Develop Reference

Combine :host() with :has() - not possible?

I have a web component with a shadow DOM and a default slot.
I need to apply certain styling based on the presence or absence of specific a light DOM descendant. Please note that I don't need a specific workaround for this specific styling, it's just an example and in the real world the example is alot more complex.
I also cannot work with regular DOM CSS like x-y:has(div) since I need to apply styles to an element in the shadow DOM based on the presence of the div in the light DOM.
Please note that the code snippet only works in browsers that support constructable stylesheets (e.g. Safari won't).
const styleStr = `
:host {
display: block;
border: 3px dotted red;
}
:host(:has(div)) {
border-color: green;
}
`;
let css;
try {
css = new CSSStyleSheet;
css.replaceSync(styleStr);
} catch(e) { console.error(e) }
customElements.define('x-y', class extends HTMLElement {
constructor() {
super().attachShadow({mode: 'open'}).adoptedStyleSheets.push(css);
this.shadowRoot.append(document.createElement('slot'))
}
})
<x-y>no div - should have red border</x-y>
<x-y>
<div>div, should have green border</div>
</x-y>
I was trying to find if maybe :host() is not accepting :has(), but was unable to find anything on it, neither in the spec, nor on MDN or caniuse.
Does anyone have definitive knowledge/reference about this, and can point me to some documentation?

You want to style slotted content based on an element inside the slot
Since <slot> are reflected, (deep dive: ::slotted CSS selector for nested children in shadowDOM slot)
you need to style a <slot> in its container element.
If you want that logic to be done from inside the Component,
you could do it from the slotchange Event, which checks if a slotted element contains that DIV
Then creates a <style> element in the container element
Disclaimer: Provided code is a Proof of Concept, not production ready
<my-component>
Hello Web Component
</my-component>
<!-- <my-component> will add a STYLE element here -->
<my-component>
<!-- <my-component> will assign a unique ID to the DIV -->
<div>Web Component with a DIV in the slot</div>
</my-component>
<script>
customElements.define("my-component", class extends HTMLElement {
constructor() {
super().attachShadow({mode: "open"}).innerHTML = `<slot/>`;
let slot = this.shadowRoot.querySelector("slot");
slot.addEventListener("slotchange", (evt) => {
[...slot.assignedNodes()].forEach(el => {
if (el.nodeName == "DIV") {
el.id = "unique" + new Date() / 1;
// inject a <style> before! <my-component>
this.before( Object.assign( document.createElement("STYLE"), {
innerHTML : `#${el.id} { background:lightgreen } `
}));
}
});
});
}
})
</script>
PS. Don't dynamically add any content inside <my-component>, because that slotchange will fire again...

Stenciljs: What is the right way to prepend/append Slot elements?

I am having some trouble with the lifecycle methods in web components.
We want to dynamically order child elements being passed in as slots.
To illustrate, this web component takes a prop, iconPos, and will determine whether the icon will be placed at the start or end of the slot.
<my-component iconPos="start">
<img src="/path/icon.svg" />
<div>{this.list}</div>
</my-component>
I haven't had any luck getting it working with ref:
dc6b89e7.js:2926 TypeError: Cannot read properties of undefined (reading 'prepend')
Here's what I have so far:
#State() slotElement!: HTMLDivElement;
#Prop() iconPos: 'start' | 'end';
...
private createSlots() {
switch (this.iconPos) {
case 'start':
this.slotElement.prepend(<img />);
break;
case 'end':
this.slotElement.append(<img />);
break;
default:
throw new Error(
`Invalid value \`${this.iconPos}\`, passed into \`iconPos\`. Expected valid values are \`start\`, \`end\``.
);
}
}
render() {
return (
// iconPos="start"
<parent-component>
<div ref={(el) => (this.slotElement= el as HTMLDivElement)}>
<slot></slot>
</div>
</parent-component>
)
}
I would prefer to not use a CSS solution if possible. Any help would be much appreciated!

Slotted content is NOT MOVED to <slot> elements; it is reflected!!
So all styling and element operations must be done in "lightDOM"
For (very) long read see:
::slotted CSS selector for nested children in shadowDOM slot
That means you have to append your elements in ligtDOM with:
this.append(this.firstElementChild)
You can't read the <my-component> innerHTML before it is parsed; so you need to wait till the innerHTML elements are created. Thus you will see the DOM change.
A better method might be to not use <slot> and declare your icon and content as attributes, and have the Web Component create the HTML.
<style>
span::after { content: attr(id) }
#FOO { background: lightgreen }
</style>
<my-component>
<span id="FOO"></span>
<span id="BAR"></span>
</my-component>
<my-component reversed>
<span id="FOO"></span>
<span id="BAR"></span>
</my-component>
<script>
window.customElements.define('my-component', class extends HTMLElement {
constructor() {
super().attachShadow({mode:'open'})
.innerHTML = `<style>::slotted(span){background:gold}</style>
${this.nodeName}<slot></slot><br>`;
}
connectedCallback() {
setTimeout(() => { // make sure innerHTML is parsed!
if (this.hasAttribute("reversed")) {
this.append(this.firstElementChild);
}
})
}
});
</script>

Conditional styling on class in Svelte

I'm trying to use Svelte to do some conditional styling and highlighting to equations. While I've been successful at applying a global static style to a class, I cannot figure out how to do this when an event occurs (like one instance of the class is hovered over).
Do I need to create a stored value (i.e. some boolean that gets set to true when a class is hovered over) to use conditional styling? Or can I write a function as in the example below that will target all instances of the class? I'm a bit unclear why targeting a class in styling requires the :global(classname) format.
App.svelte
<script>
// import Component
import Katex from "./Katex.svelte"
// math equations
const math1 = "a\\htmlClass{test}{x}^2+bx+c=0";
const math2 = "x=-\\frac{-b\\pm\\sqrt{b^2-4ac}}{2a}";
const math3 = "V=\\frac{1}{3}\\pi r^2 h";
// set up array and index for reactivity and initialize
const mathArray = [math1, math2, math3];
let index = 0;
$: math = mathArray[index];
// changeMath function for button click
function changeMath() {
// increase index
index = (index+1)%3;
}
function hoverByClass(classname,colorover,colorout="transparent")
{
var elms=document.getElementsByClassName(classname);
console.log(elms);
for(var i=0;i<elms.length;i++)
{
elms[i].onmouseover = function()
{
for(var k=0;k<elms.length;k++)
{
elms[k].style.backgroundColor=colorover;
}
};
elms[i].onmouseout = function()
{
for(var k=0;k<elms.length;k++)
{
elms[k].style.backgroundColor=colorout;
}
};
}
}
hoverByClass("test","pink");
</script>
<h1>KaTeX svelte component demo</h1>
<h2>Inline math</h2>
Our math equation: <Katex {math}/> and it is inline.
<h2>Displayed math</h2>
Our math equation: <Katex {math} displayMode/> and it is displayed.
<h2>Reactivity</h2>
<button on:click={changeMath}>
Displaying equation {index}
</button>
<h2>Static math expression within HTML</h2>
<Katex math={"V=\\pi\\textrm{ m}^3"}/>
<style>
:global(.test) {
color: red
}
</style>
Katex.svelte
<script>
import katex from "katex";
export let math;
export let displayMode = false;
const options = {
displayMode: displayMode,
throwOnError: false,
trust: true
}
$: katexString = katex.renderToString(math, options);
</script>
<svelte:head>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex#0.12.0/dist/katex.min.css" integrity="sha384-AfEj0r4/OFrOo5t7NnNe46zW/tFgW6x/bCJG8FqQCEo3+Aro6EYUG4+cU+KJWu/X" crossorigin="anonymous">
</svelte:head>
{#html katexString}

If I understand it correctly you have a DOM structure with arbitrary nested elements and you would want to highlight parts of the structure that share the same class.
So you would have a structure like this:
<div>
<p>This is some text <span class="a">highlight</span></p>
<span class="a">Another highlight</span>
<ul>
<li>Some listitem</li>
<li class="a">Some listitem</li>
<li class="b">Some listitem</li>
<li class="b">Some listitem</li>
</ul>
</div>
And if you select an element with class="a" all elements should be highlighted regardles where they are in the document. This arbitrary placement makes using the sibling selector in css not possible.
There is no easy solution to this, but I will give you my attempt:
This is the full code with some explanation
<script>
import { onMount } from 'svelte'
let hash = {}
let wrapper
onMount(() => {
[...wrapper.querySelectorAll('[class]')].forEach(el => {
if (hash[el.className]) return
else hash[el.className] = [...wrapper.querySelectorAll(`[class="${el.className}"]`)]
})
Object.values(hash).forEach(nodes => {
nodes.forEach(node => {
node.addEventListener('mouseover', () => nodes.forEach(n => n.classList.add('hovered')))
node.addEventListener('mouseout', () => nodes.forEach(n => n.classList.remove('hovered')))
})
})
})
</script>
<div bind:this={wrapper}>
<p>
Blablabla <span class="a">AAA</span>
</p>
<span class="a">BBBB</span>
<ul>
<li>BBB</li>
<li class="a b">BBB</li>
<li class="b">BBB</li>
<li class="b">BBB</li>
</ul>
</div>
<style>
div :global(.hovered) {
background-color: red;
}
</style>
The first thing I did was use bind:this to get the wrapping element (in your case you would put this around the {#html katexString}, this will make that the highlight is only applied to this specific subtree.
Doing a querySelector is a complex operation, so we will gather all the related nodes in a sort of hashtable during onMount (this kind of assumes the content will never change, but since it's rendered with #html I believe it's safe to do so).
As you can see in onMount, I am using the wrapper element to restrict the selector to this section of the page, which is a lot faster than checking the entire document and is probably what you want anyway.
I wasn't entirely sure what you want to do, but for simplicity I am just grabbing every descendant that has a class and make a hash section for each class. If you only want certain classes you could write out a bunch of selectors here instead:
hash['selector-1'] = wrapper.querySelectorAll('.selector-1');
hash['selector-2'] = wrapper.querySelectorAll('.selector-2')];
hash['selector-3'] = wrapper.querySelectorAll('.selector-3');
Once this hashtable is created, we can loop over each selector, and attach two event listeners to all of the elements for that selector. One mouseover event that will then again apply a new class to each of it's mates. And a mouseout that removes this class again.
This still means you have to add hovered class. Since the class is not used in the markup it will be removed by Svelte unless you use :global() as you found out yourself. It is indeed not that good to have global classes because you might have unintended effect elsewhere in your code, but you can however scope it as I did in the code above.
The line
div > :global(.hovered) { background-color: red; }
will be processed into
div.svelte-12345 .hovered { background-color: red; }
So the red background will only be applied to .hovered elements that are inside this specific div, without leaking all over the codebase.
Demo on REPL
Here is the same adapted to use your code and to use a document-wide querySelector instead (you could probably still restrict if wanted by having the bind one level higher and pass this node into the component)
Other demo on REPL

Is it bad practice to create a web-component inside another and append it to said other?

Essentially I have a web component "x" and I dynamically create a form component inside the "x" which will be appended to "x".
I could just do it in the place I create "x", after creating "x", of course.
Basically this:
class X extends LitElement {
render() {
return html`
<div>
<slot name="form-component">${this.appendFormComponent()}</slot>
</div>
<slot></slot>
`
}
appendFormComponent() {
const formComponent = document.createElement('input')
formComponent.slot = "form-component"
this.append(formComponent)
}
// side note, is running this append inside the render function a terrible
// idea and where should I do it instead? I mean doing it in the render
// function does appear to work...
}

As you suspected, this is definitely a terrible idea because you are mixing imperative paradigm with declarative paradigm. However, if you really need to do this and since you are using LitElement, you can nicely abstract the declarative and imperative UI code using appropriate lifecycle methods:
class X extends LitElement {
render() {
return html`
<div>
<slot name='form-component'></slot>
</div>
<slot></slot>
`;
}
// Executed only once
firstUpdated() {
const formComponent = document.createElement('input');
formComponent.slot = 'form-component';
this.append(formComponent);
}
}
Also, the approach you are attempting is probably problematic. Your problem would be easily solved by render function only:
class X extends LitElement {
render() {
return html`
<div>
<slot name='form-component'>
<!-- Notice the use of INPUT TAG here -->
<input type='text' />
</slot>
</div>
<slot></slot>
`;
}
}
Using something like firstUpdated with document.createElement should be used to create UI components which have offset elements that break the UI as Function of State notion. Such components are date pickers, multi select dropdown, dialog boxes, etc. which directly append DOM elements to the body for managing Z-index and fixed positioning accurately.
Further, as per your comments, if you have a dynamic function which needs to be assigned to the input text, simply create a wrapper function like:
class X extends LitElement {
// Input change event handler
onChange() {
// A guard to check presence of dynamic function
if (this.someDynamicFuction) {
this.someDynamicFuction();
}
}
render() {
return html`
<div>
<slot name='form-component'>
<!-- Notice the use of INPUT TAG here -->
<input type='text' #change=${this.onChange} />
</slot>
</div>
<slot></slot>
`;
}
}

custom web component event call back function in tag

class UioKey extends HTMLElement {
...
eKey(){windows.alert('class eKey function')}
}
function eKey(){
eKey(){windows.alert('document eKey function')}
<template id="uio-key-temp">
<div class="uio-key">
<div class="i" onclick="eKey()"></div><span></span>
</div>
</template>
when clikcing on the .i div agot the document ekey that is firing, i want
the class ekey() to be fired
if i omit the dodument eKey() fuction i got function eKey() undefined

onclick will only work with globally defined functions.
Here is a very quick hack that allows you to use a class function.
// Class for `<uio-key>`
class UioKey extends HTMLElement {
constructor() {
super();
let shadow = this.attachShadow({mode: 'open'});
shadow.innerHTML = '<div><div on-click="eKey">div</div><span>span</span></div>';
let a = shadow.querySelectorAll('[on-click]');
a.forEach(
el => {
const handlerName = el.getAttribute('on-click');
el.addEventListener('click', this[handlerName]);
}
);
}
eKey() {
window.alert('class eKey function');
}
}
// Define our web component
customElements.define('uio-key', UioKey);
<hr/>
<uio-key></uio-key>
<hr/>
I use a custom attribute on-click as a way to grab all elements that want a click handler then I take the value of that attribute and use it as the class function name and pass it into the addEventListener function.

Alternatly to #Intervalia's answer, you could use the getRootNode() method and then the host property to access the Custom Element object from inside the Shadow DOM.
class UioKey extends HTMLElement {
constructor() {
super()
this.attachShadow( { mode: 'open' } )
.innerHTML = uio-key-temp.innerHTML
}
eKey(){
window.alert('class eKey function' )
}
}
customElements.define( 'uio-key', UioKey )
<template id="uioKeyTemp">
<style> .i { background: gray ; height: 10pt } </style>
<div class="uio-key">
<div class="i" onclick="this.getRootNode().host.eKey()"></div>
</div>
</template>
<uio-key></uio-key>
Note that it's always a good practice to use inline scripts because in some situations they can be disabled (for security reasons).