Falsework

+++ THIS IS A WORK IN PROGRESS +++

A temporary support structure for building frontends with vanilla web components. From Wikipedia:

Falsework consists of temporary structures used in construction to support a permanent structure until its construction is sufficiently advanced to support itself.

Falsework (this "software") provides of a base class for web components and a few utilities based on Ornament.

This is not a framework, but just a set of suggestions that help to eliminate the most tedious bits of building vanilla web components. In terms of code, Falsework barely exists. You can use it as-is, but I find it much more useful as a mere starting point that a replace bits and pieces in every time I use it. The code is meant to be copied and pasted, more than it is meant to be installed as a software package.

Example

The following example uses TypeScript to show off, but Falsework does in no way require TypeScript:

import {
  type Captured,
  FalseworkElement,
  attr,
  number,
  capture,
} from "@sirpepe/falsework";

// Extending from FalseworkElement automatically registers the class as a custom
// element with the tag name derived from the class name (in this case
// "click-counter").
export class ClickCounter extends FalseworkElement {
  // Defines content attributes alongside corresponding getter/setter pairs
  // for a JS api AND attribute change handling AND type checking (and in this
  // case also bounds checking and non-NaN checking for numbers).
  @attr(number({ min: 0 })) accessor up = 0;
  @attr(number({ min: 0 })) accessor down = 0;

  // Captures events from the Shadow DOM, filtered by event name and selector
  @capture("click", "button.vote-up, button:vote-down")
  #handleUpdate(evt: Captured<MouseEvent, "button">): void {
    // Note that TypeScript understands that evt.target is neither null nor just
    // EventTarget, thanks to the helper type Captured<E, T>.
    if (evt.target.matches(".vote-up")) {
      this.up++;
    } else if (evt.target.matches(".vote-down")) {
      this.down++;
    }
  }

  // Defines CSS to inject into the shadow root for each component. This is
  // optional.
  get css() {
    return `.result { font-weight: bold }`;
  }

  // Defines the template to render in the component's shadow DOM, at most one
  // per frame. Re-rendering happens automatically every time the class state
  // (content attributes defined with @attr and/or IDL attributes defined with
  // @prop) changes.
  get template() {
    return this.html`
<p>
  <button class="vote-up">👍 ${this.up}</button>
  <span class="result">${this.up - this.down}</span>
  <button class="vote-down">👎 ${this.down}</button>
</p>`;
  }
}

Guide

General philosophy

Exit strategy

Every good library should come with an exit strategy as well as install instructions. Here is how you can get rid of Falsework if you want to migrate away:

• Components built with Falsework will generally turn out to be very close to vanilla web components (and therefore native HTML elements), so they will most probably just keep working until the heat death of the universe. As almost-native elements, your components can probably be used in projects that use other frameworks, such as Lit or React without an above-average amount much hassle.
• The best way to use Falsework is quite possibly as a mere template. You can (and maybe should) simply copy much of its source code and modify it to your liking. Maybe swap out the rendering logic, partially apply some decorators or extend the base class with additional logic specific to your use case. Falsework consists of barely anything, and most things can be tweaked or replaced.

I personally don't use Falsework as a library, but rather as a template for custom bare-bones frameworks that are slightly modified between projects. The best way to migrate away from Falsework is to never use it as-is in the first place. It is, after all, not a Framework.

Installation

If you want to ignore the previous section, you can install @sirpepe/falsework with your favorite package manager. To get the decorator syntax working, you will probably need @babel/plugin-proposal-decorators (with option version set to "2023-05") or TypeScript 5.0+ (with the option experimentalDecorators turned off).

API

Ornament

Falsework is based on Ornament and exposes Ornament's entire API. Decorators like @attr() are the way to go for defining content attributes.

Class FalseworkElement

A base class for components:

import { FalseworkElement } from "@sirpepe/falsework";

class HelloWorld extends FalseworkElement {}

Extending from FalseworkElement makes your class compatible with Ornament's decorators (like @attr()) and also provides the following additional features:

Rendering

Falsework by default bundles uhtml for rendering. If you define a getter template on your class, its return value gets rendered into the component's shadow DOM each time the component's data (accessors decorated with @prop() or @attr()) changes. You can also define a getter css for styles that need to be injected into the shadow DOM.

import { FalseworkElement } from "@sirpepe/falsework";

export class HelloWorld extends FalseworkElement {
  get css() {
    return `:host { font-weight: bold }`;
  }
  get template() {
    // this.html === uhtml's html template tag
    return this.html`<p>Hello World</p>`;
  }
}

To customize the shadow root options on a per-class-basis, override the getter shadowRootInit on your classes:

import { FalseworkElement } from "@sirpepe/falsework";

export class HelloInput extends FalseworkElement {
  get shadowRootInit() {
    return { mode: "open", delegatesFocus: true };
  }
}

The default shadow root options are { mode: "closed" };

Automagic String Tag

Decorators

Decorator @component(tagName?)

Decorator @capture(eventNames, selector)

Symbols

SHADOW_ROOT

Key for the shadow roots on FalseworkElement instances.

Types

This section is only relevant for TypeScript users.

type Captured<Event?, Target?>

Dealing with events in TypeScript sucks, because target on Event and its subclasses is always null | EventTarget. This is due to two factors that do not really matter when we use @capture():

1. target on Event is only null before the event gets dispatched. But if we capture an event using @capture(), this is never actually the case - the events have to have been dispatched in order for us to be able to capture them.
2. When not null, target on Event is EventTarget because all events can be dispatched on any compatible target. But if we use @capture(), the decorator filters targets by CSS selector, so we know that target is at least Element, or some more specific subtype.

The type Captured<Event?, Target?> expresses an instance of event subtype Event with the target Target. Both Event and Target are optional and default to Event and Element respectively.

import { type Captured } from "@sirpepe/falsework";

type T1 = Captured<Event, HTMLButtonElement>;
// = "Event" with "target" typed as "Element"

type T2 = Captured<MouseEvent, HTMLButtonElement>;
// = "MouseEvent" with "target" typed as "HTMLButtonElement"

type T3 = Captured<"click", "button">;
// = "MouseEvent" (Event type for "click" events) with "target" typed as HTMLButtonElement

type T4 = Captured<MouseEvent>;
// = "MouseEvent" with "target" typed as "Element" (default)

type T5 = Captured
// = "Event" (default) with "target" typed as "Element" (default)

Captured is useful a type for the argument on class methods decorated by @capture(). It lets you use all the fields on the event (especially target) without you having to do any manual type checking and @capture() performs compile-time type checks to ensure that it is not applied to a method that expects the wrong event or target:

class TestComponent extends FalseworkElement {
  // Type checks: "click" returns MouseEvent, input[type=button] selects <button>
  @capture("click", "input[type=button]")
  a(evt: Captured<MouseEvent, "input">) {
    // allows use of properties specific to MouseEvent and <input>
    console.log(evt.button, evt.target.value);
  }

  // Does NOT type check: incompatible event name "focus" and type "MouseEvent"
  @capture("focus", "input[type=button]")
  b(evt: Captured<MouseEvent, "input">) {}

  // Does NOT type check: incompatible event targets "<input>" and "<video>"
  @capture("focus", "input[type=button]")
  c(evt: Captured<FocusEvent, "video">) {}
}