How To Query Through <slot> Using Cypress While Testing Web Components - cypress

Question

After years of testing one global DOM for end-to-end testing, I'm finding it very difficult, if not impossible, to test web components that use slots. Before I explain the problem, I want to say that I cannot change the generated markup to improve things as they are.
<wc-1 attributes-etc="">
<wc-2 attributes-etc="">
<slot>
<wc-3 attributes-etc="">
<slot>
...eventually get to an input...
<input type="text" name="firstName" />
There are a buttload of nested web components from some kind of form builder, and there are also plenty of slots used. The web components have attributes but the slots never do, so I use the web component name for querying.
document.querSelector('wc-1')
.shadowRoot.querySelector('wc-2')
.shadowRoot.querySelector('slot')
// Yields <slot>...</slot>
All fine to this point and Cypress has a .shadow() command I used, but I'm testing with just devtools here to see all the properties the slot has.
document.querSelector('wc-1')
.shadowRoot.querySelector('wc-2')
.shadowRoot.querySelector('slot')
.shadowRoot
// Yields "null".
// I don't know how to get to the .lightDOM? of wc-2?
Any property I try ends up being null or having 0 elements in the returned value. Using other front-end tools and the global DOM, I can always cy.get('div[data-testid="the-nested-element-i-want"]').type('important words') in one command.
So my main question is: How do people test these things once web components start piling up? Or don't do this and just test the web components in isolation/unit tests since it's so hard to query the nested shadow DOMs?
The main goal is to eventually get to a form input to cy.get('input[name"firstName"]').type('John'). Can someone give me the chained docuement.querySelector() command to get to <wc-3> in my example?

Answer 1

The answer involves assignedNodes(): https://developer.mozilla.org/en-US/docs/Web/API/HTMLSlotElement/assignedNodes
The assignedNodes() property of the HTMLSlotElement interface returns a sequence of the nodes assigned to this slot...
It made no difference for me to use that vs. assignedElements(). So, all you have to do is use that method once you've queried down to the slot you need. For my example, the answer is:
const wc-3 = document.querySelector('wc-1').shadowRoot
.querySelector('wc-2').shadowRoot
.querySelector('slot').assignedNodes()
.map((el) => el.shadowRoot)[0]
And then you can keep going down the chain...I know I only have one un-named slot, so that's why I grab it from the returned .map().
Props to this Q&A for pointing me on the right direction: Web components: How to work with children?

Answer 2

There will be no DOM content in your <slot>, as there is no DOM content moved to slots.
lightDOM content is reflected in slots, but remains invisible! in lightDOM.
(that is why you also style slotted content in lightDOM)
From the docs:
𝘾𝙤𝙣𝙘𝙚𝙥𝙩𝙪𝙖𝙡𝙡𝙮, 𝙙𝙞𝙨𝙩𝙧𝙞𝙗𝙪𝙩𝙚𝙙 𝙣𝙤𝙙𝙚𝙨 𝙘𝙖𝙣 𝙨𝙚𝙚𝙢 𝙖 𝙗𝙞𝙩 𝙗𝙞𝙯𝙖𝙧𝙧𝙚.
𝙎𝙡𝙤𝙩𝙨 𝙙𝙤𝙣'𝙩 𝙥𝙝𝙮𝙨𝙞𝙘𝙖𝙡𝙡𝙮 𝙢𝙤𝙫𝙚 𝘿𝙊𝙈; 𝙩𝙝𝙚𝙮 𝙧𝙚𝙣𝙙𝙚𝙧 𝙞𝙩 𝙖𝙩 𝙖𝙣𝙤𝙩𝙝𝙚𝙧 𝙡𝙤𝙘𝙖𝙩𝙞𝙤𝙣 𝙞𝙣𝙨𝙞𝙙𝙚 𝙩𝙝𝙚 𝙨𝙝𝙖𝙙𝙤𝙬 𝘿𝙊𝙈.
So to test if something is "in" a slot
you need to check for slot=? attributes on lightDOM elements
and double check if that <slot name=? > actually exists in shadowDOM
Or vice versa
Or hook into the slotchange Event, but that is not Testing
pseudo code:
for the vice-versa approach; can contain errors.. its pseudo code..
function processDOMnode( node ){
if (node.shadowRoot){
// query shadowDOM
let slotnames = [...node.shadowRoot.querySelectorAll("slot")].map(s=>s.name);
// query lightDOM
slotnames.forEach( name =>{
let content = node.querySelectorAll(`[slot="${name}"]`);
console.log( "slot:" , name , "content:" , content );
});
// maybe do something with slotnames in lightDOM that do NOT exist in shadowDOM
// dive deeper
this.shadowRooot.children.forEach(shadownode => processDOMnode(shadownode));
}
}