From 9bacacc60d739afefa24dc769700072bb22fe048 Mon Sep 17 00:00:00 2001
From: SGavrl <277843151+SGavrl@users.noreply.github.com>
Date: Fri, 31 Jul 2026 06:47:56 +0000
Subject: [PATCH] Run custom element upgrades through real constructors
---
crates/obscura-js/js/bootstrap.js | 71 ++++++++++++-----
crates/obscura-js/src/runtime.rs | 127 ++++++++++++++++++++++++++++++
2 files changed, 179 insertions(+), 19 deletions(-)
diff --git a/crates/obscura-js/js/bootstrap.js b/crates/obscura-js/js/bootstrap.js
index 50376b4..e8f4d10 100644
--- a/crates/obscura-js/js/bootstrap.js
+++ b/crates/obscura-js/js/bootstrap.js
@@ -1035,6 +1035,13 @@ function _eventTargetDispatch(target, event) {
return !event.defaultPrevented;
}
+// During custom-element upgrade, HTMLElement's constructor must return the
+// already-existing element being upgraded. A class constructor cannot be
+// invoked with `.call(existingElement)`, so the registry and Element
+// constructor coordinate through the same construction-stack shape used by
+// browser custom-element implementations.
+const _customElementConstructionStack = [];
+
class Node {
static ELEMENT_NODE = 1;
static ATTRIBUTE_NODE = 2;
@@ -1746,7 +1753,21 @@ globalThis.NamedNodeMap = NamedNodeMap;
class Element extends Node {
constructor(nid) {
- super(nid);
+ const entry = _customElementConstructionStack[_customElementConstructionStack.length - 1];
+ const matchesUpgrade = entry && new.target === entry.constructor;
+ const upgrading = matchesUpgrade && !entry.constructed ? entry.element : null;
+ super(upgrading ? upgrading._nid : nid);
+ if (matchesUpgrade && entry.constructed) {
+ throw new TypeError("Custom element is already being constructed");
+ }
+ if (upgrading) {
+ // Keep an already-constructed marker on the stack until the outer class
+ // constructor returns. Recursive `new`/`super` calls for the same
+ // definition must not steal the element currently being upgraded.
+ entry.constructed = true;
+ Object.setPrototypeOf(upgrading, new.target.prototype);
+ return upgrading;
+ }
this._style = _styleProxy(new CSSStyleDeclaration(this));
}
// Element wrappers always back a nodeType-1 node (_wrap/_wrapEl only build an
@@ -3340,15 +3361,24 @@ class Document extends Node {
return _makeXPathResult(type, _xpathFindNodes(expression, contextNode || this));
}
createElement(t) {
- const el = _wrapEl(+_dom("create_element", t.toLowerCase()));
- if (el && t.toLowerCase() === 'template') {
+ const localName = String(t).toLowerCase();
+ const el = _wrapEl(+_dom("create_element", localName));
+ if (el && localName === 'template') {
el._templateContent = this.createDocumentFragment();
el._templateContent._fragmentContext = 'template';
}
+ const definition = globalThis.customElements?._registry?.get(localName);
+ if (el && definition) globalThis.customElements._upgradeElement(el, definition);
return el;
}
createElementNS(ns, t) {
- const el = this.createElement(t);
+ const namespace = ns == null ? null : String(ns);
+ if (namespace === "http://www.w3.org/1999/xhtml") {
+ const el = this.createElement(t);
+ if (el) el._ns = namespace;
+ return el;
+ }
+ const el = _wrapEl(+_dom("create_element", String(t).toLowerCase()));
if (el) el._ns = ns;
return el;
}
@@ -5920,25 +5950,28 @@ class CustomElementRegistry {
if (el.__customUpgraded) return;
el.__customUpgraded = true;
try {
- // Web Components spec: copy own props from the prototype onto the
- // element. JS-side classes define behavior via methods on the
- // prototype; we don't truly swap prototypes (Element is shared),
- // so attach the prototype methods directly to the instance.
- const proto = cls.prototype;
- for (const key of Object.getOwnPropertyNames(proto)) {
- if (key === 'constructor') continue;
- const desc = Object.getOwnPropertyDescriptor(proto, key);
- if (desc) Object.defineProperty(el, key, desc);
+ // Upgrade preserves object identity but installs the definition's
+ // prototype before running its class constructor. HTMLElement's
+ // constructor consumes this entry and returns `el`, so derived class
+ // fields and constructor-side state initialize on the real DOM wrapper.
+ const constructionEntry = { element: el, constructor: cls, constructed: false };
+ _customElementConstructionStack.push(constructionEntry);
+ let constructed;
+ try {
+ constructed = Reflect.construct(cls, []);
+ } finally {
+ const pending = _customElementConstructionStack.lastIndexOf(constructionEntry);
+ if (pending !== -1) _customElementConstructionStack.splice(pending, 1);
+ }
+ if (constructed !== el) {
+ throw new TypeError("Custom element constructor did not produce the element being upgraded");
}
- // Run constructor-side init on the element. Real custom elements
- // run the class constructor, but Element instances aren't a `cls`
- // subclass here; calling `.call(el)` runs whatever init logic the
- // class defines without needing a new allocation.
- try { cls.call(el); } catch (e) {}
if (typeof el.connectedCallback === 'function' && globalThis.document?.contains?.(el)) {
try { el.connectedCallback(); } catch (e) {}
}
- } catch (e) {}
+ } catch (e) {
+ el.__customUpgradeFailed = true;
+ }
}
get(name) { return this._registry.get(name); }
getName(cls) {
diff --git a/crates/obscura-js/src/runtime.rs b/crates/obscura-js/src/runtime.rs
index 5c6fd13..49aa206 100644
--- a/crates/obscura-js/src/runtime.rs
+++ b/crates/obscura-js/src/runtime.rs
@@ -1897,6 +1897,133 @@ mod tests {
assert_eq!(ok, serde_json::json!("OK"));
}
+ #[test]
+ fn custom_element_upgrade_runs_class_constructor_on_existing_element() {
+ let mut rt = setup_runtime(
+ r#""#,
+ );
+ let result = rt
+ .evaluate(
+ r#"
+ const before = document.getElementById("component");
+ class SvelteLike extends HTMLElement {
+ constructor() {
+ super();
+ this.$$s = [];
+ this.attachShadow({ mode: "open" });
+ }
+ connectedCallback() {
+ for (const subscription of this.$$s) subscription();
+ this.$$s.push(() => {});
+ this.shadowRoot.textContent = "ready";
+ }
+ }
+ customElements.define("svelte-like", SvelteLike);
+ return [
+ document.getElementById("component") === before,
+ before instanceof SvelteLike,
+ before.constructor === SvelteLike,
+ before.$$s.length,
+ before.shadowRoot && before.shadowRoot.textContent
+ ];
+ "#,
+ )
+ .unwrap();
+ assert_eq!(
+ result,
+ serde_json::json!([
+ true,
+ true,
+ true,
+ 1,
+ "ready"
+ ])
+ );
+ }
+
+ #[test]
+ fn create_element_synchronously_constructs_an_existing_definition() {
+ let mut rt = setup_runtime("");
+ let result = rt
+ .evaluate(
+ r#"
+ const testStart = true;
+ class CreatedLater extends HTMLElement {
+ constructor() {
+ super();
+ this.constructorState = ["initialized"];
+ this.attachShadow({ mode: "open" });
+ this.shadowRoot.textContent = "constructed";
+ }
+ connectedCallback() {
+ this.constructorState.push("connected");
+ }
+ }
+ customElements.define("created-later", CreatedLater);
+ const element = document.createElement("created-later");
+ const foreign = document.createElementNS(
+ "http://www.w3.org/2000/svg", "created-later"
+ );
+ return [
+ element instanceof CreatedLater,
+ element.constructor === CreatedLater,
+ element.localName,
+ element.constructorState,
+ element.shadowRoot && element.shadowRoot.textContent,
+ element.isConnected,
+ foreign instanceof CreatedLater
+ ];
+ "#,
+ )
+ .unwrap();
+ assert_eq!(
+ result,
+ serde_json::json!([
+ true,
+ true,
+ "created-later",
+ ["initialized"],
+ "constructed",
+ false,
+ false
+ ])
+ );
+ }
+
+ #[test]
+ fn throwing_custom_element_constructor_marks_upgrade_failed_without_connecting() {
+ let mut rt = setup_runtime(
+ r#""#,
+ );
+ let result = rt
+ .evaluate(
+ r#"
+ let constructorCalls = 0;
+ let connectedCalls = 0;
+ class ThrowsDuringUpgrade extends HTMLElement {
+ constructor() {
+ super();
+ constructorCalls++;
+ throw new Error("expected constructor failure");
+ }
+ connectedCallback() {
+ connectedCalls++;
+ }
+ }
+ customElements.define("throws-during-upgrade", ThrowsDuringUpgrade);
+ const element = document.getElementById("target");
+ customElements.upgrade(document);
+ return [
+ constructorCalls,
+ connectedCalls,
+ element.__customUpgradeFailed === true
+ ];
+ "#,
+ )
+ .unwrap();
+ assert_eq!(result, serde_json::json!([1, 0, true]));
+ }
+
#[test]
fn test_document_title() {
let mut rt = setup_runtime("Test");