With Rimmel you can create a whole new world of powerful HTML templates and webapps using Observables and Promises as first-class citizens.

• When a DOM event is triggered an Observer reacts
• When an Observable emits the DOM gets updated

<button onclick="${anObserver}">click me</button>

<span class="${anObservable}"></span>
<div style="color: ${anObservable};"></div>
<div>${anObservable}</div>

No need for JSX, Virtual DOM, Babel, HyperScript, Webpack, React.
No need to "set up" or "tear down" observables in your components, so you can keep them pure.
No need to unsubscribe or dispose of observers or perform any manual memory cleanup.

Rimmel works with standard JavaScript/TypeScript template literals tagged with rml out of the box.

The modern "Hello World" for reactive interfaces is the click counter: one button, you click it, he counts it.
This is how it works:

The click event is "sourced" into counter, which is an Observable Subject that takes Event objects and emits numbers.

The result is then "sinked" into the <span> element at the end.

    Rimmel Hello World on Codepen.

Most other reactive or non-reactive JavaScript UI libraries and frameworks out there are designed for the imperative programming paradigm. Occasionally they may support a few aspects of functional programming. Third-party adapters can also help with it, but the truth is that FRP was just an afterhthought and its use is severely limited in practice.

Rimmel is different in that it does primarily focus on the functional-reactive paradigm (FRP, for short). Although some imperative-reactive patterns work, supporting them is not the main purpose of this project.

What makes Rimmel functional-reactive is that you can treat everything as an Observable stream, in particular event handlers and data sinks.

This means you never really write code that changes the status of something else, as in:

target.property = value;

What you do instead, is you declare the stream your changes come from and what better place for that if not your templates?

<target property="${source}">

The concept of Virtual DOM originates from the assumption that the DOM is slow, which might be the case for some frameworks that make large numbers of unnecessary updates or re-renders, in which case it may become less expensive to run those computations outside of the DOM.

Since Rimmel never makes any unnecessary DOM update and the concept of a component re-rendering doesn't exist by itself, the whole idea of a Virtual DOM is simply not needed.

Rimmel uses Sinks, which are memory and/or performance-optimised direct DOM manipulation functions, attached to the Promises or Observable streams you provide. When there is new data, it gets sinked to the DOM immediately. You are in control of when or how often you emit data, so you can also throttle/sample/debounce as you see fit for your needs.

Several UI libraries and frameworks around have a concept of component re-rendering, which implies running the whole component's function again in order to check its output and diff it against the DOM. Since Rimmel makes use of reactive streams for state management, everything turned out to be dramatically simpler.

There is only one time a component is "rendered" and that is when it first lands on the page. After that, only "updates" happen, which are performed with native DOM calls.

If you need to render something based on a condition, plain-old standard JS can help you. No need for further abstractions:

document.body.innerHTML = rml`
  <main>
    ${
      condition == true
        ? rml`<div>yes, it's true</div>`
        : rml`<div>no, it's false</div>`
    }
  </main>

You only want to use this method if your condition is static, as in non-reactive and the content won't need to reflect your changes over time. Otherwise read on.

If you need to render based on a live, reactive condition (so when the condition changes, the other thing is rendered) you can just use a normal Observable stream:

// Use RxJS's `map` operator to emit any HTML based on whatever conditions you like
const conditionalStream = new Subject().pipe(
  map(data => {
    if(data.hasSomething) {
      return rml`<child1>`
    } else {
      return rml`<child2>`
    }
}

document.body.innerHTML = rml`
  <main>
    ${conditionalStream}
  </main>

Never forget to tag your templates with rml, otherwise they won't be reactive. This is a standard piece of HTML string:

const str = `<div>hello</div>`

And this is a reactive template managed by Rimmel:

const str = rml`<div>hello</div>`

Forward refs are a construct used in Imperative-Reactive UI libraries to enable referencing and later modifying DOM elements that don't exist yet (you're writing components that to act on DOM elements that will only exist after they are mounted).

The simple, yet effective functional paradigm used by Rimmel enables you to define any change to a DOM element as a Promise or Observable stream. The mounting and data binding is completely managed by Rimmel. This means when they resolve or emit, the elements will already be there to receive them, effectively making the use of Forward Refs redundant.

When you're dealing with structured data, like lists or grids of repeated data, the best way to handle it depends on multiple factors, so it's left as an extension opportunity.

You can create your custom, advanced Sinks to render data structures of any complexity and manage any specific aspects of its interactions in the most optimal way for your needs. This is a perfect case for reactive grids, spreadsheets, SVG or Canvas drawings, interactive 3D scenes, etc.

To display and manage the UI for any dynamic list where you plan to support CRUD operations, perhaps drag'n'drop, you may consider using a Collection from ObservableTypes, which is a rendering-aware extension of Array, every method of which is mapped to an optimised UI rendering command, and it also exposes the Observable and Observer interfaces for seamless reactivity and integration with Rimmel and RxJS.

Want a more involved example?
Let's make a component featuring a Red, Green and Blue slider that get transformed into an #RRGGBB colour string, displayed in a text box and used as the fill colour of an SVG circle:

Here is the corresponding code:

const toHex = n => n.toString(16).padStart(2, '0');
const toRGBString = rgbArr => `#${rgbArr.map(toHex).join('')}`;
const toNumericStream = x => new Subject().pipe(
  map(e => parseInt(e.target.value, 10)),
  startWith(x),
);

const ColorPicker = (initial = [0, 0, 0]) => {
  const [R, G, B] = initial.map(toNumericStream);
  const RGB = combineLatest([R, G, B]).pipe(
    map(toRGBString),
  );

  return rml`
    R: <input type="range" value="${initial[0]}" oninput="${R}">
       <span>${R}</span>

    G: <input type="range" value="${initial[1]}" oninput="${G}">
       <span>${G}</span>

    B: <input type="range" value="${initial[2]}" oninput="${B}">
       <span>${B}</span>

    Current <span>${RGB}</span>

    <svg viewbox="0 0 40 40" width="40" height="40">
      <circle fill="${RGB}" cx="20" cy="20" r="20" />
    </svg>
  `;
}

document.getElementById('root-node').innerHTML = ColorPicker([255, 128, 64])

    Hello World Plus on Codepen

As you can see, there are three main streams, one for each colour slider in the HTML. When they emit, their values are merged together through combineLatest, which passes them through as an array to toRGBString which will return the string as we need it.

Finally, we have two sinks where the data ends up; one as the innerHTML of the , ther other as the fill colour of the SVG shape.

"State", as the word itself suggests, is something static, so it doesn't belong to the dynamic, interactive, reactive webapps we make every day.

The rationale is that "state", as represented by plain old values such as numbers, strings and objects that are stored somewhere in memory is something you almost never need to read. Not now, not in 2 seconds, not in 45 minutes, not tomorrow. You only need those when certain events happen, in order to respond.

After that everything should go quiet, including your CPU, to keep your laptop cool until the next UI event occurs.

This is, in summary, the discrete-functional-reactive paradigm behind Observables and RxJS (as opposed to the functional-reactive paradigm in general in which state is more like a continuous flow of data).

Event-driven reactivity as modelled by Observables is therefore the perfect way to describe state as it changes through the lifetime of an application at the occurrence of various discrete UI events.

Modelling your state as one or more observable streams will give you fine-grained control over async events and their coordination, thanks to the full range of RxJS operators you can use.

All Rimmel does is binding your observable streams to the UI with a seamless integration that will result in improved code quality, scale, testability and performance.

Component lifecycle events such as onmount, beforeunmount, present in most other imperative frameworks quickly become useless, redudant and discouraged here. Streams and other listeners get connected and disconnected automatically for you when a component is mounted/unmounted. If you think about it, this is exactly what you would normally do in your init/onmount functions, so you no longer have to deal with these tiny details.

Since you only declare streams now and let Rimmel connect them to the DOM and each-other, your code will be immensely more concise, cleaner and more testable.

Rimmel still has a rml:onmount event, but its use is only left as a last resort to integrate imperative, non-Rimmel components (some old jQuery plugins, etc?)

You may have already realised that writing UI components with Rimmel means you no longer have to deal with side effects, which makes it trivial to make your code purely functional.

This might sound unusual, but Rimmel can actually coexist with other frameworks. Your Rimmel component can be embedded in a React component and have children made in Vue, or even jQuery plugins or sit inside a larger jQuery application, or the other way around.

If you are planning to perform a progressive framework migration, this is one way you can do it, one component at a time.

There are two key concepts used by Rimmel: sources and sinks.

Sources are things that generate data which you can optionally process and transform along the way. What remains goes somewhere. That somewhere is usually referred to as a sink.

Sources typically include any DOM events such as onclick or onmousemove, fetch() calls, just like promises in general, async functions and, most notably, Observables.

Sinks are most often the place where you want to display any information in the UI. Your main document, some HTML element, etc.

With RML/Rimmel you can treat most DOM elements as sources, sinks, or both.

Rimmel supports event listeners from all DOM elements. Static values are treated as non-observable values and no data-binding will be created. Observers such as Subjects and BehaviorSubjects will receive events as emitted by the DOM.

// Observable Subjects
const stream = new Subject<MouseEvent>();
target.innerHTML = rml`<button onclick="${stream}></button>`;


// Plain functions
const fn = (e: MouseEvent) => alert('hover');
target.innerHTML = rml`<a onmouseover="${fn}></button>`;

// Event Delegation
const buttonClicks = new Subject<MouseEvent>().pipe(
  filter(e => e.target.tagName == 'BUTTON')
);

target.innerHTML = rml`
   <div onclick="${buttonClicks}>
     <button>button1</button>
     <button>button2</button>
     <button>button3</button>
   </div>
`;

In normal circumstances your event handlers receive a native DOM Event object, such as MouseEvent, PointerEvent, etc.

To enable a better separation of concerns, as of Rimmel 1.2 you can use Event Mappers, or Event Adapters to feed your event handlers or Observable streams the exact data they need, in the format they expect it, rather than the generic, raw DOM Event objects.

Do you only need the relative [x, y] mouse coordinates when hovering an element?
Use <div onmousemove="${ OffsetXY(handler) }">

Do you want the last typed character when handling keyboard events? Use <input oninput="${ Key(handler) }">

Rimmel comes with a handful of Event Adapters out of the box, but you can create your own with ease.

If you know how to use the pipe() function from RxJS, then you almost know how to use source(...operators, target) from Rimmel. It works like pipe(), except it applies the same operators to data coming in, rather than going out of an Observable stream.

source(...operators, targetObserver)

import { rml, source } from 'rimmel';

const DatasetValue = map((e: Event) => Number(e.target.dataset.value));
const ButtonClick = filter((e: Event) => e.target.tagName == 'BUTTON');

const Component = () => {
  const total = new Subject<number>().pipe(
    scan((a, b) => a b, 0)
  );

  return rml`
    <div onclick="${source(ButtonClick, DatasetValue, total)}">

      <button data-value="1">add one</button>
      <button data-value="2">add two</button>
      <button data-value="3">add three</button>

    </div>

    Sum: <span>${total}</span>
  `;
};

As you can see, the main data model, which is the observable stream called total, receives number and emits number. The DatasetValue Event Adapter translates raw DOM events into the plain numbers required by the model. Finally, we're leveraging the DOM's standard Event Delegation by only adding one listener to the container, rather than to each button. We're making sure only button clicks are captured by using the ButtonClick filter

Rimmel supports two categories of sinks: standard (specialised) sinks and dynamic sinks. Standard sinks are the simplest and most intuitive ones: those you define in an everyday template from which the type of data binding required can be easily inferred. They are optimised for performance, updating a specific part of the DOM.

These include:

• Class (add/remove/change classes to an element)
• Dataset (add/remove/change dataset values)
• Value (set the value of an input field)
• Style (apply styles from a CSS Object/key-value pair)
• Closed (set the closed attribute of a dialog box)
• Removed (destroy an element when a source emits)
• Disabled (disable a UI control)
• Readonly (make a UI control read-only)
• InnerText, TextContent, InnerHTML, PrependHTML, AppendHTML, Sanitize
• Attribute (set any generic HTML attribute not listed above)
• Custom Sinks (create your own for specific, optimised rendering of complex data)

Some sinks are implicit as the surrounding HTML and/or CSS context makes it clear what they should do. E.g.:

<!-- set class names -->
<div class="${stream}">
<!-- set inline style -->
<div style="${stream}">
<!-- set background colour -->
<div style="background-color: ${stream};">

Other sinks can be specified explicitly when it's not obvious from the context or when several options are available to display the same content:

<!-- Remove dangerous scripts from the stream before rendering -->
<div>${Sanitize(stream)}</div>

<!-- Sink the stream's content into `.innerText`, rahter than `.innerHTML` (which is the default) on an element -->
<div>${InnerText(stream)}</div>

<!-- You take control and decide exactly how that stream should be rendered -->
<!-- Useful for complex data structures -->
<div>${YourCustomSink(stream)}</div>

// InnerHTML (this is implicit, from the context)
const stream = new Subject<HTMLString>();
target.innerHTML = rml`<div>${stream}</div>`;

// InnerText (the easy way to prevent XSS)
const stream = new Subject<HTMLString>();
target.innerHTML = rml`<div>${InnerText(stream)}</div>`;

// Class
const stream = new Subject<CSSClassObject>();
target.innerHTML = rml`<div class="${stream}"></div>`;

// Style
const stream = new Subject<CSSStyleObject>();
target.innerHTML = rml`<div style="${stream}"></div>`;

// Data Attribute
const stream = new Subject<string>();
target.innerHTML = rml`<div data-attribute="${stream}"></div>`;

// Disabled Attribute
const stream = new Subject<boolean>();
target.innerHTML = rml`<button disabled="${stream}">clickable</button>`;

// Removed (this is a RML attribute, not a standard HTML one)
const stream = new Subject<boolean>();
target.innerHTML = rml`<div rml:removed="${stream}"></div>`;

// Closed
const stream = new Subject<boolean>();
target.innerHTML = rml`<dialog closed="${stream}">...</dialog>`;

// Generic Attribute
const stream = new Subject<string>();
target.innerHTML = rml`<div some-attribute="${stream}"></div>`;

// "Sanitize" - display render-safe HTML
import { rml, Sanitize } from 'rimmel';

const stream = new Subject<HTMLString>();
target.innerHTML = rml`<div>${Sanitize(stream)}</div>`;

// A "Custom" Sink to just make output uppercase
const UpperCase = sink(
  map(strHTML => strHTML.toUpperCase())
);

const stream = new Subject<HTMLString>();
target.innerHTML = rml`<div>${UpperCase(stream)}</div>`;

Dynamic sinks, on the other hand, are optimised for size and designed for convenience. They can take anyhing as input and figure out how to update the DOM at runtime.

Mixins are an exciting by-product of dynamic sinks, which allow you to inject pretty much anything at any time (event listeners, classes, attributes, etc) into the target "host" element by means of emitting a DOM Object­— a plain-old object whose properties and methods represent DOM attributes and event listeners.

const Mixin = () => {
  const onmouseover = () => console.log('mouseover')

  const onclick = new Subject()

  // Emit 'clickable' first,
  // then 'clicked' afterwards
  const classes = onclick.pipe(
    mapTo('clicked-class'),
    startWith('clickable'),
  );

  // <! -----------------------------
  // The following DOM Object will be
  // "merged" into the target element
  // <! -----------------------------
  return {
    onclick,
    onmouseover,
    class: classes,
    'data-new-attribute': 'some value',
  }
}

// ----------------------------
// And this is how you call it:
// ----------------------------
const component = () => {
  return rml`
    <div ...${Mixin()}></div>
  `;
}

When the above component is rendered on the page, the mixin will inject everything else into it, including the onclick and the onmouseover event handlers, a statically defined data-new-attribute and a "merge-in" observable stream, classes to set classes dynamically.

Whenever the classes stream emits, you will be able to set/unset class names in the component.

You may have noticed that mixins doesn't get any reference to the elements you merge them into.

In the imperative paradigm you would indeed have one, so you can imperatively set hostElement.classList.add('big').

In order to stick with good functional principles, mixins only return this DOM Object, which is a declarative way to tell Rimmel what you want it to set on the host element.

  const returnValue = <DOMObject>{
    onclick,
    onmouseover,
    'class': classes,
    'data-new-attribute': 'some value',
    'rml:closed': closedStream,
    'disabled': disabledStream,
    'readonly': readonlyStream,
    'innerHTML': contentStream1,
    'appendHTML': contentStream2,
    // etc...
  }

A simple use case for mixins is drag'n'drop, in the form of an isolated, reusable piece of functionality.

The code above is a simple function that performs its business and returns a DOM object. Whatever it contains is merged into the host element. Static key-values (e.g.: class) are merged on the spot, before mounting. Promises and Observables get merged whenever they resolve/emit.

    Draggable Mixin on Codepen

Performance is always key, isn't it?

Actually, it depends. Some studies, in fact, show that a little bit of waiting in certain conditions can actually improve the overall user experience! (Bryant&Veroff, Kahneman&Tversky, Brickman&Campbell, Schultz)

Anyway, Rimmel is fast. You can slow it down with the Rx.delay() operator if you want, but if you don't, it's fast and here's why:

• It doesn't use a Virtual DOM. If you ever believed the story that the DOM is slow and re-running your components' code every time you blink an eye is the fast and good thing to do, you may have been the victim of a scam.
  

• Rimmel updates the DOM using "precharged" Sinks, which are just some little element-bound functions that won't need to perform any lookup, but are just ready to update the DOM as fast as possible.
  As a result, your updates in certain cases may happen even faster than the normal document.getElementById(target).style.color = newColor.
  

We call this the Vanilla Speed.

• Lightweight bundle size. V1 was just 2.5KB of code. Now it's a little more as we're sorting out a few things with specialised sinks and we are in a bit of a feature rush, but the aim is to fall back below 1KB with the launch of the new template compiler.

• The rest is on you. Rimmel is a minimalistic UI library, but the true reason for its immense power is RxJS being behind its reactivity.

Do you have a Combobox with 1M rows?
A large spreadsheet with 1k x 10k reactive cells?
An HFT stock ticker with 10000 subscriptions?

These are obviously cases where "fast updates" or raw speed are irrelevant and what truly matters is adequate frontend architecture.

These are perfect cases to create Custom Sinks implementing relevant design patterns to make extreme scenarios still work optimally in an ergonomic, functional-reactive style that's easy to test and keep well organised.

If you come from some other libraries or frameworks, including "vanilla RxJS", you know you're somewhat responsible of cleaning up memory. The indiscriminate use of Observable subscriptions without proper cleanup can cause memory leaks in certain scenarios.

Using Observables with Rimmel is trivial. All DOM subscriptions and event listeners are handled by the library behind the scenes, registered when a component is mounted and unregistered when it's removed.

Do you have async data like an API call and a placeholder to display whilst waiting? We have good news: a BehaviorSubject is all you need.

A BehaviorSubject receives a special treatment from Rimmel in that its initial value will be rendered immediately, synchronously, whilst subsequent emissions will replace it as normal. This helps avoid unnecessary reflows and jank on the page.

import { ajax } from 'rxjs/ajax';

const WaitingComponent = () => {
  const stream = new BehaviorSubject('loading...');

  ajax.getJSON('https://api.example.com/data')
    .pipe(map(data => `<pre>${JSON.stringify(data, null, 2)}</pre>`))
    .subscribe(stream)
  ;

  return rml`
    <div>${stream}</div>
  `;
}

    Suspense on Stackblitz.

We are creating a few experimental AI assistants like RimmelGPT.js, to help you convert existing components, create new ones or just get started and have fun.

(Please note these are still highly experimental and various forms of hallucination can happen under different circumstances — YMMV)

Want to delve deeper into more advanced scenarios? Have a look at the following todo app all based on RxJS streams, ObservableTypes and Rimmel.

Here's a version of Tic Tac Toe made with Observable Streams

To work with Rimmel locally, check it out then either with bun or other runtimes:

bun install
bun run build
bun test

There is a "kitchen sink" app you can use to play around locally, which should showcase most of what you can do with Rimmel:

cd examples/kitchen-sink
vite

• Completion handlers (what should happen when observables complete?)
• Error sinks (what if streams throw? Anything better than just "error boundaries"?)
• Performance benchmarks (we know it's fast, but, let's see the numbers!)
• SSR, Server Components, Full-Stack Components, Transferable Promises, Transferable Observables
• Scheduler support for real-time apps (trading front-ends, ad-tech, gaming, ...)
• Support text node and HTML comment sinks
• EventEmitter support?
• Separate memory-optimised and speed-optimised sinks
• Convenience vs Performance sinks
• Compiled Templates (because it's never fast enough, right?)
• JSX/ESX support?
• Plugin support
• Sink pipelines (just like you have rendering pipelines in computer graphics)
• RML Security (leverage pipelines to weed out XSS and other dirt)

There are discussions going on around making HTML and/or the DOM natively support Observables at WHATWG DOM/544 and the more recent Observable DOM.

Rimmel is closely following these initiatives and aims to align with them as they develop.

If you like Rimmel and would like to help the functional-reactive world grow in the JavaScript land, come say hi and let's talk.