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chromium/third_party/blink/web_tests/external/wpt/html/browsers/the-window-object/named-access-on-the-window-object/existing-prop.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Named access on the window object - Existing property</title>
<link rel="author" title="Matthew Phillips" href="mailto:[email protected]">
<link rel="help" href="https://html.spec.whatwg.org/#named-access-on-the-window-object">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<body>
<script>
"use strict";

test(() => {
  window["existing-one"] = 5;

  const img = document.createElement("img");
  img.setAttribute("id", "existing-one");
  document.body.appendChild(img);

  assert_equals(window["existing-one"], 5);
}, "A named property must not be set if a property already exists");

test(() => {
  window["existing-undefined"] = undefined;

  const img = document.createElement("img");
  img.setAttribute("id", "existing-undefined");
  document.body.appendChild(img);

  assert_equals(window["existing-undefined"], undefined);
}, "A named property must not be set if a property already exists, even if undefined");

test(() => {
  const img = document.createElement("img");
  img.setAttribute("id", "user-val");
  document.body.appendChild(img);

  window["user-val"] = 5;

  assert_equals(window["user-val"], 5);
}, "If a property is occupied by a named property but then the user sets it, the user's value shadows it");

test(() => {
  const img = document.createElement("img");
  img.setAttribute("id", "undefined");
  document.body.appendChild(img);

  /* eslint-disable no-void */
  assert_equals(undefined, void 0);
  assert_equals(window.undefined, void 0);
  /* eslint-enable no-void */
}, "A named property must not be set if the property already exists in the proxy");
</script>