dom/nodes/MutationObserver-callback-arguments.html

<!DOCTYPE HTML>
<meta charset=utf-8>
<title>MutationObserver: callback arguments</title>
<link rel="help" href="https://dom.spec.whatwg.org/#notify-mutation-observers">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="mo-target"></div>
<div id="log"></div>
<script>
"use strict";

async_test(t => {
  const moTarget = document.querySelector("#mo-target");
  const mo = new MutationObserver(function(records, observer) {
    t.step(() => {
      assert_equals(this, mo);
      assert_equals(arguments.length, 2);
      assert_true(Array.isArray(records));
      assert_equals(records.length, 1);
      assert_true(records[0] instanceof MutationRecord);
      assert_equals(observer, mo);

      mo.disconnect();
      t.done();
    });
  });

  mo.observe(moTarget, {attributes: true});
  moTarget.className = "trigger-mutation";
}, "Callback is invoked with |this| value of MutationObserver and two arguments");
</script>