs as fast as hand-written JavaScript. Its immutable data structures do usually consume more memory and are slower than raw objects and arrays, but the UI optimizations afforded by these data structures can make ClojureScript interfaces effectively faster than a corresponding JavaScript interface.

One metric that matters a great deal to JavaScript programmers is code size. When working in a server-side environment, the code size is usually not a concern - the code is read from disk and immediately read into memory. However, with front-end JavaScript applications, the code usually must be read over the internet, potentially over a low-bandwidth mobile network. In this situation, every byte counts, and we are used to laboring over our code and trying to make it as small as possible, even at the cost of clarity. Minification helps tremendously, but we still must be mindful about including more libraries. Often, the benefit added by a library is offset by the kilobytes that it adds to page load time.

One of the most interesting features of the ClojureScript compiler is that it produces Google Closure modules, and it then makes use of the Closure Compiler to optimize the JavaScript. Since the ClojureScript compiler guarantees that the JavaScript it produces is valid Google Closure modules, we can safely make use of the Closure Compiler's most aggressive optimizations when preparing production assets. In addition to the typical removal of whitespace and renaming variables, the Closure Compiler will analyze an entire codebase and remove any code paths that can never be called. Effectively, this means that we can pull in a large library, and if we use only a couple of functions from this library, only those functions and the functions they call are included in our codebase. In an environment where code size is so critical, this is clearly a significant advantage.

Quick Review

  • Can you think of any pieces of code that you find yourself writing for almost every JavaScript project? Would any of these be solved by a more complete standard library?
  • What is the advantage of working in a language that compiles to Javascript? Can you think of any disadvantages?

Why Clojure needs JavaScript

As useful as the Clojure language is, it needs JavaScript. The most significant things that JavaScript enable for the Clojure language are client-side web development, the rich ecosystem of libraries and technologies, and a much lighter-weight platform with a smaller footprint than the Java Virtual Machine. That said, ClojureScript compiles to JavaScript, so it runs where JavaScript does, including the client, server, desktop, and Internet of Things (IoT) devices.

Client-Side Development

Clojure was originally a server-side language. It was certainly possible to write desktop GUIs using Swing or another Java UI toolkit, but the vast majority of Clojure was written for the server. Clojure is excellent as a server-side programming language, but as we have discussed, it brings some significant advantages to UI development as well. With the advent of ClojureScript, Clojure is now a general-purpose language that can be used for almost any application - on the server or client. As Rich Hickey stated when he announced ClojureScript, "Clojure rocks, and JavaScript reaches."

Additionally, with technologies like Electron, we have the option of writing desktop applications in JavaScript as well; and since ClojureScript compiles to JavaScript, we can take advantage of the same technologies to write desktop applications in ClojureScript as well. While Clojure itself enables developers to write Java GUI applications, many developers prefer the lighter-weight style afforded by these JavaScript UI technologies.

ClojureScript on the Desktop

The developers of the LightTable editor - one of the most popular editors supporting the Clojure language - opted to build their UI using ClojureScript and deploy inside Electron. This enabled them to build an incredibly flexible, customizable UI without the complexity of a traditional desktop UI.

Finally, there are a few technologies that allow JavaScript applications to run as mobile apps. React Native is gaining a lot of traction in this area, making it an excellent choice for ClojureScript, since most ClojureScript UIs are built on React as a platform. While this area of JavaScript mobile native apps is relatively new territory, it is showing a lot of promise. The next generation of mobile apps may be predominantly JavaScript apps, which means that ClojureScript will be a first-class citizen for mobile clients as well.

JavaScript Ecosystem

JavaScript is more than just a language - it is a community that has opinions on best practices, libraries, tooling, and development process. It is in this community that ClojureScript lives. While we as ClojureScript developers benefit from the vast number of JavaScript libraries available, the more significant benefit provided by JavaScript is its community. We can learn from the collective experience of the community what is the good, the bad, and the ugly of front-end development. The relationship between JavaScript and Clojure is truly symbiotic, with both communities benefitting from the ideas and insights of the other.

While we have seen that ClojureScript is a very practical and useful language, let's face it - it is easy for a functional programming language to lose touch with the concerns of working programmers. Theoretical languages are useful, and most useful programming language features started out as research projects, but theoretical purity is not our top concern when writing web apps. Get-it-done-ability is a much higher priority, and from its inception, JavaScript has been about getting things done as straightforwardly as possible. Being a citizen of the JavaScript community helps ClojureScript stay focused on pragmatic concerns that help us build better web applications.

Smaller Footprint

The JVM is an excellent platform for developing high-performance cross-platform applications. It is not so excellent when it comes to running in resource-constrained environments or scripting. While the slogan "Write once, run anywhere" was used by Sun Microsystems to promote Java, it is ironically JavaScript that has become a "universal" runtime. From the browser to the server to the Raspberry Pi and embedded devices, JavaScript will run just about anywhere. Running Java on something like a Raspberry Pi, on the other hand, is a practical impossibility. ClojureScript is a great option for writing applications where Java is too much bloat. Its ability to run on almost any device is another aspect of JavaScript's "reach" that we can take advantage of from ClojureScript.

Scripting is another area where Java is fairly weak. Whether as a scripting language embedded in a larger application or as a system shell scripting language, Java is too large and complex, and the startup time of the JVM makes it impractical for short-lived programs like simple scripts. JavaScript is a great scripting languages. Node.js allows us to write system scripts as well as web servers.

Quick Review

  • What is the most common platform for ClojureScript - web, desktop, mobile, or IoT devices? Can it be used outside this platform?
  • How well does ClojureScript interoperate with existing JavaScript tools and libraries?


In this chapter, we have explored the relationship of ClojureScript to its host language, JavaScript. We have seen learned:

  • How ClojureScript improves on JavaScript's development experience
  • How JavaScript's lightweight and ubiquitous runtime allows us to write ClojureScript for practically any platform.
  • Why client-side web development is a great fit for ClojureScript.

Now that we have a good understanding of both what ClojureScript is and how it is related to the JavaScript platform, we are ready to see the language in action. In the next section, we will work through the process of writing a ClojureScript application, learning the common tools and practices as we go.

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