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Mid-level, pareto-optimal, treeshakable and tiny (< 5k) TypeScript-positive toolkit for web component infrastructure

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Ornament

📢 What's new in 2.1.0? Check out the Changelog!

Build your own frontend framework with Ornament, a mid-level, pareto-optimal, treeshakable and tiny TypeScript-positive toolkit for web component infrastructure! Escape from heavyweight frameworks, constant rewrites and the all-encompassing frontend FOMO with a declarative, simple, and type-safe API for web components:

import {
  define,
  attr,
  string,
  number,
  connected,
  reactive,
} from "@sirpepe/ornament";

// Register the element with the specified tag name
@define("my-greeter")
class MyGreeter extends HTMLElement {
  // No built-in rendering functionality. Shadow DOM or light DOM? Template
  // strings, JSX, or something else entirely? You decide!
  #shadow = this.attachShadow({ mode: "open" });

  // Define content attributes alongside corresponding getter/setter pairs
  // for a JS api and attribute change handling and type checking. If you use
  // TypeScript, the type checks will work at compile time *and* at run time
  @attr(string()) accessor name = "Anonymous";
  @attr(number({ min: 0 })) accessor age = 0;

  // Mark the method as reactive to have it run every time one of the attributes
  // change, and also run it when the component first connects to the DOM.
  @reactive()
  @connected()
  greet() {
    this.#shadow.innerHTML = `Hello! My name is ${this.name}, my age is ${this.age}`;
  }
}

Ornament makes web components fun and easy when compared to the equivalent boilerplate monstrosity that one would have to write by hand otherwise:

😱 Unveil the horror 😱 
class MyGreeter extends HTMLElement {
  #shadow = this.attachShadow({ mode: "open" });

  // Internal "name" and "age" states, initialized from the element's content
  // attributes, with default values in case the content attributes are not set.
  // The value for "age" has to be figured out with some imperative code in the
  // constructor to keep NaN off our backs.
  #name = this.getAttribute("name") || "Anonymous";
  #age;

  constructor() {
    super(); // mandatory boilerplate
    let age = Number(this.getAttribute("age"));
    if (Number.isNaN(age)) {
      // Remember to keep NaN in check
      age = 0;
    }
    this.#age = 0;
  }

  // Remember to run the reactive method when connecting to the DOM
  connectedCallback() {
    this.greet();
  }

  // Method to run each time `#name` or `#age` changes
  greet() {
    this.#shadow.innerHTML = `Hello! My name is ${this.#name}, my age is ${this.#age}`;
  }

  // DOM getter for the IDL attribute, required to make JS operations like
  // `console.log(el.name)` work
  get name() {
    return this.#name;
  }

  // DOM setter for the IDL attribute with type checking and/or conversion *and*
  // attribute updates, required to make JS operations like `el.name = "Alice"`
  // work.
  set name(value) {
    value = String(value); // Remember to convert/check the type!
    this.#name = value;
    this.setAttribute("name", value); // Remember to sync the content attribute!
    this.greet(); // Remember to run the method!
  }

  // DOM getter for the IDL attribute, required to make JS operations like
  // `console.log(el.age)` work
  get age() {
    return this.#age;
  }

  // DOM setter for the IDL attribute with type checking and/or conversion *and*
  // attribute updates, required to make JS operations like `el.age = 42` work.
  set age(value) {
    value = Number(value); // Remember to convert/check the type!
    if (Number.isNaN(value) || value < 0) {
      // Remember to keep NaN in check
      value = 0;
    }
    this.#age = value;
    this.setAttribute("age", value); // Remember to sync the content attribute!
    this.greet(); // Remember to run the method!
  }

  // Attribute change handling, required to make JS operations like
  // `el.setAttribute("name", "Bob")` update the internal element state.
  attributeChangedCallback(name, oldValue, newValue) {
    // Because `#name` is a string, and attribute values are always strings as
    // well we don't need to convert the types at this stage, but we still need
    // to manually make sure that we fall back to "Anonymous" if the new value
    // is null (if the attribute got removed) or if the value is (essentially)
    // an empty string
    if (name === "name") {
      if (newValue === null || newValue.trim() === "") {
        newValue = "Anonymous";
      }
      this.#name = newValue;
      this.greet(); // Remember to run the method!
    }
    // But for "#age" we do again need to convert types, check for NaN, enforce
    // the min value of 0...
    if (name === "age") {
      const value = Number(value); // Remember to convert/check the type!
      if (Number.isNaN(value) || value < 0) {
        // Remember to keep NaN in check
        value = 0;
      }
      this.#age = value;
      this.greet(); // Remember to run the method!
    }
  }

  // Required for attribute change monitoring to work
  static get observedAttributes() {
    return ["name", "age"]; // remember to always keep this up to date
  }
}

// Finally remember to register the element
window.customElements.define("my-greeter", MyGreeter);

Ornament makes only the most tedious bits of building vanilla web components (attribute handling and lifecycle reactions) easy by adding some primitives that really should be part of the standard, but aren't. Ornament is not a framework, but something that you want to build your own framework on top of. Combine Ornament's baseline web component features with something like uhtml or Preact for rending, add your favorite state management library, write some glue code and enjoy your very own frontend web framework.

Guide

Installation

Install @sirpepe/ornament with your favorite package manager. To get the decorator syntax working in 2024, you will probably need @babel/plugin-proposal-decorators (with option version set to "2023-11") or TypeScript 5.0+ (with the option experimentalDecorators turned off).

Apart from that, Ornament is just a bunch of functions. No further setup required, no extra concepts to learn.

General philosophy

The native APIs for web components are verbose and imperative, but lend themselves to quite a bit of streamlining with the upcoming syntax for ECMAScript Decorators. The native APIs are also missing a few important primitives. Ornament's goal is to provide the missing primitives and to streamline the developer experience. Ornament is not a framework but instead aims to be:

  • as stable as possible by remaining dependency-free, keeping its own code to an absolute minimum, and relying on iron-clad web standards where possible
  • fast and lean by being nothing more than just a bag of relatively small and simple functions
  • supportive of gradual adoption and removal by being able to co-exist with vanilla web component code
  • malleable by being easy to extend, easy to customize, and easy to get rid of
  • universal by adhering to (the spirit of) web standards, thereby staying compatible with vanilla web component code as well as all sorts of web frameworks
  • equipped with useful type definitions (and work within the constraints of TypeScript)

Ornament is infrastructure for web components and not a framework itself. It makes dealing with the native APIs bearable and leaves building something actually sophisticated up to you. Ornament does not come with any of the following:

  • state management (even though it is simple to connect components to signals or event targets)
  • rendering (but it works well with uhtml, Preact and similar libraries)
  • built-in solutions for client-side routing, data fetching, or anything beyond the components themselves
  • any preconceived notions about what should be going on server-side
  • specialized syntax for every (or any specific) use case

You can (and probably have to) therefore pick or write your own solutions for the above features. Check out the examples folder for inspiration!

Exit strategy

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

  • Components built with Ornament will generally turn out to be very close to vanilla web components, so they will most probably just keep working when used with other frameworks/libraries. You can theoretically just keep your components and replace them only when the need for change arises. A compatibility wrapper for frameworks that are not quite friendly to web components (eg. React) may be required.
  • If you want to replace Ornament with hand-written logic for web components, you can replace all attribute and update handling piecemeal. Ornament's decorators co-exist with native attributeChangedCallback() and friends just fine. Ornament extends what you can do with custom elements, it does not abstract anything away.
  • Much of your migration will depend on how you build on top of Ornament. You should keep reusable components and app-specific state containers separate, just as you would do in e.g. React. This will make maintenance and eventual migration much easier, but this is really outside of Ornament's area of responsibility.

In general, migrating away should not be too problematic. The components that you will build with Ornament will naturally tend to be self-contained and universal, and will therefore more or less always keep chugging along.

Decorators

API overview

Decorator Class element static #private Symbols Summary
@define() Class - - - Register a custom element class with a tag name and set it up for use with Ornament's other decorators
@enhance() Class - - - Set up a custom element class for use with Ornament's other decorators, but do not register it with a tag name
@prop() Accessor ✕ ✓ ✓ Define an accessor to work as an IDL attribute with a given data type
@attr() Accessor ✕ ✓1 ✓1 Define an accessor to work as a content attribute and associated IDL attribute with a given data type
@reactive() Method, Field2 ✕ ✓ ✓ Run a method or class field function when accessors decorated with @prop() or @attr() change value (with optional conditions)
@init() Method, Field2 ✕ ✓ ✓ Run a method or class field function after the class constructor finishes
@connected() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element connects to the DOM
@disconnected() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element disconnects from the DOM
@adopted() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element is adopted by a new document
@formAssociated() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element is associated with a form element
@formReset() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element's form owner resets
@formDisabled() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element's ancestor fieldset is disabled
@formStateRestore() Method, Field2 ✕ ✓ ✓ Run a method or class field function when the element's formStateRestoreCallback fires
@subscribe() Accessor, Method, Field2 ✕ ✓ ✓ Update a reactive accessor or run a method or class field function to react to changes to a signal or to events on an EventTarget
@observe() Method, Field2 ✕ ✓ ✓ Run a method or class field function as a callback for an IntersectionObserver, MutationObserver, or ResizeObserver
@debounce() Method, Field2 ✓ ✓ ✓ Debounce a method or class field function, (including static)

@define(tagName: string, options: ElementDefinitionOptions = {}, registry: CustomElementRegistry = window.customElements)

Class decorator to register a class as a custom element, basically an alternative syntax for customElements.define();

import { define } from "@sirpepe/ornament";

@define("my-test")
class MyTest extends HTMLElement {}

console.log(document.createElement("my-test")); // instance of MyTest

@define() also sets up attribute observation for use with the @attr() decorator, prepares the hooks for lifecycle decorators like @connected() and ensures that property upgrades for previously undefined elements happen in a predictable fashion.

What are safe upgrades?

HTML tags can be used even if the browser does not (yet) know about them, and this also works with web components - the browser can upgrade custom elements event after the parser has processed them as unknown elements. But this can lead to unexpected behavior when properties are set on elements that have not yet been properly defined, shadowing relevant accessors on the prototype:

const x = document.createElement("hello-world");
// "x" = unknown element = object with "HTMLElement.prototype" as prototype

x.data = 42;
// "x" now has an _own_ property data=42

// Implements an accessor for hello-world. The getters and
// setters end up as properties on the prototype
class HelloWorld extends HTMLElement {
  accessor data = 23;
}

window.customElements.define("hello-world", HelloWorld);
// It is now clear that "x" should have had "HelloWorld.prototype" as its
// prototype all along

window.customElements.upgrade(x);
// "x" now gets "HelloWorld.prototype" as its prototype (with the accessor)

console.log(x.data);
// logs 42, bypassing the getter - "x" itself has an own property "data", the
// accessor on the prototype is shadowed

Ornament ensures safe upgrades, always making sure that no prototype accessors for attributes are ever shadowed by properties defined before an element was properly upgraded.

Notes for TypeScript

You should add your custom element's interface to HTMLElementTagNameMap to make it work with native DOM APIs:

@define("my-test")
export class MyTest extends HTMLElement {
  foo = 1;
}

declare global {
  interface HTMLElementTagNameMap {
    "my-test": MyTest;
  }
}

let test = document.createElement("my-test");
console.log(test.foo); // only type checks with the above interface declaration

If you want to run your component code in a non-browser environment like JSDOM, you can pass the JSDOM's CustomElementRegistry as the third argument to @define().

@enhance()

Class decorator to set up attribute observation and lifecycle hooks without registering the class as a custom element.

import { enhance } from "@sirpepe/ornament";

@enhance()
class MyTest extends HTMLElement {}

// MyTest can only be instantiated when it has been registered as a custom
// element. Because we use @enhance() instead of @define() in this example, we
// have to take care of this manually.
window.customElements.define("my-test", MyTest);

console.log(document.createElement("my-test")); // instance of MyTest

This decorator is only really useful if you need to handle element registration in some other way than what @define() provides. It is safe to apply @enhance() more than once on a class, or on both (or either) a base class and subclass:

import { enhance } from "@sirpepe/ornament";

// Not useful, but also not a problem
@enhance()
@enhance()
@enhance()
class MyTest0 extends HTMLElement {}

// Works
@enhance()
class Base1 extends HTMLElement {}
class MyTest1 extends Base1 {}

// Works
class Base2 extends HTMLElement {}
@enhance()
class MyTest2 extends Base2 {}

// Works
@enhance()
class Base3 extends HTMLElement {}
@enhance()
class MyTest3 extends Base3 {}

@prop(transformer: Transformer<any, any>)

Accessor decorator to define an IDL property on the custom element class without an associated content attribute. Such a property is more or less a regular accessor with two additional features:

  • it uses transformers for type checking and validation
  • changes cause class members decorated with @reactive() to run

Example:

import { define, prop, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Applies the number transformer to ensure that foo is always a number
  @prop(number()) accessor foo = 23;

  // Automatically runs when "foo" (or any accessor decorated with @prop() or
  // @attr()) changes
  @reactive()
  log() {
    console.log(`Foo changed to ${this.foo}`);
  }
}

let testEl = document.createElement("my-test");
console.log(testEl.foo); // logs 23
testEl.foo = 42; // logs "Foo changed to 42"
console.log(testEl.foo); // logs 42
testEl.foo = "asdf"; // throw exception (thanks to the number transformer)

Accessors defined with @prop() work as a JavaScript-only API. Values can only be accessed through the accessor's getter, invalid values are rejected by the setter with exceptions. @prop() can be used on private accessors or symbols without problem.

Note that you can still define your own accessors, getters, setters etc. as you would usually do. They will still work as expected, but they will not cause @reactive() methods to run.

@attr(transformer: Transformer<any, any>, options: AttrOptions = {})

Accessor decorator to define an IDL attribute with a matching content attribute on the custom element class. This results in something very similar to accessors decorated with @prop(), but with the following additional features:

  • Its value can be initialized from a content attribute, if the attribute is present
  • Changes to the content attribute's value (eg. via setAttribute()) update the value of the IDL attribute to match (depending on the options and the transformer)
What's the deal with content attributes?

Getting attribute handling on Web Components right is hard, because many different APIs and states need to interact in just the right way and the related code tends to end up scattered across various class members. Attributes on HTML elements have two faces: the content attribute and the IDL attribute. Content attributes are always strings and are defined either via HTML or via DOM methods like setAttribute(). IDL attributes can be accessed via object properties such as someElement.foo and may be of any type. Both faces of attributes need to be implemented and properly synced up for an element to be truly compatible with any software out there - a JS frontend framework may work primarily with IDL attributes, while HTML authors or server-side rendering software will work with content attributes.

Keeping content and IDL attributes in sync can entail any or all of the following tasks:

  • Updating the content attribute when the IDL attribute gets changed (eg. update the HTML attribute id when running element.id = "foo" in JS)
  • Updating the IDL attribute when the content attribute gets changed (eg. element.id should return "bar" after element.setAttribute("id", "bar"))
  • Converting types while updating content and/or IDL attributes (an attribute may be a number as an IDL attribute, but content attributes are by definition always strings)
  • Rejecting invalid types on the IDL setter (as opposed to converting types from content to IDL attributes which, like all of HTML, never throws an error)
  • Connecting IDL and content attributes with different names (like how the content attribute class maps to the IDL attribute className)
  • Fine-tuning the synchronization behavior depending on circumstances (see the interaction between the value content and IDL attributes on <input>)
  • Remembering to execute side effects (like updating Shadow DOM) when any IDL and/or content attribute changes

This is all very annoying to write by hand, but because the above behavior is more or less the same for all attributes, it is possible to to simplify the syntax quite a bit:

import { attr, define number } from "@sirpepe/ornament";

@define("my-test")
class MyTest extends HTMLElement {
  @attr(number({ min: -100, max: 100 })) accessor value = 0;

  @reactive()
  log() {
    console.log(this.value);
  }
}

The line starting with with @attr gets you a content and a matching IDL attribute named value, which...

  • Always reflects a number between -100 and 100
  • Initializes from the content attribute and falls back to the initializer value 0 if the attribute is missing or can't be interpreted as a number
  • Automatically updates the content attribute with the stringified value of the IDL attribute when the IDL attribute is updated
  • Automatically updates the IDL attribute when the content attribute is updated (it parses the attribute value into a number and clamps it to the specified range)
  • Implements getters and setters for the IDL attributes, with the getter always returning a number and the setter rejecting invalid values (non-numbers or numbers outside the specified range of [-100, 100])
  • Causes the method marked @reactive() to run on update

You can use @prop() for standalone IDL attribute (that is, DOM properties without an associated content attributes), swap out the number() transformer for something else, or combine any of the above with hand-written logic.

import { define, attr, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Applies the number transformer to ensure that content attribute values get
  // parsed into numbers and that new non-number values passed to the IDL
  // attribute's setter get rejected
  @attr(number()) accessor foo = 23; // 23 = fallback value

  // Automatically runs when "foo", or any accessor decorated with @prop() or
  // @attr(), changes (plus once on element initialization)
  @reactive() log() {
    console.log(`Foo changed to ${this.foo}`);
  }
}

document.body.innerHTML = `<my-test foo="42"></my-test>`;
let testEl = document.querySelector("my-test");
console.log(testEl.foo); // logs 42 (initialized from the attribute)
testEl.foo = 1337; // logs "Foo changed to 1337"
console.log(testEl.foo); // logs 1337
console.log(testEl.getAttribute("foo")); // logs "1337"
testEl.foo = "asdf"; // throw exception (thanks to the number transformer)
testEl.setAttribute("foo", "asdf"); // works, content attributes can be any string
console.log(testEl.foo); // logs 23 (fallback value)

Accessors defined with @attr() work like all other supported attributes on built-in elements. Content attribute values (which are always strings) get parsed by the transformer, which also deals with invalid values in a graceful way (ie without throwing exceptions). Values can also be accessed through the IDL property's accessor, where invalid values are rejected with exceptions by the setter.

@attr() can only be used on private accessors or symbols only if the following holds true:

  1. The option as must be set
  2. A non-private, non-symbol getter/setter pair for the attribute name defined in the option as must exist on the custom element class

Content attributes always have public IDL attribute APIs, and ornament enforces this. A private/symbol attribute accessor with a manually-provided public facade may be useful if you want to attach some additional logic to the public API (= hand-written getters and setters) while still having the convenience of of using @attr on an accessor.

Note that you can still define your own attribute handling with attributeChangedCallback() and static get observedAttributes() as you would usually do. This will keep working work as expected, but changes to such attributes will not cause @reactive() methods to run.

Options for @attr()

  • as (string, optional): Sets an attribute name different from the accessor's name, similar to how the class content attribute works for the className IDL attribute on built-in elements. If as is not set, the content attribute's name will be equal to the accessor's name. as is required when the decorator is applied to a symbol or private property.
  • reflective (boolean, optional): If false, prevents the content attribute from updating when the IDL attribute is updated, similar to how value works on input elements. Defaults to true.

@reactive(options: ReactiveOptions = {})

Method and class field decorator that runs class members when accessors decorated with @prop() or @attr() change their values:

import { define, reactive, prop, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @prop(number()) accessor foo = 0;
  @prop(number()) accessor bar = 0;

  @reactive()
  log() {
    console.log(`foo is now ${this.foo}, bar is now ${this.bar}`);
  }
}

let testEl = document.createElement("my-test");
testEl.foo = 1;
testEl.bar = 2;

// first logs "foo is now 1, bar is now 0"
// then logs "foo is now 1, bar is now 2"

Decorated members are called with no arguments. They react to changes to the instances' internal state and should therefore be able to access all relevant data through this. In many cases you may want to apply @reactive() to methods decorated with @debounce() to prevent excessive calls.

The predicate and/or keys options can be used to control whether the decorated method or function reacts to an update. For the decorated member to run, the following needs to be true:

  1. options.keys must either have been omitted or must contain the IDL or content attribute name that changed
  2. options.excludeKeys must either have been omitted or must not contain the IDL or content attribute name that changed
  3. options.predicate must either have been omitted or must return true when called immediately before the function is scheduled to run

Options for @reactive()

  • keys (Array<string | symbol>, optional): List of attributes (defined by @prop() or @attr()) to monitor. Can include private names and symbols. Defaults to monitoring all content and IDL attributes defined by @prop() or @attr().
  • excludeKeys (Array<string | symbol>, optional): List of attributes (defined by @prop() or @attr()) not to monitor. Can include private names and symbols. Defaults to an empty array.
  • predicate (Function (instance: T) => boolean): If provided, controls whether or not the decorated method is called for a given change. Note that this function is not part of the class declaration itself and can therefore not access private fields on instance.

@init()

Method and class field decorator that runs class members when the class constructor finishes. This has the same effect as adding method calls to the end of the constructor's body.

import { define, init } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  constructor() {
    super();
    console.log(23);
  }

  @init()
  log() {
    console.log(42);
  }
}

let testEl = document.createElement("my-test");
// first logs 23, then logs 42

This decorator is particularly useful if you need to run @reactive() methods once on component initialization.

Decorated members are run with no arguments and always right after the constructor finishes, even methods and class field functions decorated with @debounce().

@connected()

Method and class field decorator that runs class members when the component connects to the DOM and the component's connectedCallback() fires:

import { define, connected } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @connected() log() {
    console.log("Connected!");
  }
}

let testEl = document.createElement("my-test");
document.body.append(testEl);
// testEl.log logs "Connected!"

Decorated members are run with no arguments. You can also still use the regular connectedCallback().

@disconnected()

Method and class field decorator that runs decorated class members when the component disconnects from the DOM and the component's disconnectedCallback() fires:

import { define, adopted } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @disconnected() log() {
    console.log("Disconnected!");
  }
}

let testEl = document.createElement("my-test");
document.body.append(testEl);
testEl.remove();
// testEl.log logs "Disconnected!"

Decorated members are run with no arguments. You can also still use the regular disconnectedCallback().

@adopted()

Method and class field decorator that runs decorated class members when the component is moved to a new document and the component's adoptedCallback() fires:

import { define, adopted } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @adopted() log() {
    console.log("Adopted!");
  }
}

let testEl = document.createElement("my-test");
const newDocument = new Document();
newDocument.adoptNode(testEl);
// testEl.log logs "Adopted!"

Decorated members are run with no arguments. You can also still use the regular adoptedCallback().

@formAssociated()

Method and class field decorator that runs decorated class members when a form-associated component's form owner changes and its formAssociatedCallback() fires:

import { define, formAssociated } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  static formAssociated = true;
  @formAssociated() log(newOwner) {
    console.log(newOwner); // null or HTMLFormElement
  }
}

let testEl = document.createElement("my-test");
let form = document.createElement("form");
form.append(testEl);
// testEl.log logs "form"

Decorated members are passed the new form owner (if any) as an argument. You can also still use the regular formAssociatedCallback().

@formReset()

Method and class field decorator that runs decorated class members when a form-associated component's form owner resets and its formResetCallback() fires:

import { define, formReset } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  static formAssociated = true;
  @formReset() log() {
    console.log("Reset!");
  }
}

let testEl = document.createElement("my-test");
let form = document.createElement("form");
form.append(testEl);
form.reset();
// ... some time passes...
// testEl.log logs "Reset!"

Decorated members are run with no arguments. You can also still use the regular formResetCallback().

Note that form reset events are observably asynchronous, unlike all other lifecycle events. This is due to the form reset algorithm itself being async.

@formDisabled()

Method and class field decorator that runs decorated class members when a form-associated component's fieldset gets disabled and its formDisabledCallback() fires:

import { define, formDisabled } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  static formAssociated = true;
  @formDisabled() log(state) {
    console.log("Disabled via fieldset:", state); // true or false
  }
}

let testEl = document.createElement("my-test");
let fieldset = document.createElement("fieldset");
let form = document.createElement("form");
form.append(fieldset);
fieldset.append(testEl);
fieldset.disabled = true;
// testEl.log logs "Disabled via fieldset: true"

Decorated members are passed the new form disabled state as an argument. You can also still use the regular formDisabledCallback().

@formStateRestore()

Method and class field decorator that causes runs decorated class methods when a form-associated component's formStateRestoreCallback() fires. This does not work in Chrome-based browsers as of November 2023.

@subscribe(...args)

Accessor, method or class field decorator that subscribes to either Event Targets or signals, depending on the arguments. If the decorated class member is a method or a function, it runs when the EventTarget emits a new event or when the signal receives a new value. If the decorated member is an accessor, it gets updated with the last event object (for event targets) or signal values (for signals) and in turn causes methods decorated with @reactive() to run.

Subscribe to EventTargets: @subscribe(targetOrTargetFactory: EventTarget | ((instance: T) => EventTarget) | Promise<EventTarget>, eventNames: string, options: EventSubscribeOptions = {})

Subscribe the decorated class member to one or more events an EventTarget. EventTarget is an interface that objects such as HTMLElement, Window, Document and many more objects implement. You can also create a vanilla event target or extend the EventTarget class:

import { define, subscribe } from "@sirpepe/ornament";

const myTarget = new EventTarget();

@define("my-test")
class Test extends HTMLElement {
  @subscribe(myTarget, "foo")
  log(evt) {
    // evt = Event({ name: "foo", target: myTarget })
    // this = Test instance
    console.log(`'${evt.type}' event fired!`);
  }
}

let testEl = document.createElement("my-test");

myTarget.dispatchEvent(new Event("foo"));

// testEl.log logs "'foo' event fired!"

To subscribe to multiple events, pass a single string with the event names separated by whitespace:

import { define, subscribe } from "@sirpepe/ornament";

const myTarget = new EventTarget();

@define("my-test")
class Test extends HTMLElement {
  @subscribe(myTarget, "foo bar") #a() {} // subscribed to both "foo" and "bar"
}

You can also provide a target-producing factory or promise in place of the target itself:

import { define, subscribe } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // "window" is a perfectly valid event target
  @subscribe(window, "update") #a() {} // same effect as below
  @subscribe(() => window, "update") #b() {} // same effect as above
  @subscribe(Promise.resolve(window), "update") #c() {} // same effect as above
}

The target-producing factory function can be used to access targets that depend on the element instance, such as the element's shadow root. The factory function gets called each time an element initializes, with its first argument set to the instance.

Notes for TypeScript

An event target can actually be delivered by an arbitrarily long chain of nested functions and promises. This is annoying to handle on the type level, you'll just have to any your way around that or provide this capability in a type-safe wrapper.

Making the @subscribe() decorator type-safe for use with events is a gnarly prospect. Given an event target and an event name, the decorator can't know what type of event the method must expect. Therefore the following is possible by default:

import { define, subscribe } from "@sirpepe/ornament";

let target = document.createElement("div");

@define("my-test")
class Test extends HTMLElement {
  @subscribe(target, "click")
  #handleClicks(evt: MouseEvent) {} // This type checks, as it should

  @subscribe(target, "click")
  #handleAnimations(evt: AnimationEvent) {} // This type checks too!
}

A mapping between event names and corresponding event types (such as "click" → MouseEvent) exists for specific cases. For example HTMLElementEventMap contains the mappings for events emitted by HTML elements. But because @subscribe() can work with any event target, the existence or relevancy of such a mapping can't be assumed. The only way around this is to create an abstraction for specific use cases where such a mapping is available. This can be based on @subscribe() itself:

// Create a variant of @subscribe() specific to DOM events
const listen = <
  T extends HTMLElement,
  K extends keyof HTMLElementEventMap,
>(
  source: HTMLElement,
  ...eventNames: K[]
) =>
  subscribe<T, HTMLElement, HTMLElementEventMap[K]>(
    source,
    eventNames.join(" "),
  );

const eventSource = document.createElement("div");
class Test extends HTMLElement {
  // Works: "click" is a MouseEvent
  @listen(eventSource, "click")
  handleClick(evt: MouseEvent) {}

  // Works: all event types listed by name are covered in the union
  @listen(eventSource, "transitionstart", "animationstart")
  handleAnimationStart(evt: AnimationEvent | TransitionEvent) {}

  // Type error: "focus" is not a mouse event
  @listen(eventSource, "focus")
  handleFocus(evt: MouseEvent) {}

  // Type error: type "TransitionEvent" is not covered
  @listen(eventSource, "transitionend", "animationend")
  handleAnimationEnd(evt: AnimationEvent) {}

  // Type error: "asdf" is not a DOM event
  @listen(eventSource, "asdf")
  handleAsdf(evt: Event) {}
Options for @subscribe() for EventTarget
  • targetOrTargetFactory (EventTarget | Promise<EventTarget> | ((instance: T) => EventTarget) | Promise<EventTarget>): The event target (or event-target-returning function/promise) to subscribe to
  • eventNames (string): The event(s) to listen to. To subscribe to multiple events, pass a single string with the event names separated by whitespace
  • options (object, optional): Event handling options, consisting of...
    • predicate (function (instance: T, event: Event) => boolean, optional): If provided, controls whether or not the decorated method is called for a given event. Gets passed the element instance and the event object, and must return a boolean. Note that this method always handles the raw event object, before and eventual transform() is applied.
    • transform (function <U>(instance: T, event: Event) => U, optional): If provided, transforms the event object into something else. The decorated class element must be compatible with the type returned from transform().
    • activateOn (Array<string>, optional): Ornament event on which to activate the subscription (that is, when to actually start listening on the EventTarget). Defaults to ["init", "connected"].
    • deactivateOn (Array<string>, optional): Ornament event on which to deactivate the subscription (when to call removeEventListener() on the EventTarget). Defaults to ["disconnected"].
    • capture (boolean, optional): option for addEventListener()
    • once (boolean, optional): option for addEventListener()
    • passive (boolean, optional): option for addEventListener()
    • signal (AbortSignal, optional): option for addEventListener()

Subscribe to Signals: @subscribe(signal: SignalLike<any>, options: SignalSubscribeOptions = {})

Subscribe to a signal:

import { define, subscribe } from "@sirpepe/ornament";
import { signal } from "@preact/signals-core";

const counter = signal(0);
@define("my-test")
class Test extends HTMLElement {
  @subscribe(counter)
  test() {
    console.log(counter.value);
  }
}
const instance = new Test();
counter.value = 1;
counter.value = 2;
counter.value = 3;
// logs 0, 1, 2, 3

Any signal object that implements the following API should work:

type Signal<T> = {
  // Takes an update callback and returns an unsubscribe function
  subscribe(callback: () => void): () => void;
  // Represents the current value
  value: T;
};

Because signals permanently represent reactive values, subscribing itself causes the method to be called with the then-current signal value. This is in contrast to subscribing to Event Targets, which do not represent values, but just happen to throw events around.

Options for @subscribe() for signals
  • signal (Signal): The signal to subscribe to
  • options (object, optional): Update handling options, consisting of...
    • predicate (function (instance: T, value) => boolean, optional): If provided, controls whether or not the decorated method is called for a given signal update. Gets passed the element instance and the signal's value, and must return a boolean. Note that this method always handles the raw signal value, before and eventual transform() is applied.
    • transform (function <U>(instance: T, value) => U, optional): If provided, transforms the signal value into something else. The decorated class element must be compatible with the type returned from transform().
    • activateOn (Array<string>, optional): Ornament event on which to activate the subscription (that is, when to actually subscribe to the Signal). Defaults to ["init", "connected"].
    • deactivateOn (Array<string>, optional): Ornament event on which to unsubscribe from the signal. Defaults to ["disconnected"].

@debounce(options: DebounceOptions = {})

Method and class field decorator for debouncing method/function invocation:

import { define, debounce } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Debounce a class method
  @debounce()
  test1(x) {
    console.log(x);
  }
  // Debounce a class field function
  @debounce() test2 = (x) => console.log(x);
}

const el = new Test();

el.test1(1);
el.test1(2);
el.test1(3);
// only logs "3"

el.test2("a");
el.test2("b");
el.test2("c");
// only logs "c"

@debounce() works with class methods, static methods, and class field functions.

Notes for TypeScript

Debouncing a method or class field function makes it impossible for the method or function to return anything but undefined. TypeScript does currently not allow decorators to modify its target's type, so @debounce() can't do that. If you apply @debounce() to a method (x: number) => number, TypeScript will keep using this signature, even though the decorated method will no longer be able to return anything but undefined.

Options for @debounce()

  • fn (function, optional): The debounce function to use. Defaults to debounce.raf(). The following debounce functions are available:
    • debounce.raf(): uses requestAnimationFrame()
    • debounce.timeout(ms: number): uses setTimeout()
    • debounce.asap(): runs the function after the next microtask

@observe(ctor: ObserverConstructor, options: ObserverOptions = {})

Method and class field decorator that sets up a MutationObserver, ResizeObserver, or IntersectionObserver with the element instance as the target and the decorated method as the callback. This enables the web component to observe itself:

import { define, observe } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Pass the observer constructor and relevant options
  @observe(MutationObserver, { childList: true })
  reactToChanges(mutations, observer) {
    // "mutations" = array of MutationRecord objects
    // "observer" = the observer instance
    console.log(mutations);
  }
}

const el = new Test();
el.innerText = "Test"; // cause mutation

// Test.reactToChanges() gets called asynchronously by the observer

@observe() always observes the element that the decorated method belongs to and its reactions are always observably (heh) asynchronous. The decorator does little more than create an observer object with the options provided and the decorated method as the callback function. In theory this should work with every kind of DOM-related observer but has only been tested with MutationObserver, ResizeObserver and IntersectionObserver so far.

Options for @observe()

  • Ctor (function): The observer constructor function (probably one of MutationObserver, ResizeObserver, and IntersectionObserver)
  • options (object, optional): A mixin type consisting of
    • All options for the relevant observer type (see MDN for options for MutationObserver, ResizeObserver, IntersectionObserver)
    • predicate (function (instance: T, records, observer) => boolean): If provided, controls whether or not an observer callback invocation calls the decorated method. Gets passed the observer's callback arguments (an array of records and the observer object) as well as the element instance and must return a boolean.
    • activateOn (Array<string>, optional): Ornament event on which to start observing the element. Defaults to ["init", "connected"].
    • deactivateOn (Array<string>, optional): Ornament event on which to stop observing the element. Defaults to ["disconnected"].

Transformers

Transformers define how the accessor decorators @attr() and @prop() implement attribute handling and type transformations. This includes converting content attributes from and to IDL attributes, type checks on IDL setters, and running side effects.

Transformers overview

Transformer Type Options
string() string
href() string (URL) location
bool() boolean
number() number min, max, allowNaN, nullable
int() bigint min, max, nullable
json() Any (JSON serializable for use with @attr()) reviver, replacer
list() Array separator, transform
literal() Any values, transform
any() any
event() function | null

A transformer is just a bag of functions with the following type signature:

export type Transformer<
  T extends HTMLElement,
  Value,
  IntermediateValue = Value,
> = {
  // Validates and/or transforms a value before it is used to initialize the
  // accessor. Can also be used to run a side effect when the accessor
  // initializes. Defaults to the identity function.
  init: (
    this: T,
    value: Value,
    context: ClassAccessorDecoratorContext<T, Value>,
    isContentAttribute: boolean,
  ) => Value;
  // Turns content attribute values into IDL attribute values. Must never throw
  // exceptions, and instead always just deal with its input. Must not cause any
  // observable side effects. May return NO_VALUE in case the content attribute
  // can't be parsed, in which case the @attr() decorator must not change the
  // IDL attribute value
  parse: (this: T, value: string | null) => Value | typeof NO_VALUE;
  // Decides if setter inputs, which may be of absolutely any type, should be
  // accepted or rejected. Should throw for invalid values, just like setters on
  // built-in elements may. Must not cause any observable side effects.
  validate: (
    this: T,
    value: unknown,
    isContentAttribute: boolean,
  ) => asserts value is IntermediateValue;
  // Transforms values that were accepted by validate() into the proper type by
  // eg. clamping numbers, normalizing strings etc.
  transform: (this: T, value: IntermediateValue) => Value;
  // Turns IDL attribute values into content attribute values (strings), thereby
  // controlling the attribute representation of an accessor together with
  // updateContentAttr(). Must never throw, defaults to the String() function
  stringify: (this: T, value: Value) => string;
  // Determines whether a new attribute value is equal to the old value. If this
  // method returns true, reactive callbacks will not be triggered. Defaults to
  // simple strict equality (===).
  eql: (this: T, a: Value, b: Value) => boolean;
  // Optionally run a side effect immediately before the accessor's setter is
  // invoked. Required by the event transformer.
  beforeSet: (
    this: T,
    value: Value,
    context: ClassAccessorDecoratorContext<T, Value>,
    attributeRemoved: boolean,
  ) => void;
  // Optionally transform the getter's response. Required by the href
  // transformer.
  transformGet: (this: T, value: Value) => Value;
  // Decides if, based on a new value, an attribute gets updated to match the
  // new value (true/false) or removed (null). Only gets called when the
  // transformer's eql() method returns false. Defaults to a function that
  // always returns true.
  updateContentAttr: (
    oldValue: Value | null,
    newValue: Value | null,
  ) => boolean | null;
};

Because transformers need to potentially do a lot of type juggling and bookkeeping, they are somewhat tricky to get right, but they are also always only a few self-contained lines of code. If you want to extend Ornament, you should simply clone one of the built-in transformers and modify it to your liking.

Notes for all transformers

For use with both @prop() and @attr()

In principle all transformers can be used with both @prop() and @attr(). Very few transformers are limited to use with either decorator, such as event() (which makes very little sense outside of content attributes).

The accessor's initial value serves as fallback value in case no other data is available (eg. when a content attribute gets removed). Transformers validate their initial value and most transformers contain reasonable default values ("" for string(), 0 for number() etc.).

For use with @attr()

A content attribute's IDL attribute value can be unset to the accessor's initial value by removing a previously set content attribute:

As an example:

import { define, attr, string } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(string()) accessor foo = "default value";
  @attr(string()) accessor bar = "default value";
  @attr(string()) accessor baz;
}

document.body.innerHTML += `<my-test foo="other value"></my-test>`;

The attributes foo, bar and baz behave as follows:

  • The element initializes with a content attribute foo already set in HTML. The IDL attribute foo will therefore (because it uses the string type via the string() transformer) contain "other value". Should the content attribute foo get removed, the IDL attribute will contain "default value".
  • The content attribute bar is not set in HTML, which will result in the IDL attribute bar containing the accessor's default value "default value".
  • The content attribute baz is also not set in HTML and the accessor has no initial value, so the string() transformer's built-in fallback value "" gets used.

Transformer string()

Implements a string attribute or property. Modeled after built-in string attributes such as id and lang, it will always represent a string and stringify any and all non-string values.

import { define, attr, string } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(string()) accessor foo = "default value";
}

Behavior overview for transformer string()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute to x String(x) IDL attribute value
Set content attribute Content attribute value As set (equal to IDL attribute value)
Remove content attribute Initial value or "" Removed

Transformer href({ location = window.location }: { location?: Location } = {})

Implements a string attribute or property that works exactly like the href attribute on <a> in that it automatically turns relative URLs into absolute URLs.

import { define, attr, href } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(href()) accessor foo = "";
}

let testEl = new Test();

// Assuming that the page is served from localhost:
console.log(testEl.foo); // > ""
testEl.foo = "asdf";
console.log(testEl.foo); // > "http://localhost/asdf"
testEl.foo = "https://example.com/foo/bar/";
console.log(testEl.foo); // > "https://example.com/foo/bar/"

If you want to run your component code in a non-browser environment like JSDOM, you can pass the JSDOM's window.location as the option location.

Behavior overview for transformer href()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute to absolute URL (string) Absolute URL IDL attribute value
Set IDL attribute to any other value x Relative URL to String(x) IDL attribute value
Set content attribute to absolute URL (string) Absolute URL As set
Set content attribute to any other string x Relative URL to x As set
Remove content attribute Initial value or "" Removed

Transformer number(options: NumberOptions = {})

Implements a number attribute with optional range constraints.

import { define, attr, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // With default options (see below)
  @attr(number()) accessor foo = 0;

  // With all options set
  @attr(number({ min: 0, max: 10 })) accessor bar = 0;
}

Non-numbers get converted to numbers. The transformer allows null and undefined (with the latter converting to null) if the option nullable is set to true. If converting a non-number to a number results in NaN and the option allowNaN is not set to true, the property setter and the accessor's initializer throw exceptions.

Options for transformer number()

  • min (number, optional): Smallest possible value. Defaults to -Infinity. Content attribute values less than min get clamped, IDL attribute values get validated and (if too small) rejected with an exception. Can be omitted or set to null or undefined to signify no minimum value.
  • max (number, optional): Largest possible value. Defaults to Infinity. Content attribute values greater than max get clamped, IDL attribute values get validated and (if too large) rejected with an exception. Can be omitted or set to null or undefined to signify no maximum value.
  • allowNaN (boolean, optional): Whether or not NaN is allowed. Defaults to false.
  • nullable (boolean, optional): Whether or not null and undefined (with the latter converting to null) are allowed. Defaults to false.

Behavior overview for transformer number()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute to value x minmax(opts.min, opts.max, toNumber(x, allowNaN)) String(IDL attribute value)
Set IDL attribute to out-of-range value RangeError String(IDL attribute value)
Set IDL attribute to null or undefined null is nullable is true, otherwise 0 Removed if nullable is true, otherwise String(IDL attribute value)
Set content attribute to value x minmax(opts.min, opts.max, toNumber(x, allowNaN)) As set
Set content attribute to non-numeric value No change, or NaN if option allowNaN is true As set
Set content attribute to out-of-range value No change As set
Remove content attribute null is nullable is true, otherwise initial value or 0 Removed

Transformer int(options: IntOptions = {})

Implements a bigint attribute. Content attribute values are expressed as plain numeric strings without the trailing n used in JavaScript's BigInt literal syntax.

import { define, attr, int } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // With default options (see below)
  @attr(int()) accessor foo = 0n;

  // With all options set
  @attr(int({ min: 0n, max: 10n, nullable: false })) accessor bar = 0n;
}

The transformer allows null and undefined (with the latter converting to null) if the option nullable is set to true. In all other cases, the IDL attribute setter throws an exception when its input cannot be converted to BigInt.

Options for transformer int()

  • min (bigint, optional): Smallest possible value. Defaults to the minimum possible bigint value. Content attribute values less than min get clamped, IDL attribute values get validated and (if too small) rejected with an exception. Can be omitted or set to null or undefined to signify no minimum value.
  • max (bigint, optional): Largest possible value. Defaults to the maximum possible bigint value. Content attribute values greater than max get clamped, IDL attribute values get validated and (if too large) rejected with an exception. Can be omitted or set to null or undefined to signify no maximum value.
  • nullable (boolean, optional): Whether or not null and undefined (with the latter converting to null) are allowed. Defaults to false.

Behavior overview for transformer int()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute to value x minmax(ops.min, opts.max, BigInt(x)) String(IDL attribute value)
Set IDL attribute to out-of-range value RangeError String(IDL attribute value)
Set IDL attribute to null or undefined null is nullable is true, otherwise 0n Removed if nullable is true, otherwise String(IDL attribute value)
Set IDL attribute to non-int value BigInt(x) String(IDL attribute value)
Set content attribute to value x minmax(opts.min, opts.max, BigInt(x)) As set
Set non-int content attribute Clamp to Int if float, otherwise no change As set
Remove content attribute null is nullable is true, otherwise initial value or 0 Removed

Transformer bool()

Implements a boolean attribute. Modeled after built-in boolean attributes such as disabled. Changes to the IDL attribute values toggle the content attribute and do not just change the content attribute's value.

import { define, attr, bool } from "@sirpepe/ornament";

class DemoElement extends HTMLElement {
  @attr(bool()) accessor foo = false;
}

In this case, the IDL attribute foo always represents a boolean. Any non-boolean value gets coerced to booleans. If the content attribute foo gets set to any value (including the empty string), foo returns true - only a missing content attribute counts as false. Conversely, the content attribute will be set to the empty string when the IDL attribute is true and the attribute will be removed when the IDL attribute is false.

If you want your content attribute to represent "false" as a string value, you can use the literal() transformer with the strings "true" and "false".

Behavior overview for transformer bool()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute to value x Boolean(x) Removed when IDL attribute is false, otherwise set to empty string
Set content attribute to x true As set
Remove content attribute false Removed

Transformer list(options: ListOptions = {})

Implements an attribute with an array of values, defined by another transformer. The list() transformer passes individual items to the transformer passed in the options and deals with content attributes by splitting and/joining stringified array contents:

import { define, attr, list, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(list({ transform: number(), separator: "," })) accessor numbers = [0];
}

This parses the content attribute numbers as a comma-separated list of strings, which are in turn parsed into numbers by the number() transformer passed to the list() transformers options. If the content attribute gets set to something other than a comma-separated list of numeric strings, the attribute's value resets back to the initial value [0]. Any attempt at setting the IDL attribute to values other arrays of will result in an exception outright. Depending on the transformer the array's content may be subject to further validation and/or transformations.

Note that when parsing a content attribute string, values are trimmed and empty strings are filtered out before they are passed on to the inner transformer:

import { define, attr, list, number } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(list({ transform: number() })) accessor foo = [0];
}
const el = new Test();
el.setAttribute("foo", "   1, , ,,2   ,3     ");
console.log(el.foo); // > [1, 2, 3]

Options for list(options?)

  • separator (string, optional): Separator string. Defaults to ","
  • transform (Transformer): Transformer to use, eg. string() for a list of strings, number() for numbers etc.

Behavior overview for transformer list()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute Exception is not an array, otherwise array with content guarded by options.transformer.validate IDL attribute values joined with options.separator
Set content attribute Attribute value is split on the separator, then trimmed, then non-empty strings are passed into options.transformer.parse As set
Remove content attribute Initial value or empty array Removed

Transformer literal(options: LiteralOptions = {})

Implements an attribute with a finite number of valid values. Should really be called "enum", but that's a reserved word in JavaScript. It works by declaring the valid list of values and a matching transformer. If, for example, the list of valid values consists of strings, then the string() transformer is the right transformer to use:

import { define, attr, literal, string } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(literal({ values: ["A", "B"], transform: string() })) accessor foo =
    "A";
}

In this case, the content attribute can be set to any value (as is usual in HTML), but if the content attribute gets set to a value other than A or B, the IDL attribute's value will remain unchanged. Any attempt at setting the IDL attribute to values other than A or B will result in an exception.

The default value is either the value the accessor was initialized with or, if the accessor has no initial value, the first element in values.

Options for literal(options?)

  • values (array): List of valid values. Must contain at least one element.
  • transform (Transformer): Transformer to use, eg. string() for a list of strings, number() for numbers etc.

Behavior overview for transformer literal()

Operation IDL attribute value Content attribute (when used with @attr())
Set IDL attribute value to x Exception if not in options.values, otherwise defined by options.transformer.validate Defined by options.transformer.stringify
Set content attribute to x Parsed by options.transformer.parse. If the result is in options.values, result, otherwise no change As set
Remove attribute Initial value or first element in options.values Removed

Transformer json(options: JSONOptions = {})

Implements an attribute that can take any value. When used with @attr(), the value must be serializable with JSON in order to be reflected as a content attribute. When used with @prop(), no restrictions apply.

import { define, attr, prop, json } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Must be valid JSON when used with @attr()
  @attr(json()) accessor foo = { user: "", email: "" };
  // When used with prop, any value can be used
  @prop(json()) accessor foo = { value: 42n };
}

Content attributes, defined with @attr(), are parsed with JSON.parse(). In this case, any invalid JSON is represented with the data used to initialize the accessor. Using the IDL attribute's setter with inputs than can't be serialized with JSON.stringify() throws errors. This transformer is really just a wrapper around JSON.parse() and JSON.stringify() without any object validation. Equality is checked with ===.

Notes for TypeScript Even though the transformer will accept literally any JSON-serializable value at runtime, TypeScript may infer a more restrictive type from the accessor's initial value. Decorators can't currently change the type of class members they are applied to, so you man need to provide a type annotation.

Options for json(options?)

  • reviver (function, optional): The reviver argument to use with JSON.parse(), if any. Only of use when used with @attr()
  • replacer (function, optional): The replacer argument to use with JSON.stringify(), if any. Only of use when used with @attr()

Behavior overview for transformer json() (when used with @attr())

Operation IDL attribute value Content attribute
Set IDL attribute value to x JSON.parse(JSON.stringify(x)) JSON.stringify(idlValue, null, options.reviver)
Set content attribute to x No change if invalid JSON, otherwise JSON.parse(x, options.receiver) As set
Remove content attribute Initial value or undefined Removed

Behavior overview for transformer json() (when used with @prop())

Operation IDL attribute value Content attribute
Set IDL attribute value to x x -

Transformer any()

Implements a transformer that does no type checking at all and falls back to the global String function for serializing to content attributes. Use this if you really don't care about types.

import { define, prop, any } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @prop(any()) accessor whatever: any = 42;
}

Transformers returned from calling any() make for great prototypes for your own custom transformer. Just note that transformers are bags of functions and not classes, so you will need to use Object.setPrototypeOf() and friends to "extend" transformers.

Notes for TypeScript Even though the transformer will accept literally any value at runtime, TS may infer a more restrictive type from the accessor's initial values. Decorators can't currently change the type of class members they are applied to, so you may need to provide an `any` type annotation.

Transformer event()

Implements old-school inline event handler attributes in the style of onclick="console.log(42)". To work properly, this should only be used in conjunction with @attr() (with reflectivity enabled) and on a non-private, non-static accessor that has a name starting with on:

import { define, attr, eventHandler } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(event()) accessor onfoo: ((evt: Event) => void) | null = null;
}

This can then be used in HTML:

<my-test onfoo="console.log('Foo event:', event)"></my-test>
<script>
  document.querySelector("my-test").dispatchEvent(new Event("foo"));
  // Logs "'Foo event:', Event{type: "foo"}"
</script>

Or in JavaScript:

const testEl = document.createElement("my-test");
testEl.onfoo = (event) => console.log("Foo event:", event);
testEl.dispatchEvent(new Event("foo"));
// Logs "'Foo event:', Event{type: "foo"}"

Regular "proper" addEventListener() is obviously also always available.

It should be noted that for built-in events that bubble, inline event handlers can be added to any element in order to facilitate event delegation. These event handlers are considered global event handlers, and all custom inline event handlers are obviously not global - they can only be used on the components that explicitly implement them.

Behavior overview for transformer event()

The behavior of event() matches the behavior of built-in event handlers like onclick.

Metadata

HTML elements do usually not expose any metadata, even though knowing the names and data types for content attributes would be quite useful sometimes. Ornament exposes a few metadata helper functions that help in scenarios where meta-programming components (eg. SSR) is required.

getTagName(instanceOrCtor)

Given an instance or custom element constructor, this function returns the element's tag name. It returns null if the object in question is not a custom element defined via @define():

import { define, getTagName } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {}

console.log(getTagName(Test));
// > "my-test"

console.log(getTagName(new Test()));
// > "my-test"

This serves roughly the same function as the standard CustomElementRegistry.getName() method but does not require access to the specific CustomElementRegistry that the element is registered with.

listAttributes(instanceOrCtor)

Lists the content attribute names that were defined via @attr() on the custom element (or constructor) in question:

import { define, attr, string, listAttributes } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @attr(string()) accessor foo = "";
  @attr(string(), { as: "asdf" }) accessor bar = "";
}

console.log(listAttributes(Test));
// > ["foo", "asdf"]

console.log(listAttributes(new Test()));
// > ["foo", "asdf"]

This is roughly analogous to the observedAttributes static property on custom element classes, but only lists content attributes defined with ornament's @attr() - manually defined attributes and IDL attributes defined with @prop() are excluded.

getAttribute(instanceOrCtor, contentAttributeName)

Returns the IDL attribute name and transformer used to define a content attribute:

import { define, attr, number, getAttribute } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Requires non-negative values
  @attr(number({ min: 0 }), { as: "asdf" }) accessor bar = 0;
}

const { prop, transformer } = getAttribute(Test, "asdf");

console.log(prop);
// > "bar" - the backend accessor for the content attribute "asdf"

transformer.parse("-1");
// > 0; input clamped to valid value

transformer.validate(-1, true);
// Throws an error; the transformer only accepts nonnegative numbers

This is particularly useful if you need access to the parsing and stringification logic for content attributes for eg. SSR.

Event Bus

Ornament runs intra-component communication over an internal event bus. You will almost certainly never need to access it directly, but there is is an API just in case.

Event Cause Event type Payload (args property on the event object)
init Constructor ran to completion OrnamentEvent<"init"> []
connected connectedCallback() fired OrnamentEvent<"connected"> []
disconnected disconnectedCallback() fired OrnamentEvent<"disconnected"> []
adopted adoptedCallback() fired OrnamentEvent<"adopted"> []
prop IDL attribute change (@prop or @attr) OrnamentEvent<"prop"> [Name: string | symbol, NewValue: any]
attr Content attribute change (@attr) OrnamentEvent<"attr"> [Name: string, OldValue: string | null, NewValue: string | null]
formAssociated formAssociatedCallback() fired OrnamentEvent<"formAssociated"> `[Owner: HTMLFormElement | null]
formReset formResetCallback() fired OrnamentEvent<"formReset"> []
formDisabled formDisabledCallback() fired OrnamentEvent<"formDisabled"> [Disabled: boolean]
formStateRestore formStateRestoreCallback() fired OrnamentEvent<"formStateRestore"> [Reason: "autocomplete" | "restore"]
Notes for TypeScript

You can declare additions to the global interface OrnamentEventMap to extend this list with your own events.

trigger(instance, name, ...payload)

Dispatches an event on the event bus for the component instance. The arguments payload must be all the for the args property on the event object on the event object (eg. a single boolean for for formDisabled).

import { trigger } from "@sirpepe/ornament";

// Dispatches an "connected" event. This will run all methods on "someElement"
// that were decorated with @connect().
trigger(someElement, "connected"); // note no args

// Dispatches an "prop" event. This will run all methods on "someElement"
// that were decorated with @reactive(), provided the "foo" key is not excluded
// in the setup of the @reactive decorator
trigger(someElement, "prop", "foo", 42); // note args for prop name and value

listen(instance, name, callback, options?)

Listens to events on the event bus for the component instance. The event bus is an instance of EventTarget, which means that you can pass any and all event listener options as the last argument.

import { listen } from "@sirpepe/ornament";

// Listen for "prop" event on the event bus for "someElement"
listen(someElement, "prop", (evt) => {
  const [name, value] = event.args;
  window.alert(`Attribute ${name} was changed to ${value}!`);
});

class OrnamentEvent<K extends keyof OrnamentEventMap>

Event type used on the internal event bus. Only really useful if you want to create your own events while using TypeScript.

Symbols

NO_VALUE

Transformers can return a special symbol to indicate that they were unable to parse an input. This symbol is exported by Ornament as NO_VALUE and is also available behind the key "ORNAMENT_NO_VALUE" in the global symbol registry.

METADATA

Ornament, being a collection of decorators, stores its metadata in Decorator Metadata. To avoid collisions with other libraries, the actual metadata is hidden behind a symbol that is exported by Ornament as ORNAMENT_METADATA_KEY or available behind the key "ORNAMENT_METADATA_KEY" in the global symbol registry. The contents of the metadata record should not be considered part of Ornament's stable API and could change at any moment. Use the metadata API instead.

Troubleshooting

TypeError: Cannot read private member from an object whose class did not declare it

This usually happens when methods decorated with @init() run at inopportune times. Consider the following example:

import { define, init } from "@sirpepe/ornament";

function otherDecorator(target) {
  return class OtherMixin extends target {
    #secret = 42;
    get foo() {
      return this.#secret; // <- Fails because @init() runs too early
    }
  };
}

@otherDecorator
@define("foo-bar") // If this was before @otherDecorator it would work
class Test extends HTMLElement {
  @init() // Runs after the constructor of Test has run, does not wait for the mixin class
  method() {
    console.log(this.foo);
  }
}

new Test();

In this scenario, @define() sets up to trigger @init() once the constructor of Test has finished. Inside Test, the method method() accesses the getter foo which is provided by the decorator @otherDecorator. The getter in turn tries to accesses the private field #secret, but fails with an exception. This happens because @define() installs logic that triggers the init event on the constructor for class Test, but the resulting class gets extended in turn by OtherMixin.

OtherMixinConstructor(
  DefineMixinConstructor(
    TestConstructor(
      HTMLElementConstructor()
    )
    // <---- init event happens here, after TestConstructor has run
  )
  // <---- finishes only after the init event has happened
)

This results in the event running before the private field #secret is fully initialized. The simplest way to remedy this situation is to apply @otherDecorator first. You might also want to consider using @connected() instead of @init().

This is not a bug in Ornament, but rather a simple effect of how mixin classes are subclasses of their targets. Because @init() is equivalent to calling the decorated method in the constructor, the effect can be reproduced without involving Ornament at all:

function decorator(target) {
  return class DecoratorMixin extends target {
    #secret = 42;
    get feature() {
      return this.#secret;
    }
  };
}

@decorator
class A {
  constructor() {
    console.log(this.feature);
  }
}

new A();

Cookbook

Debounced reactive

@reactive() causes its decorated method to get called for once for every attribute and property change. This is sometimes useful, but sometimes you will want to batch method calls for increased efficiency. This is easy if you combine @reactive() with @debounce():

import { define, prop, reactive, debounce int } from "@sirpepe/ornament";

@define("my-test")
export class TestElement extends HTMLElement {
  @prop(int()) accessor value = 0;

  @reactive()
  @debounce()
  #log() {
    console.log("Value is now", this.value);
  }
}

let el = new TestElement();
el.value = 1;
el.value = 2;
el.value = 2;

// Only logs "Value is now 3"

Rendering shadow DOM with uhtml

Ornament does not directly concern itself with rendering Shadow DOM, but you can combine Ornament with suitable libraries such as uhtml:

import { render, html } from "uhtml";
import { define, prop, reactive, debounce int } from "@sirpepe/ornament";

@define("counter-element")
export class CounterElement extends HTMLElement {
  @prop(int()) accessor value = 0;

  @reactive()
  @debounce()
  #render() {
    render(
      this.shadowRoot ?? this.attachShadow({ mode: "open" }),
      html`
        Current value: ${this.value}
        <button .click={() => ++this.value}>Add 1</button>
      `
    );
  }
}

This component uses an event handler to update the decorated accessor value, which in turn causes the @reactive() method #render() to update the UI accordingly - debounced with @debounce() for batched updates.

Rendering shadow DOM with Preact

You can also use Preact to render shadow DOM:

import { define, attr, number, reactive, connected } from "@sirpepe/ornament";
import { Fragment, h, render } from "preact";

@define("click-counter")
class ClickCounter extends HTMLElement {
  #shadow = this.attachShadow({ mode: "closed" });

  @attr(number({ min: 0 }), { reflective: false }) accessor up = 0;
  @attr(number({ min: 0 }), { reflective: false }) accessor down = 0;

  @connected()
  @reactive()
  render() {
    render(
      <>
        <button onClick={() => this.up++}>+1</button>
        Total: <b>{this.up + this.down}</b>
        <button onClick={() => this.down--}>-1</button>
      </>,
      this.#shadow,
    );
  }
}

In the case of Web Components and Ornament, it makes some sense to use class members for local state instead of hooks.

Read-only property

You can create a writable private accessor with @prop() and manually expose a public getter. This keeps reactive functions working, but only allows readonly access from outside the component:

import { define, attr, reactive, string } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  // Writable, but private
  @prop(string()) accessor #foo = "Starting value";

  // Provides public readonly access to #foo
  get foo() {
    return this.#foo;
  }

  change() {
    this.#foo++;
  }

  // Reacts to changes to #foo, which can only be caused by calling the method
  // `change()`
  @reactive()
  log() {
    console.log(this.#foo);
  }
}

Custom logic in IDL attributes

The point of the accessor keyword is to generate a getter, setter, and private property in a way that makes it easy to apply a decorator to everything at once. But because the getters and setters are auto-generated, there is no non-decorator way to attach custom logic to accessor members. To work around this for IDL attributes defined via @attr() or @prop(), you can build a and decorate a private or symbol accessor that you then expose with a custom facade:

@define("my-test")
class Test extends HTMLElement {
  // Implements a content attribute "foo" with getters and setters at #secret
  @attr(string(), { as: "foo" }) accessor #secret = "A";

  // To provide public IDL attributes, we just write a getter/setter pair with
  // names matching the content attribute
  get foo() {
    console.log("Custom getter logic!");
    return this.#secret; // accesses the getter decorated with @attr()
  }

  set foo(value) {
    console.log("Custom seter logic!");
    this.#secret = value; // accesses the setter decorated with @attr()
  }
}

Notes for @attr():

  1. The option as is mandatory when you use @attr() on a private or symbol accessor
  2. Ornament throws exceptions if the class does not implement a public API for a content attribute defined with @attr() on a private or symbol accessor

Event delegation

The following example captures all input events fired by <input type="number"> in the document:

import { define, subscribe } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  @subscribe(document.documentElement, "input", {
    predicate: (evt) => evt.target.matches("input[type-number]"),
  })
  log(evt) {
    console.log(evt); // "input" events
  }
}

If you'd rather catch event happening in the component's shadow dom, the syntax gets a bit more gnarly at first:

import { define, subscribe } from "@sirpepe/ornament";

@define("my-test")
class Test extends HTMLElement {
  root = this.attachShadow({ mode: "open" });
  @subscribe((instance) => this.root, "input", {
    predicate: (evt) => evt.target.matches("input[type-number]"),
  })
  log(evt) {
    console.log(evt); // "input" events
  }
}

Decorators like @subscribe run when the class definition initializes, and at that point, no class instances (and no shadow DOM to subscribe to) exist. We must therefore provide a function that can return the event target on initialization. To make this less of an eyesore, it makes sense to create a custom decorator for event delegation based on @subscribe:

import { define, subscribe } from "@sirpepe/ornament";

const handle = (eventName, selector) =>
  subscribe((instance) => this.root, eventName, {
    predicate: (evt) => evt.target.matches(selector),
  });

@define("my-test")
class Test extends HTMLElement {
  root = this.attachShadow({ mode: "open" });

  @handle("input", "input[type-number]") // Much better!
  log(evt) {
    console.log(evt); // "input" events
  }
}

Note that the function that @subscribe takes to access event targets can not access a classes private fields. The shadow root has to be publicly accessible (unless you want to mess around with WeakMaps storing ShadowRoots indexed by element instances or something similar).

Also note that not all events bubble, so you might want to use event capturing instead:

import { define, subscribe } from "@sirpepe/ornament";

// This can now handle all events from the shadow root
const capture = (eventName, selector) =>
  subscribe((instance) => this.root, eventName, {
    predicate: (evt) => evt.target.matches(selector),
    capture: true,
  });

Also also note that only composed events propagate through shadow boundaries, which may become important if you want to nest components with shadow dom and also want to use event delegation.

Custom defaults

If you don't like Ornament's defaults, remember that decorators and transformers are just functions. This means that you can use partial application to change the default options:

import {
  define,
  attr,
  reactive as baseReactive,
  string,
} from "@sirpepe/ornament";

// @reactive with "keys" always set to ["foo"]
const reactive = (options) => baseReactive({ ...options, keys: ["foo"] });

@define("my-test")
class Test extends HTMLElement {
  @prop(string()) accessor foo = "A"; // included in options.keys
  @prop(string()) accessor bar = "A"; // excluded from options.keys

  @reactive()
  log() {
    console.log("Hello");
  }
}

let test = new Test();
test.foo = "B"; //  logs "Hello"
test.bar = "B"; //  does not log anything

The same approach works when you want to create specialized decorators from existing ones...

import { define, subscribe } from "@sirpepe/ornament";

// A more convenient decorator for event delegation
function listen(event, selector = "*") {
  return subscribe(document.documentElement, "input", (evt) =>
    evt.target.matches(selector),
  );
}

@define("my-test")
class Test extends HTMLElement {
  @listen("input", "input[type-number]")
  log(evt) {
    console.log(evt);
  }
}

... or when you want to create your own transformers:

import { define, attr, number } from "@sirpepe/ornament";

function nonnegativeNumber(otherOptions) {
  return number({ ...otherOptions, min: 0 });
}

@define("my-test")
class Test extends HTMLElement {
  @attr(nonnegativeNumber({ max: 1337 }))
  accessor foo = 42;
}

You can also compose decorators, since they are just functions over a target and a context object:

import { reactive as baseReactive, connected } from "@sirpepe/ornament";

// Combines @reactive() and @connected() into one handy decorator that runs
// methods when components connect AND when their attributes change
function reactive() {
  return function (target, context) {
    return baseReactive()(connected()(target, context), context);
  };
}

And while we are at it, why not compose and partially apply decorators:

import {
  reactive as baseReactive,
  connected,
  debounce,
} from "@sirpepe/ornament";

// Combines @reactive(), @connected() and @debounce():
// - reacts to attribute updates (only while the component is connected)
// - and runs its target method at most once per frame
// - and also when the component connects
const reactive = () => (target, context) =>
  baseReactive({ predicate: ({ isConnected }) => isConnected })(
    connected()(debounce({ fn: debounce.raf() })(target, context), context),
    context,
  );

Also remember that transformer functions return plain objects that you can modify for one-off custom transformers:

import { define, attr, string } from "@sirpepe/ornament";

// The built-in string transformer always represents strings, but we want to
// allow `null` in this case
let nullableString = {
  ...string(),
  validate(value) {
    if (value === null || typeof value === "undefined") {
      return value;
    }
    return String(value);
  },
};

@define("my-test")
class Test extends HTMLElement {
  @attr(nullableString())
  accessor foo = "Hello";
}

Ornament's building blocks are extremely basic and you should hack, combine and extend them to get the most out of your components.

Footnotes

  1. Can be #private or a symbol if a non-private non-symbol getter/setter pair for the attribute name exists and a content attribute name has been set using the as option. ↩ ↩2

  2. Class field values must be of type function ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12

About

Mid-level, pareto-optimal, treeshakable and tiny (< 5k) TypeScript-positive toolkit for web component infrastructure

sirpepe.github.io/use-ornament/

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