javascript - How can I tell if an element is in a shadow DOM?

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javascript - How can I tell if an element is in a shadow DOM?

asked Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

javascript - How can I tell if an element is in a shadow DOM?

I have a project where I'm using the shadow DOM natively (not through a polyfill). I'd like to detect if a given element is contained within a shadow DOM or a light DOM.

I've looked through all of the properties on the elements, but there don't seem to be any which vary based on the type of DOM an element is in.

How can I determine if an element is part of a shadow DOM or a light DOM?

Here is an example of what is considered "shadow DOM" and "light DOM" for the purpose of this question.

 (light root) ? Document
      (light)   ? HTML
      (light)   | ? BODY
      (light)   |   ??DIV
(shadow root)   |     ? ShadowRoot
     (shadow)   |       ? DIV 
     (shadow)   |         ? IFRAME 
 (light root)   |           ? Document
      (light)   |             ? HTML
      (light)   |             | ? BODY
      (light)   |             |   ? DIV
(shadow root)   |             |     ? ShadowRoot
     (shadow)   |             |       ? DIV
       (none)   |             ? [Unattached DIV of second Document]
       (none)   ? [Unattached DIV of first Document]

<!doctype html>
<title>
  isInShadow() test document - can not run in Stack Exchange's sandbox
</title>
<iframe src="about:blank"></iframe>
<script>

function isInShadow(element) {
  // TODO
}

function test() {
  //  (light root) ? Document
  //       (light)   ? HTML
  var html = document.documentElement;

  console.assert(isInShadow(html) === false);

  //       (light)   | ? BODY
  var body = document.body;

  console.assert(isInShadow(body) === false);

  //       (light)   |   ??DIV
  var div = document.createElement('div');
  body.appendChild(div);

  console.assert(isInShadow(div) === false);

  // (shadow root)   |     ? ShadowRoot
  var divShadow = div.createShadowRoot();

  var shadowDiv = document.createElement('div');
  divShadow.appendChild(shadowDiv);

  //      (shadow)   |       ? DIV 
  console.assert(isInShadow(shadowDiv) === true);

  //      (shadow)   |         ? IFRAME 
  var iframe = document.querySelector('iframe');
  shadowDiv.appendChild(iframe);

  console.assert(isInShadow(iframe) === true);

  //  (light root)   |           ? Document
  var iframeDocument = iframe.contentWindow.document;

  //       (light)   |             ? HTML
  var iframeHtml = iframeDocument.documentElement;

  console.assert(isInShadow(iframeHtml) === false);

  //       (light)   |             | ? BODY
  var iframeBody = iframeDocument.body;

  //
  console.assert(isInShadow(iframeHtml) === false);

  //       (light)   |             |   ? DIV
  var iframeDiv = iframeDocument.createElement('div');
  iframeBody.appendChild(iframeDiv);
   
  console.assert(isInShadow(iframeDiv) === false);
   
  // (shadow root)   |             |     ? ShadowRoot
  var iframeDivShadow = iframeDiv.createShadowRoot();

  //      (shadow)   |             |       ? DIV
  var iframeDivShadowDiv = iframeDocument.createElement('div');
  iframeDivShadow.appendChild(iframeDivShadowDiv);
    
  console.assert(isInShadow(iframeDivShadowDiv) === true);
     
  //        (none)   |             ? [Unattached DIV of second Document]
  var iframeUnattached = iframeDocument.createElement('div');
    
  console.assert(Boolean(isInShadow(iframeUnattached)) === false);

  //        (none)   ? [Unattached DIV of first Document]
  var rootUnattached = document.createElement('div');
    
  console.assert(Boolean(isInShadow(rootUnattached)) === false);
}

onload = function main() {
  console.group('Testing');
  try {
    test();
    console.log('Testing complete.');
  } finally {
    console.groupEnd();
  }
}

</script>

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1 Answer

深蓝 · Answer 1 · 2021-10-23T18:43:52+0000

If you call a ShadowRoot's toString() method, it will return "[object ShadowRoot]". According to this fact, here's my approach:

function isInShadow(node) {
    var parent = (node && node.parentNode);
    while(parent) {
        if(parent.toString() === "[object ShadowRoot]") {
            return true;
        }
        parent = parent.parentNode;
    }
    return false;
}

EDIT

Jeremy Banks suggests an approach in another style of looping. This approach is a little different from mine: it also checks the passed node itself, which I didn't do.

function isInShadow(node) {
    for (; node; node = node.parentNode) {
        if (node.toString() === "[object ShadowRoot]") {
            return true;
        }
    }
    return false;
}

function isInShadow(node) {
    for (; node; node = node.parentNode) {
        if (node.toString() === "[object ShadowRoot]") {
            return true;
        }
    }
    return false;
}

console.group('Testing');

var lightElement = document.querySelector('div');    

console.assert(isInShadow(lightElement) === false);

var shadowChild = document.createElement('div');
lightElement.createShadowRoot().appendChild(shadowChild);

console.assert(isInShadow(shadowChild) === true);

var orphanedElement = document.createElement('div');

console.assert(isInShadow(orphanedElement) === false);

var orphanedShadowChild = document.createElement('div');
orphanedElement.createShadowRoot().appendChild(orphanedShadowChild);

console.assert(isInShadow(orphanedShadowChild) === true);

var fragmentChild = document.createElement('div');
document.createDocumentFragment().appendChild(fragmentChild);

console.assert(isInShadow(fragmentChild) === false);

console.log('Complete.');
console.groupEnd();

<div></div>

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javascript - How can I tell if an element is in a shadow DOM?

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