Polymer2 Shadow dom select child element - selector

Question

I am working on a polymer2 shadow dom template project need to select children elements from parent elements. I found this article introduces a way to select child shadow dom elements that like this:
// No fun.
document.querySelector('x-tabs').shadowRoot
.querySelector('x-panel').shadowRoot
.querySelector('#foo');
// Fun.
document.querySelector('x-tabs::shadow x-panel::shadow #foo');
However, when I tried in my polymer2 project, like this:
//First: works!!
document.querySelector('container')
.shadowRoot.querySelector('app-grid')
.shadowRoot.querySelector('#apps');
//Second: Doesn't work!// got null
document.querySelector('container::shadow app-grid::shadow #apps')
// Thrird: document.querySelector('* /deep/ #apps') // Doesn't work, got null
I really need the second way or the third, which to put selectors in (), but both couldn't work. Does anyone know why the second one doesn't work? Thank you so much!

Answer 1

::shadow and /deep/ has never(?) worked in Firefox, and is depraved in Chrome 63 and later.
Source
Eric Biedelman has written a nice querySelector method for finding all custom elements on a page using shadow DOM. I wouldn't use it myself, but I have implemented it so I can "querySelect" custom elements in the console. Here is his modified code:
// EXAMPLES
// findCustomElement('app-grid') // Returns app-grid element
// findCustomElements('dom-if') // Returns an array of dom-if elements (if there are several ones)
// findCustomElement('app-grid').props // Returns properties of the app-grid element
function findCustomElement(customElementName) {
const allCustomElements = [];
customElementName = (customElementName) ? customElementName.toLowerCase() : customElementName;
function isCustomElement(el) {
const isAttr = el.getAttribute('is');
// Check for <super-button> and <button is="super-button">.
return el.localName.includes('-') || isAttr && isAttr.includes('-');
}
function findAllCustomElements(nodes) {
for (let i = 0, el; el = nodes[i]; ++i) {
if (isCustomElement(el)) {
el.props = el.__data__ || el.__data || "Doesn't have any properties";
if (customElementName && customElementName === el.tagName.toLowerCase()) {
allCustomElements.push(el);
} else if (!customElementName) {
allCustomElements.push(el);
}
}
// If the element has shadow DOM, dig deeper.
if (el.shadowRoot) {
findAllCustomElements(el.shadowRoot.querySelectorAll('*'));
}
}
}
findAllCustomElements(document.querySelectorAll('*'));
if (allCustomElements.length < 2) {
return allCustomElements[0] || customElementName + " not found";
} else if (customElementName) {
allCustomElements.props = "Several elements found of type " + customElementName;
}
return allCustomElements;
}
Remove the if (isCustomElement(el)) { statement, and you can querySelect whatever element and get an array of it if several of them exists. You can change findAllCustomElements to implement a smarter querySelect using the recursive loop on shadowDoom as base. Again, I wouldn't use this myself – and instead pass on variables from parent element(s) to children where the children have observers that activates specific behaviors – but I wanted to give you a general implementation of a fallback if nothing else works.
The problem with your question is that you don't give any specifics about WHY you want to select the children in the first place.