Choo, architecture & performance

Almost every Choo release so far has managed to include more features, and reduce the size on disk. In the upcoming v6 release we’re now sitting at 3.8Kb.

People often ask how it’s possible to pack a “batteries included” browser framework into something that’s about an order of magnitude smaller than conventional options. This post is intended to break down the bits that make up Choo, and explain why we made certain choices — all with the hopes of making frameworks feel less magical. Here goes.

note: Everything here is from our perspective. Different people have different constraints and silver bullets don’t exist. It’s fine to disagree, but keep in mind that everyone has the best intentions and it’s not a competition.


The core building block of any browser framework is creating HTML elements. We’ve chosen to use ES6 tagged template strings for this, because it allows writing domain specific languages (DSLs) in the browser without needing to change the language. In choo you can do it like this:

var html = require('choo/html')

var planet = 'earth'
var myElement = html`
Hello ${planet}

Under the hood tagged template strings are just functions, with the strings and interpolated values (e.g. planet) being passed as arguments. In turn the HTML is then parsed, and becomes a bunch of document.createElement() and Element.setAttribute() calls. The module we use for this is bel.

The downside of using tagged template strings in the browser, is that parsing the HTML and converting it to DOM elements is a little expensive. But luckily this can be optimized by using yo-yoify during compilation.

In Node, bel falls back to just rendering strings instead of DOM nodes by using pelo. When coupled with caching, this approach allows for sub-millisecond response times in many cases.


One of core values in the Unix philosophy is to fail hard and fail early. The clearer the error message, the better the debugging experience will be. Node ships with require('assert') for this purpose. It allows creating conditional statements, that if they fail will throw an Error. We use assertions all over the place; not only to check input types, but to make sure every assumption in our code is accounted for. We try and make errors friendly, and easy to track down - all with the goal to make debugging take less time, and the framework work for you.

var assert = require('assert')
assert.notEqual(typeof window, 'undefined', 'my-module: expected window to exist')

In production you might not want to ship all assertions — after all, they’re mostly to catch developer errors. Luckily there’s a transform for that™ in unassertify.

DOM diffing

Dom diffing is a form of declarative rendering. Instead of telling the browser how to create elements, you specify what you want to render and let an algorithm take care of it. In theory this might be slower than manual DOM updates, but it drastically reduces rendering bugs. As a first pass this is excellent, since it’s still possible to drop down into the DOM and perform manual mutations. DOM diffing looks somewhat like this:

var nanomorph = require('nanomorph')
var html = require('bel')

var a = html`<div>hello earthlings</div>`
var b = html`<div>tears of joy</div>`
nanomorph(a, b) // tell "a" to look like "b"

console.log(a.toString() === '<div>tears of joy</div>') // true

The module we use for diffing is nanomorph. You might have heard of the term “virtual DOM” before; this is not it. Instead of using a “virtual” DOM (e.g. nested objects), we use actual DOM nodes.

This comes with a few tradeoffs: overall it’s a little slower, but it uses less memory and doesn’t introduce a custom format to represent the DOM. This means it’s compatible with virtually all existing libraries, as long as they return DOM nodes. Yay for compatibility.


In choo we use the nanorouter library. This is a router built around a trie data structure. It supports partials (e.g. /foo/:bar) and wilcard routes (e.g. /foo/*) and doesn't do much more. It's all executed synchronously, and doesn't do anything besides routing things. Exactly what youd'd expect from a router.

var nanorouter = require('nanorouter')
var router = nanorouter()
router.route('/', myView)


Hyperlinks are one of the cornerstones of websites — it puts the Hyper in HyperText (Markup Language). Because the framework ships with a router, it’s aware of all routes. So to handle links we attach a listener at the root node for all 'click' events, and if they come from an <a> tag we handle them with the router. This makes switching pages heaps easy.

var html = require('bel')
html`<a href="/foo">click me</a>`

Events (the browser kind)

Modern browsers are quite consistent with their event implementations. Some frameworks have a notion of “synthetic events”, but we don’t do anything with that out of the box with choo (e.g. through Element.addEventListener()).

For those unfamiliar with “synthetic events”: it’s a method where instead of attaching specific listeners to elements, you attach a single listener on the root node (e.g. document.body), and catch events as they bubble up. This allows for some performance gains because you attaching listeners on elements can cost a bit of time. We consider this to be an optimization though: an optimization that can be implemented in specific cases, but not the right tradeoff to use everywhere.

var html = require('bel')
var el = html`
<button onclick=${onclick}>
click me
function onclick (e) {
console.log('button clicked')

Events (the application kind)

Many frameworks are tightly knit with some form of event bus for application logic. Probably a good example of this is React and Redux: there are other options available, but it’s usually one that has the dominant mindshare.

In choo we’ve decided to ship an event bus and router in core. Not only does this mean there’s less boilerplate when setting things up, it also means we have a clear way to send events around. The router uses it to send 'navigate' events whenever we change routes, and by emitting 'render' we can trigger re-renders in our application. This means there's one less thing to worry about.

var choo = require('choo')
var app = choo()
app.use(function (state, emitter) { = 'bar'
  emitter.on('DOMContentLoaded', function () {

The API is similar to Node’s require('events') API, with the one difference that we support .on('*'). This listener allows full inspection of each event that flows through the emitter, which is great for development logging and monitoring production loads.

Having a Node-style event emitters in core also means that more elaborate setups can be implemented on top of it. For example RxJs and pull-stream have adapters that work out of the box with choo. We use nanobus for our events.


State is just an object. It’s up to you to format it however you prefer. Some people might prefer elaborate transaction systems, while others have to minimize garbage collection. Choo leaves this entirely up to you by only exposing a mutable object, and a 'render' event to trigger re-renders.

var state = {}

Performance monitoring

Modern browsers ship something called the User Timing API, which is part of the Performance API. You can create user timings by creating two marks, and then measuring the time between them. It looks like this:

// execute code here
window.performance.measure('my-measure', 'first-mark', 'second-mark')

User timings are very useful. With them you can answer questions like: “how long did it take to render my view?” and “what is the slowest component on my page, on average?” Once you start using timings everywhere, it becomes an amazing tool to keep track of performance.

The result is something like this:

In the Choo ecosystem we use two packages for this: nanotiming to create the marks, and measure them during the browser’s idle time. And on-performance to listen to performance events as they come in. on-performance uses the window.PerformanceObserver API, which is currently only supported in Chrome and Firefox behind a flag. It seems the w3c is committed to expanding this API though, with events added for "time till first meaningful paint" and the like.

var nanotiming = require('nanotiming')
var timing = nanotiming('my-timing')
var i = 1000
while (i--) console.log(i)
var onPerformance = require('on-performance')
onPerformance(function (entry) {
console.log(, entry.duration)

Idle time

The browser event loop can create a new frame every 16 milliseconds. Creating more frames isn’t useful, because humans don’t notice if we show more than 60 frames a second. A good strategy to hit 60 FPS is to create a distinction between “important work” and “work that we can get to eventually”. This often boils down to “work important to render things” and “everything else”.

Modern browsers ship with an API that allows you to schedule work in the spare time of each frame. The window.requestIdleCallback() API triggers a callback when all timers have resolved for a frame, and all rendering is done. Not all browser support this API though, so we use the on-idle package for this.

var onIdle = require('on-idle')
onIdle(function () {
console.log('yay, spare time!')

A fair warning with this API tho: if you use it to render anything on the DOM it will cause a blocking recalc at the start of the next frame. Use this API only to prepare for renders (e.g. create DOM nodes but without appending them to the DOM), or do other work that doesn’t affect the DOM directly.

We make use of idle time for things like logging, creating HTTP requests, mutating objects and otherwise expensive operations that don’t directly affect the user interface.

We’re big fans of the RIC API because it allows for better results by working smarter, not harder.

Animation frames

Another API to work with the browser frames is window.requestAnimationFrame. In Choo we use nanoraf to create an upper limit of 60 frames a second, but do nothing by default. Re-rendering is done by calling the emit('render') event. Because re-renders are explicit, we can update the state without it also causing a re-render.

var nanoraf = require('nanoraf')
var loop = nanoraf(function () {

Wrapping up

And that’s about it. There’s many more things going on in the extended choo ecosystem (e.g. compilers, API frameworks, reusable components) but this should cover the core of things. Let us know what you thought in the comments.

✌️ -Yosh