x-element.js

import * as defaultTemplateEngine from './x-template.js';

/** Base element class for creating custom elements. */
export default class XElement extends HTMLElement {
  /**
   * Extends HTMLElement.observedAttributes to handle the properties block.
   * @returns {string[]}
   */
  static get observedAttributes() {
    XElement.#analyzeConstructor(this);
    return [...XElement.#constructors.get(this).attributeMap.keys()];
  }

  /**
   * Default templating engine. Use "templateEngine" to override.
   * @returns {{[key: string]: Function}}
   */
  static get defaultTemplateEngine() {
    return defaultTemplateEngine;
  }

  /**
   * Configured templating engine. Defaults to "defaultTemplateEngine".
   * Override this as needed if x-element's default template engine does not
   * meet your needs. A "render" method is the only required field. An "html"
   * tagged template literal is expected, but not strictly required.
   * @returns {{[key: string]: Function}}
   */
  static get templateEngine() {
    return XElement.defaultTemplateEngine;
  }

  /**
   * Declare an array of CSSStyleSheet objects to adopt on the shadow root.
   * Note that a CSSStyleSheet object is the type returned when importing a
   * stylesheet file via import attributes.
   * ```js
   * import importedStyle from './path-to.css' with { type: 'css' };
   * class MyElement extends XElement {
   *   static get styles() {
   *     const inlineStyle = new CSSStyleSheet();
   *     inlineStyle.replaceSync(`:host { display: block; }`);
   *     return [importedStyle, inlineStyle];
   *   }
   * }
   * ```
   * @returns {CSSStyleSheet[]}
   */
  static get styles() {
    return [];
  }

  /**
   * Observe callback.
   * @callback observeCallback
   * @param {HTMLElement} host
   * @param {any} value
   * @param {any} oldValue
   */

  /**
   * A property value.
   * @typedef {object} Property
   * @property {any} [type]
   * @property {string} [attribute]
   * @property {string[]} [input]
   * @property {Function} [compute]
   * @property {observeCallback} [observe]
   * @property {boolean} [reflect]
   * @property {boolean} [internal]
   * @property {boolean} [readOnly]
   * @property {any|Function} [initial]
   * @property {any|Function} [default]
   */

  /**
   * Declare watched properties (and related attributes) on an element.
   * ```js
   * static get properties() {
   *   return {
   *     property1: {
   *       type: String,
   *     },
   *     property2: {
   *       type: Number,
   *       input: ['property1'],
   *       compute: this.computeProperty2,
   *       reflect: true,
   *       observe: this.observeProperty2,
   *       default: 0,
   *     }
   *   };
   * }
   * ```
   * @returns {{[key: string]: Property}}
   */
  static get properties() {
    return {};
  }

  /**
   * Listen callback.
   * @callback delegatedListenCallback
   * @param {HTMLElement} host
   * @param {Event} event
   */

  /**
   * Declare event handlers on an element.
   * ```js
   * static get listeners() {
   *   return {
   *     click: this.onClick,
   *   }
   * }
   *```
   * Note that listeners are added to the element's render root. Listeners are
   * added during "connectedCallback" and removed during "disconnectedCallback".
   * The arguments passed to your callback are always "(host, event)".
   * @returns {{[key: string]: delegatedListenCallback}}
   */
  static get listeners() {
    return {};
  }

  /**
   * Customize shadow root initialization and optionally forgo encapsulation.
   * E.g., setup focus delegation or return host instead of host.shadowRoot.
   * @param {HTMLElement} host
   * @returns {HTMLElement|ShadowRoot}
   */
  static createRenderRoot(host) {
    return host.attachShadow({ mode: 'open' });
  }

  /**
   * Template callback.
   * @callback templateCallback
   * @param {object} properties
   * @param {HTMLElement} host
   */

  /**
   * Setup template callback to update DOM when properties change.
   * ```js
   * static template(html) {
   *   return ({ href }) => {
   *     return html`<a href="${href}">click me</a>`;
   *   }
   * }
   * ```
   * @param {Function} html
   * @param {{[key: string]: Function}} engine
   * @returns {templateCallback}
   */
  static template(html, engine) { // eslint-disable-line no-unused-vars
    return (properties, host) => {}; // eslint-disable-line no-unused-vars
  }

  /**
   * Standard instance constructor.
   */
  constructor() {
    super();
    XElement.#constructHost(this);
  }

  /**
   * Extends HTMLElement.prototype.connectedCallback.
   */
  connectedCallback() {
    XElement.#connectHost(this);
  }

  // TODO: #254: Uncomment once we leverage “moveBefore”.
  // /**
  //  * Extends HTMLElement.prototype.connectedMoveCallback.
  //  */
  // connectedMoveCallback() {}

  /**
   * Extends HTMLElement.prototype.attributeChangedCallback.
   * @param {string} attribute
   * @param {string|null} oldValue
   * @param {string|null} value
   */
  attributeChangedCallback(attribute, oldValue, value) {
    const { attributeMap } = XElement.#constructors.get(this.constructor);
    // Authors may extend "observedAttributes". Optionally chain to account for
    // attributes which we don't know about.
    attributeMap.get(attribute)?.sync(this, value, oldValue);
  }

  /**
   * Extends HTMLElement.prototype.adoptedCallback.
   */
  adoptedCallback() {}

  /**
   * Extends HTMLElement.prototype.disconnectedCallback.
   */
  disconnectedCallback() {
    XElement.#disconnectHost(this);
  }

  /**
   * Uses the result of your template callback to update your render root.
   *
   * This is called when properties update, but is exposed for advanced use cases.
   */
  render() {
    const { template, properties, renderRoot, render } = XElement.#hosts.get(this);
    const result = template(properties, this);
    try {
      render(renderRoot, result);
    } catch (error) {
      const pathString = XElement.#toPathString(this);
      // @ts-ignore — TypeScript doesn’t get that this can accept any class.
      const tagName = customElements.getName(this.constructor);
      const message = `Invalid template for "${this.constructor.name}" / <${tagName}> at path "${pathString}".`;
      throw new Error(message, { cause: error });
    }
  }

  /**
   * Listen callback.
   * @callback listenCallback
   * @param {Event} event
   */

  /**
   * Wrapper around HTMLElement.addEventListener.
   * Advanced — use this only if declaring listeners statically is not possible.
   * @param {EventTarget} element
   * @param {string} type
   * @param {listenCallback} callback
   * @param {object} [options]
   */
  listen(element, type, callback, options) {
    if (XElement.#typeIsWrong(EventTarget, element)) {
      const typeName = XElement.#getTypeName(element);
      throw new Error(`Unexpected element passed to listen (expected EventTarget, got ${typeName}).`);
    }
    if (XElement.#typeIsWrong(String, type)) {
      const typeName = XElement.#getTypeName(type);
      throw new Error(`Unexpected type passed to listen (expected String, got ${typeName}).`);
    }
    if (XElement.#typeIsWrong(Function, callback)) {
      const typeName = XElement.#getTypeName(callback);
      throw new Error(`Unexpected callback passed to listen (expected Function, got ${typeName}).`);
    }
    if (XElement.#notNullish(options) && XElement.#typeIsWrong(Object, options)) {
      const typeName = XElement.#getTypeName(options);
      throw new Error(`Unexpected options passed to listen (expected Object, got ${typeName}).`);
    }
    XElement.#addListener(this, element, type, callback, options);
  }

  /**
   * Wrapper around HTMLElement.removeEventListener. Inverse of "listen".
   * @param {EventTarget} element
   * @param {string} type
   * @param {listenCallback} callback
   * @param {object} [options]
   */
  unlisten(element, type, callback, options) {
    if (XElement.#typeIsWrong(EventTarget, element)) {
      const typeName = XElement.#getTypeName(element);
      throw new Error(`Unexpected element passed to unlisten (expected EventTarget, got ${typeName}).`);
    }
    if (XElement.#typeIsWrong(String, type)) {
      const typeName = XElement.#getTypeName(type);
      throw new Error(`Unexpected type passed to unlisten (expected String, got ${typeName}).`);
    }
    if (XElement.#typeIsWrong(Function, callback)) {
      const typeName = XElement.#getTypeName(callback);
      throw new Error(`Unexpected callback passed to unlisten (expected Function, got ${typeName}).`);
    }
    if (XElement.#notNullish(options) && XElement.#typeIsWrong(Object, options)) {
      const typeName = XElement.#getTypeName(options);
      throw new Error(`Unexpected options passed to unlisten (expected Object, got ${typeName}).`);
    }
    XElement.#removeListener(this, element, type, callback, options);
  }

  /**
   * Helper method to dispatch an "ErrorEvent" on the element.
   * @param {Error} error
   */
  dispatchError(error) {
    const { message } = error;
    const eventData = { error, message, bubbles: true, composed: true };
    this.dispatchEvent(new ErrorEvent('error', eventData));
  }

  /**
   * For element authors. Getter and setter for internal properties.
   * Note that you can set read-only properties from host.internal. However, you
   * must get read-only properties directly from the host.
   * @returns {object}
   */
  get internal() {
    return XElement.#hosts.get(this).internal;
  }

  // Called once per class — kicked off from "static get observedAttributes".
  static #analyzeConstructor(constructor) {
    const { styles, properties, listeners } = constructor;
    const propertiesEntries = Object.entries(properties);
    const listenersEntries = Object.entries(listeners);
    XElement.#validateProperties(constructor, properties, propertiesEntries);
    XElement.#validateListeners(constructor, listeners, listenersEntries);
    const propertyMap = new Map(propertiesEntries);
    const internalPropertyMap = new Map();
    // Use a normal object for better autocomplete when debugging in console.
    const propertiesTarget = {};
    const internalTarget = {};
    const attributeMap = new Map();
    for (const [key, property] of propertyMap) {
      // We mutate (vs copy) to allow cross-referencing property objects.
      XElement.#mutateProperty(constructor, propertyMap, key, property);
      if (property.internal || property.readOnly) {
        internalPropertyMap.set(key, property);
        internalTarget[key] = undefined;
      }
      propertiesTarget[key] = undefined;
      if (property.attribute) {
        attributeMap.set(property.attribute, property);
      }
    }
    const listenerMap = new Map(listenersEntries);
    XElement.#constructors.set(constructor, {
      styles, propertyMap, internalPropertyMap, attributeMap, listenerMap,
      propertiesTarget, internalTarget,
    });
  }

  // Called during constructor analysis.
  static #validateProperties(constructor, properties, entries) {
    const path = `${constructor.name}.properties`;
    for (const [key, property] of entries) {
      if (XElement.#typeIsWrong(Object, property)) {
        const typeName = XElement.#getTypeName(property);
        throw new Error(`${path}.${key} has an unexpected value (expected Object, got ${typeName}).`);
      }
    }
    for (const [key, property] of entries) {
      XElement.#validateProperty(constructor, key, property);
    }
    const attributes = new Set();
    const inputMap = new Map();
    for (const [key, property] of entries) {
      if (XElement.#propertyHasAttribute(property)) {
        // Attribute names are case-insensitive — lowercase to properly check for duplicates.
        const attribute = property.attribute ?? XElement.#camelToKebab(key);
        XElement.#validatePropertyAttribute(constructor, key, property, attribute);
        if (attributes.has(attribute)) {
          throw new Error(`${path}.${key} causes a duplicated attribute "${attribute}".`);
        }
        attributes.add(attribute);
      }
      if (property.input) {
        inputMap.set(property, property.input.map(inputKey => properties[inputKey]));
        for (const [index, inputKey] of Object.entries(property.input)) {
          if (XElement.#typeIsWrong(Object, properties[inputKey])) {
            throw new Error(`${path}.${key}.input[${index}] has an unexpected item ("${inputKey}" has not been declared).`);
          }
        }
      }
    }
    for (const [key, property] of entries) {
      if (XElement.#propertyIsCyclic(property, inputMap)) {
        throw new Error(`${path}.${key}.input is cyclic.`);
      }
    }
  }

  static #validateProperty(constructor, key, property) {
    const path = `${constructor.name}.properties.${key}`;
    if (key.includes('-')) {
      throw new Error(`Unexpected key "${path}" contains "-" (property names should be camelCased).`);
    }
    for (const propertyKey of Object.keys(property)) {
      if (XElement.#propertyKeys.has(propertyKey) === false) {
        throw new Error(`Unexpected key "${path}.${propertyKey}".`);
      }
    }
    const { type, attribute, compute, input, reflect, internal, readOnly } = property;
    if (Reflect.has(property, 'type') && XElement.#typeIsWrong(Function, type)) {
      const typeName = XElement.#getTypeName(type);
      throw new Error(`Unexpected value for "${path}.type" (expected constructor Function, got ${typeName}).`);
    }
    for (const subKey of ['compute', 'observe']) {
      if (Reflect.has(property, subKey) && XElement.#typeIsWrong(Function, property[subKey])) {
        const typeName = XElement.#getTypeName(property[subKey]);
        throw new Error(`Unexpected value for "${path}.${subKey}" (expected Function, got ${typeName}).`);
      }
    }
    for (const subKey of ['reflect', 'internal', 'readOnly']) {
      if (Reflect.has(property, subKey) && XElement.#typeIsWrong(Boolean, property[subKey])) {
        const typeName = XElement.#getTypeName(property[subKey]);
        throw new Error(`Unexpected value for "${path}.${subKey}" (expected Boolean, got ${typeName}).`);
      }
    }
    if (!internal && XElement.#prototypeInterface.has(key)) {
      throw new Error(`Unexpected key "${path}" shadows in XElement.prototype interface.`);
    }
    if (Reflect.has(property, 'attribute') && XElement.#typeIsWrong(String, attribute)) {
      const typeName = XElement.#getTypeName(attribute);
      throw new Error(`Unexpected value for "${path}.attribute" (expected String, got ${typeName}).`);
    }
    if (Reflect.has(property, 'attribute') && attribute === '') {
      throw new Error(`Unexpected value for "${path}.attribute" (expected non-empty String).`);
    }
    for (const subKey of ['initial', 'default']) {
      const value = Reflect.get(property, subKey);
      if (
        XElement.#notNullish(value) &&
        XElement.#typeIsWrong(Boolean, value) &&
        XElement.#typeIsWrong(String, value) &&
        XElement.#typeIsWrong(Number, value) &&
        XElement.#typeIsWrong(Function, value)
      ) {
        const typeName = XElement.#getTypeName(value);
        throw new Error(`Unexpected value for "${path}.${subKey}" (expected Boolean, String, Number, or Function, got ${typeName}).`);
      }
    }
    if (Reflect.has(property, 'input') && XElement.#typeIsWrong(Array, input)) {
      const typeName = XElement.#getTypeName(input);
      throw new Error(`Unexpected value for "${path}.input" (expected Array, got ${typeName}).`);
    }
    if (Reflect.has(property, 'input')) {
      for (const [index, inputKey] of Object.entries(input)) {
        if (XElement.#typeIsWrong(String, inputKey)) {
          const typeName = XElement.#getTypeName(inputKey);
          throw new Error(`Unexpected value for "${path}.input[${index}]" (expected String, got ${typeName}).`);
        }
      }
    }
    const unserializable = XElement.#serializableTypes.has(property.type) === false;
    const typeName = property.type?.prototype && property.type?.name ? property.type.name : XElement.#getTypeName(property.type);
    if (attribute && type && unserializable) {
      throw new Error(`Found unserializable "${path}.type" (${typeName}) but "${path}.attribute" is defined.`);
    }
    if (reflect && unserializable) {
      throw new Error(`Found unserializable "${path}.type" (${typeName}) but "${path}.reflect" is true.`);
    }
    if (compute && !input) {
      throw new Error(`Found "${path}.compute" without "${path}.input" (computed properties require input).`);
    }
    if (input && !compute) {
      throw new Error(`Found "${path}.input" without "${path}.compute" (computed properties require a compute callback).`);
    }
    if (Reflect.has(property, 'initial') && compute) {
      throw new Error(`Found "${path}.initial" and "${path}.compute" (computed properties cannot set an initial value).`);
    }
    if (Reflect.has(property, 'readOnly') && compute) {
      throw new Error(`Found "${path}.readOnly" and "${path}.compute" (computed properties cannot define read-only).`);
    }
    if (reflect && internal) {
      throw new Error(`Both "${path}.reflect" and "${path}.internal" are true (reflected properties cannot be internal).`);
    }
    if (internal && readOnly) {
      throw new Error(`Both "${path}.internal" and "${path}.readOnly" are true (read-only properties cannot be internal).`);
    }
    if (internal && attribute) {
      throw new Error(`Found "${path}.attribute" but "${path}.internal" is true (internal properties cannot have attributes).`);
    }
  }

  static #validatePropertyAttribute(constructor, key, property, attribute) {
    const path = `${constructor.name}.properties`;
    // Attribute names are case-insensitive — lowercase to properly check for duplicates.
    if (attribute !== attribute.toLowerCase()) {
      throw new Error(`${path}.${key} has non-standard attribute casing "${attribute}" (use lower-cased names).`);
    }
  }

  // Determines if computed property inputs form a cycle.
  static #propertyIsCyclic(property, inputMap, seen = new Set()) {
    if (inputMap.has(property)) {
      for (const input of inputMap.get(property)) {
        const nextSeen = new Set([...seen, property]);
        if (
          input === property ||
          seen.has(input) ||
          XElement.#propertyIsCyclic(input, inputMap, nextSeen)
        ) {
          return true;
        }
      }
    }
  }

  static #validateListeners(constructor, listeners, entries) {
    const path = `${constructor.name}.listeners`;
    for (const [type, listener] of entries) {
      if (XElement.#typeIsWrong(Function, listener)) {
        const typeName = XElement.#getTypeName(listener);
        throw new Error(`${path}.${type} has unexpected value (expected Function, got ${typeName}).`);
      }
    }
  }

  // Called once per-property during constructor analysis.
  static #mutateProperty(constructor, propertyMap, key, property) {
    property.key = key;
    property.attribute = XElement.#propertyHasAttribute(property)
      ? property.attribute ?? XElement.#camelToKebab(key)
      : undefined;
    property.input = new Set((property.input ?? []).map(inputKey => propertyMap.get(inputKey)));
    property.output = property.output ?? new Set();
    for (const input of property.input) {
      input.output = input.output ?? new Set();
      input.output.add(property);
    }
    XElement.#addPropertyInitial(constructor, property);
    XElement.#addPropertyDefault(constructor, property);
    XElement.#addPropertySync(constructor, property);
    XElement.#addPropertyCompute(constructor, property);
    XElement.#addPropertyReflect(constructor, property);
    XElement.#addPropertyObserve(constructor, property);
  }

  // Wrapper to improve ergonomics of coalescing nullish, initial value.
  static #addPropertyInitial(constructor, property) {
    // Should take `value` in and spit the initial or value out.
    if (Reflect.has(property, 'initial')) {
      const initialValue = property.initial;
      const isFunction = XElement.#typeIsWrong(Function, initialValue) === false;
      property.initial = value =>
        value ?? (isFunction ? initialValue.call(constructor) : initialValue);
    } else {
      property.initial = value => value;
    }
  }

  // Wrapper to improve ergonomics of coalescing nullish, default value.
  static #addPropertyDefault(constructor, property) {
    // Should take `value` in and spit the default or value out.
    if (Reflect.has(property, 'default')) {
      const { key, default: defaultValue } = property;
      const isFunction = XElement.#typeIsWrong(Function, defaultValue) === false;
      const getOrCreateDefault = host => {
        const { defaultMap } = XElement.#hosts.get(host);
        if (!defaultMap.has(key)) {
          const value = isFunction ? defaultValue.call(constructor) : defaultValue;
          defaultMap.set(key, value);
          return value;
        }
        return defaultMap.get(key);
      };
      property.default = (host, value) => value ?? getOrCreateDefault(host);
    } else {
      property.default = (host, value) => value;
    }
  }

  // Wrapper to improve ergonomics of syncing attributes back to properties.
  static #addPropertySync(constructor, property) {
    if (XElement.#propertyHasAttribute(property)) {
      property.sync = (host, value, oldValue) => {
        const { initialized, reflecting } = XElement.#hosts.get(host);
        if (reflecting === false && initialized && value !== oldValue) {
          const deserialization = XElement.#deserializeProperty(host, property, value);
          host[property.key] = deserialization;
        }
      };
    }
  }

  // Wrapper to centralize logic needed to perform reflection.
  static #addPropertyReflect(constructor, property) {
    if (property.reflect) {
      property.reflect = host => {
        const value = XElement.#getPropertyValue(host, property);
        const serialization = XElement.#serializeProperty(host, property, value);
        const hostInfo = XElement.#hosts.get(host);
        hostInfo.reflecting = true;
        serialization === undefined
          ? host.removeAttribute(property.attribute)
          : host.setAttribute(property.attribute, serialization);
        hostInfo.reflecting = false;
      };
    }
  }

  // Wrapper to prevent repeated compute callbacks.
  static #addPropertyCompute(constructor, property) {
    const { compute } = property;
    if (compute) {
      property.compute = host => {
        const { computeMap, valueMap } = XElement.#hosts.get(host);
        const saved = computeMap.get(property);
        if (saved.valid === false) {
          const args = [];
          for (const input of property.input) {
            args.push(XElement.#getPropertyValue(host, input));
          }
          if (saved.args === undefined || args.some((arg, index) => arg !== saved.args[index])) {
            const value = property.default(host, compute.call(constructor, ...args));
            XElement.#validatePropertyValue(host, property, value);
            valueMap.set(property, value);
            saved.args = args;
          }
          saved.valid = true;
        }
        return valueMap.get(property);
      };
    }
  }

  // Wrapper to provide last value to observe callbacks.
  static #addPropertyObserve(constructor, property) {
    const { observe } = property;
    if (observe) {
      property.observe = host => {
        const saved = XElement.#hosts.get(host).observeMap.get(property);
        const value = XElement.#getPropertyValue(host, property);
        if (Object.is(value, saved.value) === false) {
          observe.call(constructor, host, value, saved.value);
        }
        saved.value = value;
      };
    }
  }

  // Called once per-host during construction.
  static #constructHost(host) {
    const invalidProperties = new Set();
    // The weak map prevents memory leaks. E.g., adding anonymous listeners.
    const listenerMap = new WeakMap();
    const valueMap = new Map();
    const renderRoot = host.constructor.createRenderRoot(host);
    if (!renderRoot || renderRoot !== host && renderRoot !== host.shadowRoot) {
      throw new Error('Unexpected render root returned. Expected "host" or "host.shadowRoot".');
    }
    const { render, html, ...engine } = host.constructor.templateEngine;
    const template = host.constructor.template(html, { html, ...engine }).bind(host.constructor);
    const properties = XElement.#createProperties(host);
    const internal = XElement.#createInternal(host);
    const computeMap = new Map();
    const observeMap = new Map();
    const defaultMap = new Map();
    const { styles, propertyMap } = XElement.#constructors.get(host.constructor);
    if (styles.length > 0) {
      if (renderRoot === host.shadowRoot) {
        if (renderRoot.adoptedStyleSheets.length === 0) {
          renderRoot.adoptedStyleSheets = styles;
        } else {
          throw new Error('Unexpected "styles" declared when preexisting "adoptedStyleSheets" exist.');
        }
      } else {
        throw new Error('Unexpected "styles" declared without a shadow root.');
      }
    }
    for (const property of propertyMap.values()) {
      if (property.compute) {
        computeMap.set(property, { valid: false, args: undefined });
      }
      if (property.observe) {
        observeMap.set(property, { value: undefined });
      }
    }
    XElement.#hosts.set(host, {
      initialized: false, reflecting: false, invalidProperties, listenerMap,
      renderRoot, render, template, properties, internal, computeMap,
      observeMap, defaultMap, valueMap,
    });
  }

  // Called during host construction.
  static #createInternal(host) {
    const { propertyMap, internalPropertyMap, internalTarget } =  XElement.#constructors.get(host.constructor);
    // Everything but "get", "set", "has", and "ownKeys" are considered invalid.
    // Note that impossible traps like "apply" or "construct" are not guarded.
    const invalid = () => { throw new Error('Invalid use of internal proxy.'); };
    const get = (target, key) => {
      const internalProperty = internalPropertyMap.get(key);
      if (internalProperty?.internal) {
        return XElement.#getPropertyValue(host, internalProperty);
      } else {
        const path = `${host.constructor.name}.properties.${key}`;
        const property = propertyMap.get(key);
        if (property === undefined) {
          throw new Error(`Property "${path}" does not exist.`);
        } else {
          throw new Error(`Property "${path}" is publicly available (use normal getter).`);
        }
      }
    };
    const set = (target, key, value) => {
      const internalProperty = internalPropertyMap.get(key);
      if (internalProperty && Reflect.has(internalProperty, 'compute') === false) {
        XElement.#setPropertyValue(host, internalProperty, value);
        return true;
      } else {
        const path = `${host.constructor.name}.properties.${key}`;
        const property = propertyMap.get(key);
        if (property === undefined) {
          throw new Error(`Property "${path}" does not exist.`);
        } else if (property.compute) {
          throw new Error(`Property "${path}" is computed (computed properties are read-only).`);
        } else {
          throw new Error(`Property "${path}" is publicly available (use normal setter).`);
        }
      }
    };
    const has = (target, key) => internalPropertyMap.has(key);
    const ownKeys = () => [...internalPropertyMap.keys()];
    const handler = {
      defineProperty: invalid, deleteProperty: invalid, get,
      getOwnPropertyDescriptor: invalid, getPrototypeOf: invalid, has,
      isExtensible: invalid, ownKeys, preventExtensions: invalid,
      set, setPrototypeOf: invalid,
    };
    return new Proxy(internalTarget, handler);
  }

  // Only available in template callback. Provides getter for all properties.
  // Called during host construction.
  static #createProperties(host) {
    const { propertyMap, propertiesTarget } =  XElement.#constructors.get(host.constructor);
    // Everything but "get", "set", "has", and "ownKeys" are considered invalid.
    const invalid = () => { throw new Error('Invalid use of properties proxy.'); };
    const get = (target, key) => {
      if (propertyMap.has(key)) {
        return XElement.#getPropertyValue(host, propertyMap.get(key));
      } else {
        const path = `${host.constructor.name}.properties.${key}`;
        throw new Error(`Property "${path}" does not exist.`);
      }
    };
    const set = (target, key) => {
      const path = `${host.constructor.name}.properties.${key}`;
      if (propertyMap.has(key)) {
        throw new Error(`Cannot set "${path}" via "properties".`);
      } else {
        throw new Error(`Property "${path}" does not exist.`);
      }
    };
    const has = (target, key) => propertyMap.has(key);
    const ownKeys = () => [...propertyMap.keys()];
    const handler = {
      defineProperty: invalid, deleteProperty: invalid, get,
      getOwnPropertyDescriptor: invalid, getPrototypeOf: invalid, has,
      isExtensible: invalid, ownKeys, preventExtensions: invalid, set,
      setPrototypeOf: invalid,
    };
    return new Proxy(propertiesTarget, handler);
  }

  // Called once per-host from initial "connectedCallback".
  static #connectHost(host) {
    const initialized = XElement.#initializeHost(host);
    XElement.#addListeners(host);
    if (initialized) {
      XElement.#updateHost(host);
    }
  }

  static #disconnectHost(host) {
    XElement.#removeListeners(host);
  }

  static #initializeHost(host) {
    const hostInfo = XElement.#hosts.get(host);
    const { computeMap, initialized, invalidProperties } = hostInfo;
    if (initialized === false) {
      XElement.#upgradeOwnProperties(host);
      // Only reflect attributes when the element is connected.
      const { propertyMap } = XElement.#constructors.get(host.constructor);
      for (const property of propertyMap.values()) {
        const { value, found } = XElement.#getPreUpgradePropertyValue(host, property);
        XElement.#initializeProperty(host, property);
        if (found) {
          host[property.key] = property.default(host, property.initial(value));
        } else if (!property.compute) {
          // Set to a nullish value so that it coalesces to the default.
          XElement.#setPropertyValue(host, property, property.default(host, property.initial()));
        }
        invalidProperties.add(property);
        if (property.compute) {
          computeMap.get(property).valid = false;
        }
      }
      hostInfo.initialized = true;
      return true;
    }
    return false;
  }

  // Prevent shadowing from properties added to element instance pre-upgrade.
  static #upgradeOwnProperties(host) {
    for (const key of Reflect.ownKeys(host)) {
      const value = Reflect.get(host, key);
      Reflect.deleteProperty(host, key);
      Reflect.set(host, key, value);
    }
  }

  // Called during host initialization.
  static #getPreUpgradePropertyValue(host, property) {
    // Process possible sources of initial state, with this priority:
    // 1. imperative, e.g. `element.prop = 'value';`
    // 2. declarative, e.g. `<element prop="value"></element>`
    const { key, attribute, internal } = property;
    let value;
    let found = false;
    if (!internal) {
      // Only look for public (i.e., non-internal) properties.
      if (Reflect.has(host, key)) {
        value = host[key];
        found = true;
      } else if (attribute && host.hasAttribute(attribute)) {
        const attributeValue = host.getAttribute(attribute);
        value = XElement.#deserializeProperty(host, property, attributeValue);
        found = true;
      }
    }
    return { value, found };
  }

  static #initializeProperty(host, property) {
    if (!property.internal) {
      const { key, compute, readOnly } = property;
      const path = `${host.constructor.name}.properties.${key}`;
      const get = () => XElement.#getPropertyValue(host, property);
      const set = compute || readOnly
        ? () => {
          if (compute) {
            throw new Error(`Property "${path}" is computed (computed properties are read-only).`);
          } else {
            throw new Error(`Property "${path}" is read-only.`);
          }
        }
        : value => XElement.#setPropertyValue(host, property, value);
      Reflect.deleteProperty(host, key);
      Reflect.defineProperty(host, key, { get, set, enumerable: true });
    }
  }

  static #addListener(host, element, type, callback, options) {
    callback = XElement.#getListener(host, callback);
    element.addEventListener(type, callback, options);
  }

  static #addListeners(host) {
    const { listenerMap } = XElement.#constructors.get(host.constructor);
    const { renderRoot } = XElement.#hosts.get(host);
    for (const [type, listener] of listenerMap) {
      XElement.#addListener(host, renderRoot, type, listener);
    }
  }

  static #removeListener(host, element, type, callback, options) {
    callback = XElement.#getListener(host, callback);
    element.removeEventListener(type, callback, options);
  }

  static #removeListeners(host) {
    const { listenerMap } = XElement.#constructors.get(host.constructor);
    const { renderRoot } = XElement.#hosts.get(host);
    for (const [type, listener] of listenerMap) {
      XElement.#removeListener(host, renderRoot, type, listener);
    }
  }

  static #getListener(host, listener) {
    const { listenerMap } = XElement.#hosts.get(host);
    if (listenerMap.has(listener) === false) {
      listenerMap.set(listener, listener.bind(host.constructor, host));
    }
    return listenerMap.get(listener);
  }

  static #updateHost(host) {
    // Order of operations: compute, reflect, render, then observe.
    const { invalidProperties } = XElement.#hosts.get(host);
    const invalidPropertiesCopy = new Set(invalidProperties);
    invalidProperties.clear();
    for (const property of invalidPropertiesCopy) {
      property.reflect?.(host);
    }
    host.render();
    for (const property of invalidPropertiesCopy) {
      property.observe?.(host);
    }
  }

  // Used to improve error messaging by appending DOM path information.
  static #toPathString(host) {
    const path = [];
    let reference = host;
    while (reference) {
      path.push(reference);
      reference = reference.parentElement ?? reference.getRootNode().host;
    }
    return path
      .map(element => {
        const tag = element.localName;
        const attributes = Array.from(element.attributes)
          .map(({ name, value }) => value ? `${name}="${value}"` : name);
        return `${tag}${attributes.length ? `[${attributes.join('][')}]` : ''}`;
      })
      .join(' < ');
  }

  static async #invalidateProperty(host, property) {
    const { initialized, invalidProperties, computeMap } = XElement.#hosts.get(host);
    if (initialized) {
      for (const output of property.output) {
        XElement.#invalidateProperty(host, output);
      }
      const queueUpdate = invalidProperties.size === 0;
      invalidProperties.add(property);
      if (property.compute) {
        computeMap.get(property).valid = false;
      }
      if (queueUpdate) {
        // Queue a microtask. Allows multiple, synchronous changes.
        await Promise.resolve();
        XElement.#updateHost(host);
      }
    }
  }

  static #getPropertyValue(host, property) {
    const { valueMap } = XElement.#hosts.get(host);
    return property.compute?.(host) ?? valueMap.get(property);
  }

  static #validatePropertyValue(host, property, value) {
    if (property.type && XElement.#notNullish(value)) {
      if (XElement.#typeIsWrong(property.type, value)) {
        const path = `${host.constructor.name}.properties.${property.key}`;
        const typeName = XElement.#getTypeName(value);
        throw new Error(`Unexpected value for "${path}" (expected ${property.type.name}, got ${typeName}).`);
      }
    }
  }

  static #setPropertyValue(host, property, value) {
    const { valueMap } = XElement.#hosts.get(host);
    if (Object.is(value, valueMap.get(property)) === false) {
      value = property.default(host, value);
      XElement.#validatePropertyValue(host, property, value);
      valueMap.set(property, value);
      XElement.#invalidateProperty(host, property);
    }
  }

  static #serializeProperty(host, property, value) {
    if (XElement.#notNullish(value)) {
      if (property.type === Boolean) {
        return value ? '' : undefined;
      }
      return value.toString();
    }
  }

  static #deserializeProperty(host, property, value) {
    if (property.type === Boolean) {
      // Per HTML spec, every value other than null is considered true.
      return value !== null;
    } else if (value === null) {
      // Null as an attribute is really "undefined" as a property.
      return undefined;
    } else if (!property.type) {
      // Property doesn't have a type, leave it as a string.
      return value;
    } else {
      // Coerce type as needed.
      switch (property.type) {
        case Number:
          // Don't try and coerce something like "Number('') >> 0".
          return value.trim() ? property.type(value) : Number.NaN;
        default:
          return property.type(value);
      }
    }
  }

  // Public properties which are serializable or typeless have attributes.
  static #propertyHasAttribute(property) {
    return !property.internal && (XElement.#serializableTypes.has(property.type) || !property.type);
  }

  static #getTypeName(value) {
    return Object.prototype.toString.call(value).slice(8, -1);
  }

  static #notNullish(value) {
    return value !== undefined && value !== null;
  }

  static #typeIsWrong(type, value) {
    // Because `instanceof` fails on primitives (`'' instanceof String === false`)
    // and `Object.prototype.toString` cannot handle inheritance, we use both.
    return (
      XElement.#notNullish(value) === false ||
      (!(value instanceof type) && XElement.#getTypeName(value) !== type.name)
    );
  }

  static #camelToKebab(camel) {
    if (XElement.#caseMap.has(camel) === false) {
      XElement.#caseMap.set(camel, camel.replace(/([A-Z])/g, '-$1').toLowerCase());
    }
    return XElement.#caseMap.get(camel);
  }

  static #constructors = new WeakMap();
  static #hosts = new WeakMap();
  static #propertyKeys = new Set(['type', 'attribute', 'input', 'compute', 'observe', 'reflect', 'internal', 'readOnly', 'initial', 'default']);
  static #serializableTypes = new Set([Boolean, String, Number]);
  static #caseMap = new Map();
  static #prototypeInterface = new Set(Object.getOwnPropertyNames(XElement.prototype));
}