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How To Set An Attribute Of A Web Component Dynamically

I am stuck on trying to set an attribute for a custom web component called ws-dialog I designed. It always adds an attribute undefined on the code:
<ws-dialog class="global" undefined="add-page-content"></ws-dialog>
What I want to do here is to declare an attribute like template instead of undefined. Here is the code that I used for adding an onclick event to all dialog target elements:
function addDialogEvents(target) {
target.addEventListener("click", () => {
let dialog = document.createElement(Dialog.getName());
let body = document.querySelector("body");
dialog.template = "add-page-content";
body.append(dialog);
});
}
And here is the code for my custom component:
"use strict";
class Dialog extends HTMLElement {
connectedCallback() {
//Styles for this custom component are declared in a separate CSS module
}
updateContent(id) {
let content = document.querySelector(`template#${id}`).content;
if (content)
this.append(content);
}
attributeChangedCallback(name, oldValue, newValue) {
this.TEMPLATE = "template";
switch (name) {
case this.TEMPLATE:
this.updateContent(newValue);
break;
}
}
static get observedAttributes() {
return [this.TEMPLATE];
}
static getName() {
return "ws-dialog";
}
get template() {
return this.getAttribute(this.TEMPLATE);
}
set template(template) {
this.setAttribute(this.TEMPLATE, template);
}
}
customElements.define(Dialog.getName(), Dialog);

Here is a playground to help you see when what method is fired.
Note when and how many times observedAttributes is called.
That means your this.TEMPLATE is undefined, and thus becomes a String undefined in the Array
<script>
const log = (...args) => {
let div=document.body.appendChild(document.createElement('DIV'));
div.style = `background:${args.shift()};color:white;font:13px Arial`;
div.append(args.join(" "));
}
customElements.define('my-element', class extends HTMLElement {
log() {
log(this.getAttribute("color"), this.outerHTML.split(">")[0],'>', ...arguments);
}
static get observedAttributes() {
log('red', `my-element observedAttributes`);// NO 'this' / Element here!
return ["color"];
}
constructor() { super().log("constructor") }
connectedCallback() {
this.log("connectedCallback" , this.innerHTML || "No innerHTML" );//FireFox difference!
setTimeout(() => this.log(`delayed connectedCallback ${this.innerHTML}`), 0);
}
attributeChangedCallback(name, oldValue, newValue) { // 4th W3C parameter = Namespace (not implemented in Browsers)
this.log("attributeChangedCallback", name, oldValue || "null", newValue);
}
disconnectedCallback(){ this.log("disconnectedCallback") }
})
document.body.onload = () => {
log('magenta', 'onload event');
A.setAttribute("color", "darkolivegreen");
B.innerHTML = "<my-element id=C color=hotpink>Charlie replaced Bravo</my-element>";
B.remove();
}
</script>
<my-element id=A color=green>Alfa</my-element>
<my-element id=B color=blue>Bravo</my-element>
Notes:
Helpful diagram at: https://andyogo.github.io/custom-element-reactions-diagram/
The execution order is slightly different in FireFox only, my advice is to not Develop, only Test in Firefox
In FireFox you can access the inner Elements from the connectedCallback. In all other Browsers you need that setTimeout. Different tech interpretations of the same W3C standard. And in this case Apple engineers were correct source. Chromium is based on the same engine.
Note how (delayed) code from the C connectedCallback (or any method) runs AFTER the disconnectedCallback.
You need to write code that does not error when DOM elements no longer exist.
JSFiddle playground: https://jsfiddle.net/WebComponents/67pduja9/

how to pass objects in web component javascript [duplicate]

My understanding is that data is passed to a custom html element via its attributes and sent out by dispatching a CustomEvent.
JavaScript objects can obviously be sent out in the event's detail field, but what if the element needs a lot of data passed into it. Is there a way to provide it with an object in JavaScript.
What if the element for instance contains a variable number of parts that needs to be initialized or changed dynamically (e.g. a table with a variable number of rows)? I can imagine setting and modifying an attribute consisting of a JSON string that is parsed inside the component, but it does not feel like an elegant way to proceed:
<my-element tableRowProperties="[{p1:'v1', p2:'v2'}, {p1:'v1',p2:'v2'}, {p1:'v1',p2:'v2'}]"></my-element>
Or can you make the element listen to events from the outside that contains a payload of data?

Passing Data In
If you really want/need to pass large amounts of data into your component then you can do it four different ways:
1) Use a property. This is the simplest since you just pass in the Object by giving the value to the element like this: el.data = myObj;
2) Use an attribute. Personally I hate this way of doing it this way, but some frameworks require data to be passed in through attributes. This is similar to how you show in your question. <my-el data="[{a:1},{a:2}....]"></my-el>. Be careful to follow the rules related to escaping attribute values. If you use this method you will need to use JSON.parse on your attribute and that may fail. It can also get very ugly in the HTML to have the massive amount of data showing in a attribute.
3 Pass it in through child elements. Think of the <select> element with the <option> child elements. You can use any element type as children and they don't even need to be real elements. In your connectedCallback function your code just grabs all of the children and convert the elements, their attributes or their content into the data your component needs.
4 Use Fetch. Provide a URL for your element to go get its own data. Think of <img src="imageUrl.png"/>. If your already has the data for your component then this might seem like a poor option. But the browser provides a cool feature of embedding data that is similar to option 2, above, but is handled automatically by the browser.
Here is an example of using embedded data in an image:
img {
height: 32px;
width: 32px;
}
<img src="data:image/svg+xml;charset=utf8,%3C?xml version='1.0' encoding='utf-8'?%3E%3Csvg version='1.1' xmlns='http://www.w3.org/2000/svg' x='0px' y='0px' viewBox='0 0 314.7 314.7'%3E%3Cstyle type='text/css'%3E .st0{fill:transparent;stroke:%23231F20;stroke-width:12;} .st1{fill:%23231F20;stroke:%23231F20;stroke-width:10;stroke-linejoin:round;stroke-miterlimit:10;} %3C/style%3E%3Cg%3E%3Ccircle class='st0' cx='157.3' cy='157.3' r='150.4'/%3E%3Cpolygon class='st1' points='108,76.1 248.7,157.3 108,238.6'/%3E%3C/g%3E%3C/svg%3E">
And here is an example of using embedded data in a web component:
function readSrc(el, url) {
var fetchHeaders = new Headers({
Accept: 'application/json'
});
var fetchOptions = {
cache: 'default',
headers: fetchHeaders,
method: 'GET',
mode: 'cors'
};
return fetch(url, fetchOptions).then(
(resp) => {
if (resp.ok) {
return resp.json();
}
else {
return {
error: true,
status: resp.status
}
}
}
).catch(
(err) => {
console.error(err);
}
);
}
class MyEl extends HTMLElement {
static get observedAttributes() {
return ['src'];
}
attributeChangedCallback(attrName, oldVal, newVal) {
if (oldVal !== newVal) {
this.innerHtml = '';
readSrc(this, newVal).then(
data => {
this.innerHTML = `<pre>
${JSON.stringify(data,0,2)}
</pre>`;
}
);
}
}
}
// Define our web component
customElements.define('my-el', MyEl);
<!--
This component would go load its own data from "data.json"
<my-el src="data.json"></my-el>
<hr/>
The next component uses embedded data but still calls fetch as if it were a URL.
-->
<my-el src="data:json,[{"a":9},{"a":8},{"a":7}]"></my-el>
You can do that same this using XHR, but if your browser supports Web Components then it probably supports fetch. And there are several good fetch polyfills if you really need one.
The best advantage to using option 4 is that you can get your data from a URL and you can directly embed your data. And this is exactly how most pre-defined HTML elements, like <img> work.
UPDATE
I did think of a 5th way to get JSON data into an object. That is by using a <template> tag within your component. This still required you to call JSON.parse but it can clean up your code because you don't need to escape the JSON as much.
class MyEl extends HTMLElement {
connectedCallback() {
var data;
try {
data = JSON.parse(this.children[0].content.textContent);
}
catch(ex) {
console.error(ex);
}
this.innerHTML = '';
var pre = document.createElement('pre');
pre.textContent = JSON.stringify(data,0,2);
this.appendChild(pre);
}
}
// Define our web component
customElements.define('my-el', MyEl);
<my-el>
<template>[{"a":1},{"b":"<b>Hi!</b>"},{"c":"</template>"}]</template>
</my-el>
Passing Data Out
There are three ways to get data out of the component:
1) Read the value from a property. This is ideal since a property can be anything and would normally be in the format of the data you want. A property can return a string, an object, a number, etc.
2) Read an attribute. This requires the component to keep the attribute up to date and may not be optimal since all attributes are strings. So your user would need to know if they need to call JSON.parse on your value or not.
3) Events. This is probably the most important thing to add to a component. Events should trigger when state changes in the component. Events should trigger based on user interactions and just to alert the user that something has happened or that something is available. Traditionally you would include the relevant data in your event. This reduces the amount of code the user of your component needs to write. Yes, they can still read properties or attributes, but if your events include all relevant data then they probably won't need to do anything extra.

There is a 6th way that is really similar to #Intervalia's answer above but uses a <script> tag instead of a <template> tag.
This is the same approach used by a Markdown Element.
class MyEl extends HTMLElement {
connectedCallback() {
var data;
try {
data = JSON.parse(this.children[0].innerHTML);
}
catch(ex) {
console.error(ex);
}
this.innerHTML = '';
var pre = document.createElement('pre');
pre.textContent = JSON.stringify(data,0,2);
this.appendChild(pre);
}
}
// Define our web component
customElements.define('my-el', MyEl);
<my-el>
<script type="application/json">[{"a":1},{"b":"<b>Hi!</b>"},{"c":"</template>"}]</script>
</my-el>

If you are using Polymer based web components, the passing of data could be done by data binding. Data could be stored as JSON string within attribute of and passed via context variable.
<p>JSON Data passed via HTML attribute into context variable of and populating the variable into combobox.</p>
<dom-bind><template>
<iron-ajax url='data:text/json;charset=utf-8,
[{"label": "Hydrogen", "value": "H"}
,{"label": "Oxygen" , "value": "O"}
,{"label": "Carbon" , "value": "C"}
]'
last-response="{{lifeElements}}" auto handle-as="json"></iron-ajax>
<vaadin-combo-box id="cbDemo"
label="Label, value:[[cbDemoValue]]"
placeholder="Placeholder"
items="[[lifeElements]]"
value="{{ cbDemoValue }}"
>
<template>
[[index]]: [[item.label]] <b>[[item.value]]</b>
</template>
</vaadin-combo-box>
<vaadin-combo-box label="Disabled" disabled value="H" items="[[lifeElements]]"></vaadin-combo-box>
<vaadin-combo-box label="Read-only" readonly value="O" items="[[lifeElements]]"></vaadin-combo-box>
<web-elemens-loader selection="
#polymer/iron-ajax,
#vaadin/vaadin-element-mixin/vaadin-element-mixin,
#vaadin/vaadin-combo-box,
"></web-elemens-loader>
</template></dom-bind>
<script src="https://cdn.xml4jquery.com/web-elements-loader/build/esm-unbundled/node_modules/#webcomponents/webcomponentsjs/webcomponents-loader.js"></script><script type="module" src="https://cdn.xml4jquery.com/web-elements-loader/build/esm-unbundled/src/web-elemens-loader.js"></script>

Using a tiny lib such as Lego would allow you to write the following:
<my-element :tableRowProperties="[{p1:'v1', p2:'v2'}, {p1:'v1',p2:'v2'}, {p1:'v1',p2:'v2'}]"></my-element>
and within your my-element.html web-component:
<template>
<table>
<tr :for="row in state.tableRowProperties">
<td>${row.p1}</td>
<td>${row.p2}</td>
</tr>
</template>
<script>
this.init() {
this.state = { tableRowPropoerties: [] }
}
</script>

I know this has been answered, but here is an approach I took. I know it's not rocket science and there are probably reasons not to do it this way; however, for me, this worked great.
This is an indirect approach to pass in data where an attribute called wc_data is passed in the custom element which is a 'key' that can be used one time.
You can obviously do whatever with the wc-data like callbacks and "callins" into the custom-tag.
link to codesandbox
files:
wc_data.ts
export const wc_data: {
[name: string]: any,
get(key: string): any,
set(key: string, wc_data: any): any
} = {
get(key: string): any {
const wc_data = this[key];
delete this[key];
return wc_data;
},
set(p_key: string, wc_data: any) {
this[p_key] = wc_data;
}
}
CustomTag.ts
import { wc_data } from './wc_data';
const template = document.createElement('template');
template.innerHTML = `
<style>
.custom-tag {
font-size: 1.6em;
}
</style>
<button class="custom-tag">Hello <span name="name"></span>, I am your <span name="relation"></span></button>
`;
class CustomTag extends HTMLElement {
constructor() {
super();
this.attachShadow({ mode: 'open' });
this.shadowRoot.appendChild(template.content.cloneNode(true));
}
callin() {
console.log('callin called');
}
connectedCallback() {
const v_wc_data = wc_data.get(this.getAttribute('wc-data'));
console.log('wc_data', v_wc_data);
const v_name = this.shadowRoot.querySelector('[name="name"]');
const v_relation = this.shadowRoot.querySelector('[name="relation"]');
v_name.innerHTML = v_wc_data.name;
v_relation.innerHTML = v_wc_data.relation;
const v_button = this.shadowRoot.querySelector('button');
v_button.style.color = v_wc_data.color;
v_wc_data.element = this;
v_button.addEventListener('click', () => v_wc_data.callback?.());
}
disconnectedCallback() {
}
}
window.customElements.define('custom-tag', CustomTag);
console.log('created custom-tag element');
export default {};
SomeTsFile.ts
wc_data.set('tq', {
name: 'Luke',
relation: 'father',
color: 'blue',
element: undefined,
callback() {
console.log('the callback worked');
const v_tq_element = this.element;
console.log(this.element);
v_tq_element.callin();
},
});
some html..
<div>stuff before..</div>
<custom-tag wc_data="tq" />
<div>stuff after...</div>

Thanks to the other contributors, I came up with this solution which seems somewhat simpler. No json parsing. I use this example to wrap the entire component in a-href to make the block clickable:
customElements.define('ish-marker', class extends HTMLElement {
constructor() {
super()
const template = document.getElementById('ish-marker-tmpl').content
const wrapper = document.createElement("a")
wrapper.appendChild( template.cloneNode(true) )
wrapper.setAttribute('href', this.getAttribute('href'))
const shadowRoot = this.attachShadow({mode: 'open'}).appendChild( wrapper )
}
})
<ish-marker href="https://go-here.com">
...
// other things, images, buttons.
<span slot='label'>Click here to go-here</span>
</ish-marker>

custom elements & connectedCallback() : wait for parent node to be available before firing a function

I'm using custom elements, which are very nice.
But I'm facing a problem :
When the connectedCallback() function is called, it seems that the node is not yet at its place in the DOM, thus I cannot access its parents - and I need them.
class myElement extends HTMLElement{
constructor() {
super();
this.tracklist = undefined;
}
connectedCallback(){
this.render();
}
render(){
this.tracklist = this.closest('section');
// following code requires this.tracklist!
// ...
}
window.customElements.define('my-element', myElement);
How could I be sure the parent nodes are accessible before calling render() ?
Thanks !

It is a known issue:
connectedCallback does not mean your element is or is not fully parsed.
Custom Elements is lacking a parsedCallback method
See all the answers at:
textContent empty in connectedCallback() of a custom HTMLElement
connectedcallback-of-a-custom-htmlelement
How to have a 'connectedCallback' for when all child custom elements have been connected
TL;DR;
The accepted method is to delay your render method:
connectedCallback(){
setTimeout(this.render);
}

It seems that the connectedCallback cannot access other elements in relation to itself when it hasn't been parsed yet. This kind of makes sense if you consider that a custom-element has to be able to live anywhere in the DOM without being dependent on another element. So if there were no parent to be selected, the element would probably not work properly.
A way to do this is to modify the render method to take an argument which will set the tracklist property dynamically to the custom element. Then select the my-element element from the DOM and look for the section.
Then use the customElements.whenDefined method to connect the section and my-element together whenever the custom element is ready. This method returns a Promise that resolves whenever the custom element is defined and gives you the ability to execute a callback.
See example below:
// Do something whenever the element is ready.
window.addEventListener('load', function() {
// Wait for the document to load so the DOM has been parsed.
window.customElements.whenDefined('my-element').then(() => {
const myElement = document.querySelector('my-element');
// Only do something if the element exists on the page.
if (myElement !== null) {
const tracklist = myElement.closest('section');
myElement.render(tracklist);
console.log(myElement.tracklist);
}
});
});
// Create element.
class myElement extends HTMLElement{
constructor() {
super();
this.tracklist = null;
}
render(tracklist){
this.tracklist = tracklist;
// following code requires this.tracklist!
// ...
}
}
// Define element.
window.customElements.define('my-element', myElement);
<section>
<my-element></my-element>
</section>
If I have been unclear or you have questions, please let me know.
Have a nice day!

I haven't tested this out but seems like a Promise might work:
// DomReady.js
class DomReady extends HTMLElement {
constructor() {
super();
this.domReadyPromise = new Promise(resolve => (this.domReadyResolve = resolve));
}
connectedCallback() {
this.domReadyResolve();
}
domReady() { return this.domReadyPromise; }
}
// ParentCustom.js
class ParentCustom extends DomReady {
connectedCallback() {
super.connectedCallback();
...
}
}
// ChildCustom.js
class ChildCustom extends HTMLElement {
async connectedCallback() {
await document.querySelector('parent-custom').domReady();
}
}

Web Components, pass data to and from

My understanding is that data is passed to a custom html element via its attributes and sent out by dispatching a CustomEvent.
JavaScript objects can obviously be sent out in the event's detail field, but what if the element needs a lot of data passed into it. Is there a way to provide it with an object in JavaScript.
What if the element for instance contains a variable number of parts that needs to be initialized or changed dynamically (e.g. a table with a variable number of rows)? I can imagine setting and modifying an attribute consisting of a JSON string that is parsed inside the component, but it does not feel like an elegant way to proceed:
<my-element tableRowProperties="[{p1:'v1', p2:'v2'}, {p1:'v1',p2:'v2'}, {p1:'v1',p2:'v2'}]"></my-element>
Or can you make the element listen to events from the outside that contains a payload of data?

Passing Data In
If you really want/need to pass large amounts of data into your component then you can do it four different ways:
1) Use a property. This is the simplest since you just pass in the Object by giving the value to the element like this: el.data = myObj;
2) Use an attribute. Personally I hate this way of doing it this way, but some frameworks require data to be passed in through attributes. This is similar to how you show in your question. <my-el data="[{a:1},{a:2}....]"></my-el>. Be careful to follow the rules related to escaping attribute values. If you use this method you will need to use JSON.parse on your attribute and that may fail. It can also get very ugly in the HTML to have the massive amount of data showing in a attribute.
3 Pass it in through child elements. Think of the <select> element with the <option> child elements. You can use any element type as children and they don't even need to be real elements. In your connectedCallback function your code just grabs all of the children and convert the elements, their attributes or their content into the data your component needs.
4 Use Fetch. Provide a URL for your element to go get its own data. Think of <img src="imageUrl.png"/>. If your already has the data for your component then this might seem like a poor option. But the browser provides a cool feature of embedding data that is similar to option 2, above, but is handled automatically by the browser.
Here is an example of using embedded data in an image:
img {
height: 32px;
width: 32px;
}
<img src="data:image/svg+xml;charset=utf8,%3C?xml version='1.0' encoding='utf-8'?%3E%3Csvg version='1.1' xmlns='http://www.w3.org/2000/svg' x='0px' y='0px' viewBox='0 0 314.7 314.7'%3E%3Cstyle type='text/css'%3E .st0{fill:transparent;stroke:%23231F20;stroke-width:12;} .st1{fill:%23231F20;stroke:%23231F20;stroke-width:10;stroke-linejoin:round;stroke-miterlimit:10;} %3C/style%3E%3Cg%3E%3Ccircle class='st0' cx='157.3' cy='157.3' r='150.4'/%3E%3Cpolygon class='st1' points='108,76.1 248.7,157.3 108,238.6'/%3E%3C/g%3E%3C/svg%3E">
And here is an example of using embedded data in a web component:
function readSrc(el, url) {
var fetchHeaders = new Headers({
Accept: 'application/json'
});
var fetchOptions = {
cache: 'default',
headers: fetchHeaders,
method: 'GET',
mode: 'cors'
};
return fetch(url, fetchOptions).then(
(resp) => {
if (resp.ok) {
return resp.json();
}
else {
return {
error: true,
status: resp.status
}
}
}
).catch(
(err) => {
console.error(err);
}
);
}
class MyEl extends HTMLElement {
static get observedAttributes() {
return ['src'];
}
attributeChangedCallback(attrName, oldVal, newVal) {
if (oldVal !== newVal) {
this.innerHtml = '';
readSrc(this, newVal).then(
data => {
this.innerHTML = `<pre>
${JSON.stringify(data,0,2)}
</pre>`;
}
);
}
}
}
// Define our web component
customElements.define('my-el', MyEl);
<!--
This component would go load its own data from "data.json"
<my-el src="data.json"></my-el>
<hr/>
The next component uses embedded data but still calls fetch as if it were a URL.
-->
<my-el src="data:json,[{"a":9},{"a":8},{"a":7}]"></my-el>
You can do that same this using XHR, but if your browser supports Web Components then it probably supports fetch. And there are several good fetch polyfills if you really need one.
The best advantage to using option 4 is that you can get your data from a URL and you can directly embed your data. And this is exactly how most pre-defined HTML elements, like <img> work.
UPDATE
I did think of a 5th way to get JSON data into an object. That is by using a <template> tag within your component. This still required you to call JSON.parse but it can clean up your code because you don't need to escape the JSON as much.
class MyEl extends HTMLElement {
connectedCallback() {
var data;
try {
data = JSON.parse(this.children[0].content.textContent);
}
catch(ex) {
console.error(ex);
}
this.innerHTML = '';
var pre = document.createElement('pre');
pre.textContent = JSON.stringify(data,0,2);
this.appendChild(pre);
}
}
// Define our web component
customElements.define('my-el', MyEl);
<my-el>
<template>[{"a":1},{"b":"<b>Hi!</b>"},{"c":"</template>"}]</template>
</my-el>
Passing Data Out
There are three ways to get data out of the component:
1) Read the value from a property. This is ideal since a property can be anything and would normally be in the format of the data you want. A property can return a string, an object, a number, etc.
2) Read an attribute. This requires the component to keep the attribute up to date and may not be optimal since all attributes are strings. So your user would need to know if they need to call JSON.parse on your value or not.
3) Events. This is probably the most important thing to add to a component. Events should trigger when state changes in the component. Events should trigger based on user interactions and just to alert the user that something has happened or that something is available. Traditionally you would include the relevant data in your event. This reduces the amount of code the user of your component needs to write. Yes, they can still read properties or attributes, but if your events include all relevant data then they probably won't need to do anything extra.

There is a 6th way that is really similar to #Intervalia's answer above but uses a <script> tag instead of a <template> tag.
This is the same approach used by a Markdown Element.
class MyEl extends HTMLElement {
connectedCallback() {
var data;
try {
data = JSON.parse(this.children[0].innerHTML);
}
catch(ex) {
console.error(ex);
}
this.innerHTML = '';
var pre = document.createElement('pre');
pre.textContent = JSON.stringify(data,0,2);
this.appendChild(pre);
}
}
// Define our web component
customElements.define('my-el', MyEl);
<my-el>
<script type="application/json">[{"a":1},{"b":"<b>Hi!</b>"},{"c":"</template>"}]</script>
</my-el>

If you are using Polymer based web components, the passing of data could be done by data binding. Data could be stored as JSON string within attribute of and passed via context variable.
<p>JSON Data passed via HTML attribute into context variable of and populating the variable into combobox.</p>
<dom-bind><template>
<iron-ajax url='data:text/json;charset=utf-8,
[{"label": "Hydrogen", "value": "H"}
,{"label": "Oxygen" , "value": "O"}
,{"label": "Carbon" , "value": "C"}
]'
last-response="{{lifeElements}}" auto handle-as="json"></iron-ajax>
<vaadin-combo-box id="cbDemo"
label="Label, value:[[cbDemoValue]]"
placeholder="Placeholder"
items="[[lifeElements]]"
value="{{ cbDemoValue }}"
>
<template>
[[index]]: [[item.label]] <b>[[item.value]]</b>
</template>
</vaadin-combo-box>
<vaadin-combo-box label="Disabled" disabled value="H" items="[[lifeElements]]"></vaadin-combo-box>
<vaadin-combo-box label="Read-only" readonly value="O" items="[[lifeElements]]"></vaadin-combo-box>
<web-elemens-loader selection="
#polymer/iron-ajax,
#vaadin/vaadin-element-mixin/vaadin-element-mixin,
#vaadin/vaadin-combo-box,
"></web-elemens-loader>
</template></dom-bind>
<script src="https://cdn.xml4jquery.com/web-elements-loader/build/esm-unbundled/node_modules/#webcomponents/webcomponentsjs/webcomponents-loader.js"></script><script type="module" src="https://cdn.xml4jquery.com/web-elements-loader/build/esm-unbundled/src/web-elemens-loader.js"></script>

Using a tiny lib such as Lego would allow you to write the following:
<my-element :tableRowProperties="[{p1:'v1', p2:'v2'}, {p1:'v1',p2:'v2'}, {p1:'v1',p2:'v2'}]"></my-element>
and within your my-element.html web-component:
<template>
<table>
<tr :for="row in state.tableRowProperties">
<td>${row.p1}</td>
<td>${row.p2}</td>
</tr>
</template>
<script>
this.init() {
this.state = { tableRowPropoerties: [] }
}
</script>

I know this has been answered, but here is an approach I took. I know it's not rocket science and there are probably reasons not to do it this way; however, for me, this worked great.
This is an indirect approach to pass in data where an attribute called wc_data is passed in the custom element which is a 'key' that can be used one time.
You can obviously do whatever with the wc-data like callbacks and "callins" into the custom-tag.
link to codesandbox
files:
wc_data.ts
export const wc_data: {
[name: string]: any,
get(key: string): any,
set(key: string, wc_data: any): any
} = {
get(key: string): any {
const wc_data = this[key];
delete this[key];
return wc_data;
},
set(p_key: string, wc_data: any) {
this[p_key] = wc_data;
}
}
CustomTag.ts
import { wc_data } from './wc_data';
const template = document.createElement('template');
template.innerHTML = `
<style>
.custom-tag {
font-size: 1.6em;
}
</style>
<button class="custom-tag">Hello <span name="name"></span>, I am your <span name="relation"></span></button>
`;
class CustomTag extends HTMLElement {
constructor() {
super();
this.attachShadow({ mode: 'open' });
this.shadowRoot.appendChild(template.content.cloneNode(true));
}
callin() {
console.log('callin called');
}
connectedCallback() {
const v_wc_data = wc_data.get(this.getAttribute('wc-data'));
console.log('wc_data', v_wc_data);
const v_name = this.shadowRoot.querySelector('[name="name"]');
const v_relation = this.shadowRoot.querySelector('[name="relation"]');
v_name.innerHTML = v_wc_data.name;
v_relation.innerHTML = v_wc_data.relation;
const v_button = this.shadowRoot.querySelector('button');
v_button.style.color = v_wc_data.color;
v_wc_data.element = this;
v_button.addEventListener('click', () => v_wc_data.callback?.());
}
disconnectedCallback() {
}
}
window.customElements.define('custom-tag', CustomTag);
console.log('created custom-tag element');
export default {};
SomeTsFile.ts
wc_data.set('tq', {
name: 'Luke',
relation: 'father',
color: 'blue',
element: undefined,
callback() {
console.log('the callback worked');
const v_tq_element = this.element;
console.log(this.element);
v_tq_element.callin();
},
});
some html..
<div>stuff before..</div>
<custom-tag wc_data="tq" />
<div>stuff after...</div>

Thanks to the other contributors, I came up with this solution which seems somewhat simpler. No json parsing. I use this example to wrap the entire component in a-href to make the block clickable:
customElements.define('ish-marker', class extends HTMLElement {
constructor() {
super()
const template = document.getElementById('ish-marker-tmpl').content
const wrapper = document.createElement("a")
wrapper.appendChild( template.cloneNode(true) )
wrapper.setAttribute('href', this.getAttribute('href'))
const shadowRoot = this.attachShadow({mode: 'open'}).appendChild( wrapper )
}
})
<ish-marker href="https://go-here.com">
...
// other things, images, buttons.
<span slot='label'>Click here to go-here</span>
</ish-marker>

Accessing childNodes of custom elments?

This might be a bit confusing. I am trying to access innerHTML or childNodes from my custom element. Is it possible to get access to the original DOM structure from the web component import file? BEFORE attachShadow?
In the below example I am trying to get access to the src of the two jookah-images from my jookah-gallery import file.
Disclaimer: Im a total noob when it comes to shadow DOM and web components so if there are any major mistakes I'd love to understand why. Thanks for any help!
index.html
<jookah-gallery>
//here
<jookah-image class="gallery-image" src="http://merehead.com/blog/wp-content/uploads/gradient-design.jpeg">
<jookah-image class="gallery-image" src="https://webgradients.com/public/webgradients_png/035%20Itmeo%20Branding.png">
</jookah-gallery>
import file for jookah-gallery:
(function(owner) {
class jookahGallery extends HTMLElement {
constructor() {
super();
//this returns false
if (this.hasChildNodes()) {
console.log('there are nodes')
}else{
console.log('NO nodes')
}
//shadow DOM attach
const shadowRoot = this.attachShadow({mode: 'open'});
const template = owner.querySelector('#jookah-gallery-template');
const instance = template.content.cloneNode(true);
shadowRoot.appendChild(instance);
}
// ---------------- object events -------------------------//
connectedCallback() {
}
render(){
}
disconnectedCallback(){
}
attributeChangedCallback(){
}
// ---------------- methods ------------------------//
}
customElements.define('jookah-gallery', jookahGallery);
})(document.currentScript.ownerDocument);

According to the Spec you are not supposed to inspect, change, add and children in the constructor of your Web Component.
https://w3c.github.io/webcomponents/spec/custom/#custom-element-conformance
Instead you need to move the reading of the children into your connected callback:
class jookahGallery extends HTMLElement {
constructor() {
super();
this._childrenRead = false;
const shadowRoot = this.attachShadow({mode: 'open'});
const template = document.createElement('template');
template.innerHtml = `Place your template here`;
const instance = template.content.cloneNode(true);
shadowRoot.appendChild(instance);
}
connectedCallback() {
if (!this._childrenRead) {
this._childrenRead = true;
if (this.hasChildNodes()) {
console.log('there are nodes')
}else{
console.log('NO nodes')
}
}
}
}
customElements.define('jookah-gallery', jookahGallery);
You can also use <slot> to embed your children. But there are some CSS issues you need to be aware of when using slots.
Remember that shadowDOM is not supported in all browsers and is not a simple polyfill. So if you are only working on Chrome and Safari, go for it. If you are planning to support a broader range of browsers then you might not want to use ShadowDOM just yet.
https://alligator.io/web-components/composing-slots-named-slots/
Also read more here: How to use child elements in Web Components