Designing for reusability

One of the core principles of choo is that the framework should be as disposable as possible. We don't want to lock you into choo - in the contrary: we want choo components to work with or without choo; as long as there's a DOM available.

This guide is intended to show some patterns you can apply to your code in order to make it more reusable, and outlast the cycle of frameworks.

Note that in this guide the words "module" and "package" will be used interchangeably.

Directory structure

In order to facilitate reusability, one must plan for it. Often the hardest part about modularity is becoming comfortable with determining where the boundaries are. As a rule of thumb: any piece of logic that doesn't directly need to receive all values passed in by a function in choo could probably split off into its own little package. If you apply this rule aggressively you'll no doubtedly make the mistake of overmodularizing, but in doing so you'll most likely become more tuned as to what makes for a good package.

When building projects it can be nice to have a special directory into which generalized logic can find its place - ready to be published as a separate package outside the project. If you've never used a specific directory for this before, the lib/ dirname can be recommended.

Generally a typical choo project would have a directory structure somewhat similar to this:

assets/        images and fonts, if you have any
elements/      standalone application-specific elements
lib/           generalized components, should be moved out of project later
models/        choo models
pages/         views that are directly mounted on the router
scripts/       shell scripts, to be interfaced with through `npm scripts`
client.js      main application entry; programmatic manifest file
package.json   manifest file

This structure is only here as a reference (we're not going to tell you how to write your apps). But it's a model we've found to work well for choo apps as it only works well for slim applications, and focuses on splitting things off. So if you haven't decided on a structure yet, this might be a nice one to try.


Probably the premier candidate to modularize would be effects. Effects are generally the places where most complexity hides; calling the choo specific send(name, data, cb) function to issue commands to the rest of the application. Though very powerful, using send() all over the place will eventually cause for mess and giant fallout. Instead there's several patterns that might help:

send() at the borders only

[ talk about callbacks and never passing send() directly ]

Rise of the SDKs

[ talk about how SDKs can wrap APIs, making networking reusable ]


Views are passed three things: the current state of the world, the state of the world like it was in the last frame that was rendered and a send() method that can issue commands to update the state of the world. These things are the APIs choo exposes to its view components (aka components that make up views).

As with all other parts of the framework, we should try and minimize the extent of interaction between the components we create and the framework's API. An approach that works particularly nice is to create elements using a function which accepts an object with callbacks on it that are propagated into the element. Whenever an action is triggered, it calls the appropriate callback passed in.

Another important thing to keep in mind is that not all components need all data. Often time it's worthwhile to deconstruct state and only pass relevant fields into the components. This makes it easier to create testable components, and better defines the scope of an element.

Say we have a form element. We want it to trigger two types of validation: when input occurs we want to validate it on the client. When submit is clicked we want it to submit the data and validate it on the server. To create a generalized component we'd do:

// ./pages/main.js
const html = require('choo/html')
const Form = require('../components/form')

module.exports = function mainView (state, prev, send) {
  const form = Form({
    values: state.myFormModel,
    onInput: (data) => send('myFormModel:validate', data),
    onSubmit: (data) => send('myFormModel:submit', data)

  return html`
// ./components/form.js
const getFormData = require('get-form-data')
const html = require('choo/html')

module.exports = function formComponent (opts) {
  return `
      <input name="woof" type="text" placeholder="type here"
        value=${opts.values.woof} oninput=${onInput}>
      <input type="submit" onsubmit=${onSubmit}>

  function onInput (e) {
    const data = e.value
    opts.onInput && opts.onInput(data) // allows onInput callback to be optional

  function onSubmit (e) {
    const data = getFormData(
    opts.onSubmit && opts.onSubmit(data)
    e.preventDefault() // not called if above lines throw error, degrading to default form functionality

Binding components to the framework should generally happen in the pages/ directory (or whichever equivalent you prefer). If all components are imported exlicitly, and all actions are bound, concerns are separated pretty much as well as they can be.


reducers are basically data formatting pipelines. Data comes in, different data comes out. In such they are probably the most boring to test, and probably generally not super reusable. Except perhaps for validation logic. Client-side validation should be a completely synchronous function without an error condition. If data is invalid, an object should be returned with the appropriate error codes (or if it's your thing: using throw and try ... catch).

Validation logic can probably often be bundled together with standalone view components; separating the part that validates from the actual view, and binding both to the larger framework using JS functions.

// example needed


[ talk about why plugins are needed, tight coupling and common pitfalls ]

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