gecko-dev/testing/web-platform/tests/custom-elements/microtasks-and-constructors.html

<!DOCTYPE html>
<title>Custom elements: performing a microtask checkpoint after construction</title>
<meta name="author" title="Domenic Denicola" href="mailto:d@domenic.me">
<link rel="help" href="https://html.spec.whatwg.org/multipage/scripting.html#concept-upgrade-an-element">
<link rel="help" href="https://dom.spec.whatwg.org/#concept-create-element">
<link rel="help" href="https://github.com/whatwg/html/issues/2381">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<div id="log"></div>

<x-upgrade></x-upgrade>

<script>
"use strict";
setup({ allow_uncaught_exception: true });

window.doMicrotasks = (callback1, callback2 = callback1) => {
  Promise.resolve().then(callback1);

  const mo = new MutationObserver(callback2);
  const node = document.createTextNode("");
  mo.observe(node, { characterData: true });
  node.data = "x";
};

window.logMicrotasks = events => {
  window.doMicrotasks(() => events.push("promise microtask"),
                      () => events.push("MutationObserver microtask"));
};

window.flushAsyncEvents = () => {
  return new Promise(resolve => step_timeout(resolve, 0));
};

window.x0Events = [];
customElements.define("x-0", class extends HTMLElement {
  constructor() {
    super();
    logMicrotasks(window.x0Events);
  }
});
</script>

<x-0></x-0>

<script>
"use strict";

test(() => {
  assert_array_equals(window.x0Events, ["promise microtask", "MutationObserver microtask"]);
}, "Microtasks evaluate immediately when the stack is empty inside the parser");

customElements.define("x-bad", class extends HTMLElement {
  constructor() {
    super();
    doMicrotasks(() => this.setAttribute("attribute", "value"));
  }
});
</script>

<x-bad></x-bad>

<script>
"use strict";

test(() => {
  const xBad = document.querySelector("x-bad");
  assert_false(xBad.hasAttribute("attribute"), "The attribute must not be present");
  assert_true(xBad instanceof HTMLUnknownElement, "The element must be a HTMLUnknownElement");
}, "Microtasks evaluate immediately when the stack is empty inside the parser, causing the " +
   "checks on no attributes to fail")

promise_test(() => {
  const events = [];
  customElements.define("x-1", class extends HTMLElement {
    constructor() {
      super();
      logMicrotasks(events);
    }
  });

  document.createElement("x-1");
  events.push("after");

  return flushAsyncEvents().then(() => {
    assert_array_equals(events, ["after", "promise microtask", "MutationObserver microtask"]);
  });
}, "Microtasks evaluate afterward when the stack is not empty using createElement()");

promise_test(() => {
  const events = [];
  class X2 extends HTMLElement {
    constructor() {
      super();
      logMicrotasks(events);
    }
  }
  customElements.define("x-2", X2);

  new X2();
  events.push("after");

  return flushAsyncEvents().then(() => {
    assert_array_equals(events, ["after", "promise microtask", "MutationObserver microtask"]);
  });
}, "Microtasks evaluate afterward when the stack is not empty using the constructor");

promise_test(() => {
  const events = [];
  customElements.define("x-upgrade", class extends HTMLElement {
    constructor() {
      super();
      logMicrotasks(events);
    }
  });
  events.push("after");

  return flushAsyncEvents().then(() => {
    assert_array_equals(events, ["after", "promise microtask", "MutationObserver microtask"]);
  });
}, "Microtasks evaluate afterward when the stack is not empty due to upgrades");
</script>