[RFC]: JSX-based approach for brookjs components

Status: Withdrawn

We’re attempting to implement an API like this on top of React, instead of in a separate library.

brookjs was originally designed to be backed with a Handlebars-based templating engine, providing a clean separation between what constitutes structure (HTML in the Handlebars template), interaction (JavaScript component declaration), & style (CSS files). Using Handlebars to back components allows them to be rendered by any backend, no JavaScript required. The display of the DOM is expressed as a Handlebars template, and behavior is mapped from the template to the component’s configuration through Handlebars helpers.

While this approach for writing components is common (this is not unlike Backbone.Events), using a templating language like Handlebars limits what we can feasibly do with a component. A JSX-based approach would allow the developer to express her view of the DOM over time through Observables and expose the full power of JavaScript to do so, rather than being limited by what the templating language affords. Specifically, this could enable:

  1. static typechecking of our components, including props$
  2. simplified data flow down into child components
  3. CSS-in-JS solutions for styling
  4. stateful components with embedded reducers

This post lays out the initial JSX-based API for components.

Mental Model: Membrane Controlling Data Flow

The idea now is to think of a component as an expression of how data flows into and out of a particular section of the DOM. It’s a membrane around the stateful DOM contents within it, ensuring that any changes made to what’s inside is controlled and protected by Observables and the pure functions that interact with them.

Testing a component thus entails pushing data and events through the membrane and ensuring the right values come out. We can push props into the component, do snapshot testing with the resulting element, and verify the correct effects were emitted, and fire events against that section of the DOM and verify the correct actions were emitted. This would cover the full behavior of the component. Combining that with Storybook-based screenshot test workflow would provide a solid testing experience for brookjs components.

Changes Expressed as Observables

This is our first component, and highlights the first two APIs we’ll move into JSX:

import { h, component } from 'brookjs'
import { editClick } from '../actions'

// Changes only occur when bound to a stream
export default component({
    render: props$ => (
  • {/* Text changes when a value is emitted. */}

    {props$.map(props => props.text)}

    {/* Maps a stream of events to actions. */}
  • ) })

    The first thing to note is the text of the element is expressed a stream embedded in the JSX. Any changes in the element must be expressed as an embedded Observable. The render function is only called once and passed the stream of props$ for the component.

    The second thing is events are now expressed as onX attributes, which take functions. These functions are called with a stream of event$, which are the same functions passed into the current events helper, so there’s no change in the logic when converting to JSX. Backwards compatibility will be maintained by migrating the current Handlebars-based attributes to the new style at runtime.

    Note that we’re no longer using the render helper function in the above example. We’ll determine whether this is a new-style or old-style component based on whether it’s using the old helper functions. We can deprecate those helper functions when (and if!) we deprecate the handlebars-based mechanism for rendering components (discussed below).

    Dealing with Attributes & Children Components

    This is an example of a component using a child component:

    import { h, component } from 'brookjs'
    import TodoList from './TodoList'
    export default component({
        render: props$ => (

    Todo App

    {/** * Individual attributes respond to observables * Performance optimized inline */} props.editing).skipDuplicates()} onInput={event$ => event$.map(event => editTodo(event.target.value))} /> props.todos)} />
    ) })

    Even attributes on a component must be expressed as an Observable. Again, anything dynamic must be expressed as an Observable. This allows the developer to optimize rendering at a very granular level. Individual attributes can be filtered, and skipDuplicates becomes a version of React’s shouldComponentUpdate, but instead of needing a single function to account for the entire component, we can write a function for each individual change in a component.

    Child components look similar to React, but instead of being passed individual props, they’ll be provided a stream of props$ based on the parent’s props$. In the previous version, we had a hook for this, modifyChildProps, but because the render context was handled through Handlebars, it wasn’t actually that useful. This makes the idea intended by modifyChildProps easily expressed in the render function.

    Lists of Children

    Similar to React, an Observable can return an array of elements, so a props$ stream can be mapped to an array of children components to embed a list in the JSX:

    import { h, component } from 'brookjs'
    import TodoItem from './TodoItem'
    export default component({
        render: props$ => (

    My Todos

    {/** * `order` provides sequence todos should appear * `dict` provides a performant instance lookup */}
      {props$.map(todos => todos.order.map(key => ( todos.dict[key])} key={key} preplug={instance$ => instance$.map(action => ({ ...action, meta: { key } })} /> )))}
    ) })

    As described in the comment, the todos passed into the TodoList component is an object containing an order key and a dict key. The order is an array of keys which indicate the order the todos appear in. The dict key is an object that holds all the key / value pairs of the given todo. These two combined provide a mechanism for rendering lists of components. This is ideal because searching through an array for the correct instance based on a property in an array of items could hurt performance with a large array.

    key works the same way it does with React. It indicates a unique instance of a component. This ensures that should a component be rearranged, the correct instance is moved to the new location.

    preplug is a hook into the child component’s instance before it get plugged into the parent component which allows the developer to modify child actions and contextualize them. This allows small child components to emit generic, reusable actions while the parent components modify them based on where they come from. This is also currently doable with the current Handlebars-based API.

    This doesn’t have to be done manually; we will provide an iteration helper to do this for you as well as cache the stream for a given component, but even that case should provide a stream of order / dict objects. That would simplify list rendering to this:

    import { h, component, list } from 'brookjs'
    import TodoItem from './TodoItem'
    export default component({
        render: props$ => (

    My Todos

      {/* Must be a stream of objects with `order` & `dict` */} {list(props$, (props$, key) => ( instance$.map(action => ({ ...action, meta: { key } })} /> )}
    ) })

    The parent component is thus responsible only for putting the child component in the right order in the list, while the child component takes its stream of props$ and updates itself in response to its values.

    Bootstrapping the Application

    While we’d maintain the component-as-function approach for backwards compatibility, we’d like to move to an external function to mount the component in order to enable custom renderers, the same way React does. We could probably borrow some ideas from Ink for a custom CLI renderer and update brookjs-cli to use it.

    To those ends, we could update the application bootstrapping step to look like this:

    import { createStore, applyMiddleware } from 'redux'
    import { h, observeDelta, Kefir } from brookjs
    import { App } from './components'
    import { selectProps } from './selectors'
    const store = createStore(
        (state, action) => state, // reducer
        window.__INITIAL_STATE__ || {},
            /* register deltas here */
    const state$ = Kefir.fromESObservable(store)
     * `mount` thus takes the DOM to mount
     * and the element to bind it to, and
     * returns a stream. Note that because
     * of how streams work, nothing happens
     * until the stream is observed.
    const view$ = mount(

    This makes the application startup look functionally similar to React, except the mount function returns an Observable. Running observe, binding the view to the store, starts up the application.

    To some extent, this breaks the architecture, as the view is now "special", rather than just another delta where side-effects occur, but it’s more idiomatic in the React community.

    If the domDelta is currently in use, then no modifications need to be made. Bootstrapping would continue to look like this:

    import { createStore, applyMiddleware } from 'redux'
    import { h, observeDelta, Kefir } from brookjs
    import { App } from './components'
    import { selectProps } from './selectors'
    const el = document.getElementById('app')
    const store = createStore(
        (state, action) => state,
        window.__INITIAL_STATE__ || {},
            /* register deltas here */
            domDelta({ el, selectProps, view: App })
    // Everything is bound to the store immediately,
    // but an init action makes sure everything waits
    // until the store is fully instantiated.

    This is how brookjs-cli currently scaffolds a new app, so we might not need to change much in userland to ensure backwards compatibility. This is also basically how brookjs-cli bootstraps itself, so there’s decent precedent that this works. Under the hood, the domDelta can use the new mount function without breaking any backwards compatibility. We can also write other delta functions around other view compatibility layers.

    Backwards Compatibility for Current Components

    For now, backwards compatibility will be maintained through the old helper functions, which indicate Handlebars-style components. New components will just pass in an object of functions directly, and the component function will map the functions to the various lifecycle hooks. They also become an easy target for deprecation if we decide to remove support for Handlebars-based components.

    Deprecate Handlebars?

    We gain two advantages by getting rid of Handlebars:

    1. If a user isn’t using Handlebars components, the parse step is loaded for no reason, expanding the bundle size unnecessarily.
    2. If a user is using Handlebars, the parse function is a performance bottleneck which can be better optimized through JSX-based components.

    The only way to eliminate both these problems is to build a vdom-backed Handlebars implementation, which is not a trivial effort. We also have to maintain two compatibility layers, which is both extra code and extra overhead, as we also have to run and maintain tests for both types of components to ensure they both continue to work as expected. There isn’t enough bandwidth to implement both a JSX-based & a Handlebars-based component system.

    If the Handlebars implementation is doomed to be an inferior experience, does it make sense to deprecate it over the next few versions, as the JSX implementation comes online?

    Upgrade Path

    A Handlebars component can be upgraded to a JSX component by copying & pasting the Handlebars template into render, and replacing all of the mustache tags, helpers, & partials with Observables derived from the passed in props$ stream. Logic can be moved from their current locations as configuration to inline in the JSX. Logic won’t have to change. Userland tests should continue to function as they do currently, and we will introduce some test helpers to simplify testing and support both snapshot and screenshot testing.

    Alternative to JSX: Handlebars-based vdom

    The alternative is to continue in the current direction. Work has already begun on the Handlebars-to-vdom parser, but supporting the entire spec will also take time, so both streams can’t be pursued in parallel. Spec tests are available and we’ll be working to ensure compliance with all of it. The main benefit of a Handlebars-based approach is that it’s possible to do server-side rendering without a Node- or JavaScript-based back-end, but that requires full compliance with the Handlebars spec in order to ensure interop. Switching to JSX would deny us this advantage, requiring JavaScript to be executed in order to generate HTML from a component.

    Is server-side rendering worth what we’d gain with a JSX-based approach?

    Why Not Build on React? (Prior Art)

    The design of the API was inspired by two projects that enable embedding Observables into React components:

    Both of these wrap React components into "lifted" React components that accept Observables as React children. It then wraps those children to update when the Observable emits a new value. The syntax you find in these examples matches much of what we’d like to do with brookjs.

    Both of these are solid solutions. If you’re already using react-redux, you can integrate karet and our observeDelta to start using Observables throughout your application. If you’re already using RxJS, focal and redux-observable would be a reasonable combination as well, and would give you access to the full React ecosystem.

    In order to fulfill the full API presented above, we won’t be able to rely on React because the design conflicts with two areas we’d like to enable:

    1. Declaring events as functions of Observables, and thus returning an Observable when mounting, would either be invasive or require writing a custom React reconciler, which would be difficult to impossible to fully support the ecosystem while enabling this feature.
    2. Declaring effects as Observables. The current implementation of modifyEffect$$ provides a low-level hook into the rendering process, and this will probably not be implementable on top of React, which has its own rendering process, Fiber.

    Because of these two goals of brookjs, we probably can’t build on top of React, although investigation will continue in this area to see if it’s feasible.

    Edit this post on GitHub.

    This post is part of the thread: brookjs - an ongoing story on this site. View the thread timeline for more context on this post.