| name | building-a-html-element-sandbox-with-lit |
| description | Learn how to build a Lit web component to create a dynamic HTML element sandbox with live updates, perfect for experimenting with and showcasing web components. |
| metadata | {"url":"https://rodydavis.com/posts/html-code-sandbox","last_modified":"Tue, 03 Feb 2026 20:04:27 GMT"} |
Building a HTML Element Sandbox with Lit
In this article I will go over how to set up a Lit web component and use it to create a HTML Element sandbox that can be used to update a live component.
TLDR The final source here and an online demo.
Prerequisites
- Vscode
- Node >= 16
- Typescript
Getting Started
We can start off by navigating in terminal to the location of the project and run the following:
npm init @vitejs/app --template lit-ts
Then enter a project name html-element-sandbox and now open the project in vscode and install the dependencies:
cd html-element-sandbox
npm i lit
npm i -D @types/node
code .
Update the vite.config.ts with the following:
import { defineConfig } from "vite";
import { resolve } from "path";
export default defineConfig({
base: "/html-element-sandbox/",
build: {
lib: {
entry: "src/html-element-sandbox.ts",
formats: ["es"],
},
rollupOptions: {
input: {
main: resolve(__dirname, "index.html"),
},
},
},
});
Template
Open up the index.html and update it with the following:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<link rel="icon" type="image/svg+xml" href="/src/favicon.svg" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>HTML Element Sandbox</title>
<script type="module" src="/src/html-element-sandbox.ts"></script>
<style>
body {
margin: 0;
padding: 0;
font-family: sans-serif;
}
html-element-sandbox {
display: block;
width: 100%;
height: 100vh;
}
</style>
</head>
<body>
<html-element-sandbox>
<template>
<button
class="button"
knob-text="label"
knob-css-color="fg-color"
knob-css-background-color="bg-color"
knob-css-border-radius="shape"
knob-css-font-size="text-font-size"
knob-css-padding="padding"
knob-css---shadow-color="shadow"
>
My Button
</button>
<style>
.button {
--shadow-color: #000;
--elevation: 3px;
display: block;
width: 100%;
height: 100%;
border: none;
background-color: transparent;
cursor: pointer;
box-shadow: 0 var(--elevation) calc(var(--elevation) * 2) 0 var(--shadow-color);
}
</style>
</template>
<div slot="knobs">
<knob-string id="label" name="Label" value="BUTTON"></knob-string>
<knob-group name="Style" expanded>
<knob-color
id="bg-color"
name="Background Color"
value="#ff0000"
></knob-color>
<knob-color
id="fg-color"
name="Foreground Color"
value="#ffffff"
></knob-color>
<knob-color
id="shadow"
name="Shadow Color"
value="#000000"
></knob-color>
<knob-number
id="text-font-size"
name="Font Size"
value="16"
suffix="px"
></knob-number>
<knob-number
id="shape"
name="Border Radius"
value="100"
suffix="px"
></knob-number>
<knob-number
id="padding"
name="Padding"
value="12"
suffix="px"
></knob-number>
</knob-group>
</div>
</html-element-sandbox>
</body>
</html>
Here we are defining the markup we want to use in our sandbox. We are using the html-element-sandbox component to create a sandbox for our HTML Element.
<html-element-sandbox></html-element-sandbox>
Each knob is defined by an id and a name. The id is used to identify the knob in the template and the name is used to display the knob in the UI.
<knob-number
id="shape"
name="Border Radius"
value="30"
suffix="px"
></knob-number>
For the element inside the template we use knob-* attributes to get the values of the knobs and set the attributes, CSS style or text content.
<!-- Attributes -->
<div knob-attr-disabled="disabled"></div>
<knob-boolean id="disabled" name="Disable" value="false"></knob-boolean>
<!-- CSS Properties -->
<div knob-css-color="fg-color" knob-css-background-color="bg-color"></div>
<knob-color id="bg-color" name="Background Color" value="#ff0000"></knob-color>
<knob-color id="fg-color" name="Foreground Color" value="#ffffff"></knob-color>
<!-- Text Content -->
<div knob-text="content"></div>
<knob-string id="content" name="Text Content" value="Hello World"></knob-string>
A single knob can point to multiple elements:
<html-element-sandbox>
<template>
<div id="buttons">
<button
knob-text="label"
knob-css-color="fg-color"
knob-css-background-color="bg-color"
knob-css-border-radius="shape"
knob-css-font-size="text-font-size"
knob-css-padding="padding"
knob-css---shadow-color="shadow"
knob-attr-raised="raised"
knob-attr-contenteditable="contenteditable"
></button>
<mwc-button
knob-attr-label="label"
knob-css---mdc-theme-on-primary="fg-color"
knob-css---mdc-theme-primary="bg-color"
knob-css---mdc-shape-small="shape"
knob-attr-raised="raised"
label="My Button"
></mwc-button>
</div>
<script type="module">
import "https://www.unpkg.com/@material/[email protected]/mwc-button.js?module";
</script>
<style>
button {
--shadow-color: #000;
--elevation: 3px;
display: block;
border: none;
background-color: transparent;
cursor: pointer;
box-shadow: 0 var(--elevation) calc(var(--elevation) * 2) 0 var(--shadow-color);
}
mwc-button {
--mdc-theme-on-primary: #000;
--mdc-theme-primary: #fff;
--mdc-shape-small: none;
}
#buttons {
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
gap: 1rem;
}
</style>
</template>
<div slot="knobs">
<knob-string id="label" name="Label" value="BUTTON"></knob-string>
<knob-group name="Style" expanded>
<knob-color
id="bg-color"
name="Background Color"
value="#ff0000"
></knob-color>
<knob-color
id="fg-color"
name="Foreground Color"
value="#ffffff"
></knob-color>
<knob-color id="shadow" name="Shadow Color" value="#000000"></knob-color>
<knob-number
id="text-font-size"
name="Font Size"
value="16"
suffix="px"
></knob-number>
<knob-number
id="shape"
name="Border Radius"
value="30"
suffix="px"
></knob-number>
<knob-number
id="padding"
name="Padding"
value="12"
suffix="px"
></knob-number>
</knob-group>
<knob-group name="Attributes" expanded>
<knob-boolean id="raised" name="Raised" value="false"></knob-boolean>
<knob-list id="contenteditable" name="Content Editable" value="false">
<option value="true">true</option>
<option value="false">false</option>
</knob-list>
</knob-group>
</div>
</html-element-sandbox>
A style and script can be added to load extra content into the sandbox (e.g. a script to load a web component).
Web Component
Before we update our component we need to rename my-element.ts to html-element-sandbox.ts
Open up html-element-sandbox.ts and update it with the following:
import { css, html, LitElement } from "lit";
import { customElement, state } from "lit/decorators.js";
import "./knobs/boolean";
import "./knobs/string";
import "./knobs/number";
import "./knobs/color";
import "./knobs/list";
import "./knobs/group";
import { KnobValue } from "./knobs/base";
import { BooleanKnob } from "./knobs/boolean";
export const tagName = "html-element-sandbox";
@customElement(tagName)
export class HTMLElementSandbox extends LitElement {
static styles = css`
main {
--knobs-width: 300px;
--code-height: calc(100% * 0.4);
--mobile-height: 350px;
display: grid;
grid-template-areas: "preview" "knobs" "code";
grid-template-columns: 100%;
grid-template-rows: var(--mobile-height) auto auto;
height: 100%;
width: 100%;
}
#preview {
grid-area: preview;
display: flex;
flex-direction: column;
align-items: center;
justify-content: center;
border-bottom: 1px solid #272727;
background-color: whitesmoke;
}
@media (min-width: 600px) {
main {
grid-template-areas:
"preview knobs"
"code knobs";
grid-template-columns: calc(100% - var(--knobs-width)) var(
--knobs-width
);
grid-template-rows: calc(100% - var(--code-height)) var(--code-height);
}
#preview {
border-bottom: none;
}
slot[name="knobs"] {
overflow-y: auto;
}
pre {
overflow-y: scroll;
}
}
section {
flex: 1;
}
slot[name="knobs"] {
grid-area: knobs;
display: flex;
flex-direction: column;
border-left: 1px solid #000;
}
slot[name="code"] {
grid-area: code;
}
pre {
margin: 0;
font-family: Monaco, Courier, monospace;
padding: 16px;
background-color: #272727;
color: #c8c8c8;
}
code {
font-size: 0.8rem;
white-space: pre-wrap;
}
`;
@state() code = "";
render() {
return html`<main>
<section id="preview">
<slot></slot>
</section>
<slot name="knobs"> </slot>
<slot name="code">
<pre><code>${this.code}</code></pre>
</slot>
</main>`;
}
firstUpdated() {
this.init();
}
init() {
this.setUpKnobs();
this.code = this.getCode();
// Update the code every time a knob value changes
this.addEventListener("value", () => {
this.code = this.getCode();
});
}
setUpKnobs() {
const root = this.shadowRoot!;
const preview = root.getElementById("preview")!;
const template = this.querySelector("template");
if (template) {
const div = document.createElement("div");
div.appendChild(template.content.cloneNode(true));
// Text Knobs (knob-text)
div.querySelectorAll("[knob-text]").forEach((el) => {
const elemId = el.getAttribute("knob-text") || "";
const knob = this.querySelector(`#${elemId}`);
if (knob && knob instanceof KnobValue) {
knob.addEventListener("value", () => {
const val = knob.value;
el.textContent = val;
});
el.addEventListener("input", (e) => {
const target = e.target as HTMLElement;
knob.value = target.textContent;
});
knob.init();
}
});
div.querySelectorAll("*").forEach((el) => {
const attrs = el.attributes;
for (let i = 0; i < attrs.length; i++) {
const attr = attrs[i];
const attrName = attr.name;
// CSS Knobs (knob-css-*)
if (attrName.startsWith("knob-css-")) {
const cssKey = attrName.replace("knob-css-", "");
const knob = this.querySelector(`#${attr.value}`);
if (
knob &&
knob instanceof KnobValue &&
el instanceof HTMLElement
) {
knob.addEventListener("value", () => {
const val = knob.value;
if (knob.hasAttribute("suffix")) {
// Add suffix to the value (e.g. px)
el.style.setProperty(
cssKey,
val + knob.getAttribute("suffix")
);
} else {
// No suffix, just set the value
el.style.setProperty(cssKey, val);
}
});
knob.init();
}
}
// Attribute Knobs (knob-attr-*)
if (attrName.startsWith("knob-attr-")) {
const attrKey = attrName.replace("knob-attr-", "");
const knob = this.querySelector(`#${attr.value}`);
if (knob && knob instanceof KnobValue) {
knob.addEventListener("value", () => {
const val = knob.value;
if (knob instanceof BooleanKnob) {
if (val) {
// <div hidden>
el.setAttribute(attrKey, "");
} else {
// <div>
el.removeAttribute(attrKey);
}
} else {
// <div value="foo">
el.setAttribute(attrKey, val);
}
});
knob.init();
}
}
}
});
preview.appendChild(div);
}
}
getCode() {
const root = this.shadowRoot!;
const preview = root.getElementById("preview")!;
if (preview.children.length > 0) {
const child = preview.children[1];
if (child && child.children.length > 0) {
const lines = this.elementToString(child.children[0]);
// Trim empty lines
const linesArray = lines.split("\n");
const filteredLines = linesArray.filter((line) => line.trim() !== "");
return filteredLines.join("\n");
}
}
return "";
}
elementToString(node: Element) {
const sb: string[] = [];
const tag = node.tagName.toLowerCase();
sb.push(`<${tag}`);
const attrs = node.attributes;
// Add attributes
for (let i = 0; i < attrs.length; i++) {
const attr = attrs[i];
if (attr.name.startsWith("knob-")) continue;
// If the attribute is a boolean attribute, add it only if it's true
if (attr.value === "") {
sb.push(` ${attr.name}`);
} else {
sb.push(` ${attr.name}="${attr.value}"`);
}
}
sb.push(">");
if (node.childNodes.length > 0) {
for (let i = 0; i < node.childNodes.length; i++) {
const child = node.childNodes[i];
// If the child is a text node, add the content
if (child instanceof Text) {
sb.push(child.textContent || "");
} else if (child instanceof Element) {
// If the child is an element, recurse
sb.push(this.elementToString(child));
}
}
}
sb.push(`</${tag}>`);
return sb.join("\n");
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: HTMLElementSandbox;
}
}
Knobs
First let up create a base class that will be used to create all other knobs. Create src/knobs/base.ts and update with with the following:
import { css, html, LitElement, TemplateResult } from "lit";
import { property } from "lit/decorators.js";
export class Knob extends LitElement {
constructor(name: string) {
super();
this.name = name;
}
@property() name: string;
}
export abstract class KnobValue<T> extends Knob {
constructor(name: string, public val: T) {
super(name);
this._value = val;
this.notify();
}
static styles = css`
.knob {
display: flex;
flex-direction: row;
align-items: center;
padding: 0.5rem;
}
.knob label {
flex: 1;
}
`;
_value: T;
get value(): T {
return this._value;
}
set value(value: T) {
this._value = value;
this.notify();
}
notify() {
const value = this.value;
this.onValue(value);
this.dispatchEvent(
new CustomEvent("value", {
detail: value,
bubbles: true,
composed: true,
})
);
this.requestUpdate();
}
render() {
return html`
<div class="knob">
<label>${this.name}</label>
${this.buildInput()}
</div>
`;
}
onValue(_val: T) {}
init() {
this.notify();
}
resolveValue(val: T) {
return val;
}
abstract buildInput(): TemplateResult;
}
Boolean knob
Create src/knobs/boolean.ts and update with the following:
import { KnobValue } from "./base";
import { html } from "lit";
import { customElement, property } from "lit/decorators.js";
export const tagName = "knob-boolean";
@customElement(tagName)
export class BooleanKnob extends KnobValue<boolean> {
constructor(name: string, val: boolean) {
super(name, val);
}
static styles = KnobValue.styles;
@property({
type: Boolean,
attribute: "value",
})
_value = false;
buildInput() {
return html`<input
type="checkbox"
.checked=${this.resolveValue(this.value)}
@change=${this.onChange}
/>`;
}
onChange(e: Event) {
const target = e.target as HTMLInputElement;
this.value = target.checked;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: BooleanKnob;
}
}
Number Knob
Create src/knobs/number.ts and update with the following:
import { KnobValue } from "./base";
import { html } from "lit";
import { customElement, property } from "lit/decorators.js";
export const tagName = "knob-number";
@customElement(tagName)
export class NumberKnob extends KnobValue<number> {
constructor(name: string, val: number) {
super(name, val);
}
static styles = KnobValue.styles;
@property({
type: Number,
attribute: "value",
converter: {
fromAttribute: (val: string) => parseFloat(val),
toAttribute: (val: boolean) => val.toString(),
},
})
_value = 0;
buildInput() {
return html`<input
type="number"
.valueAsNumber=${this.resolveValue(this.value)}
@change=${this.onChange}
/>`;
}
onChange(e: Event) {
const target = e.target as HTMLInputElement;
this.value = target.valueAsNumber;
}
resolveValue(val: number): number {
return val;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: NumberKnob;
}
}
String Knob
Create src/knobs/string.ts and update with the following:
import { KnobValue } from "./base";
import { html } from "lit";
import { customElement, property } from "lit/decorators.js";
export const tagName = "knob-string";
@customElement(tagName)
export class StringKnob extends KnobValue<string> {
constructor(name: string, val: string) {
super(name, val);
}
static styles = KnobValue.styles;
@property({ type: String, attribute: "value" })
_value = "";
buildInput() {
return html`<input
type="text"
.value=${this.resolveValue(this.value)}
@input=${this.onChange}
/>`;
}
onChange(e: Event) {
const target = e.target as HTMLInputElement;
this.value = target.value;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: StringKnob;
}
}
Color Knob
Create src/knobs/color.ts and update with the following:
import { html } from "lit";
import { customElement } from "lit/decorators.js";
import { StringKnob } from "./string";
export const tagName = "knob-color";
@customElement(tagName)
export class ColorKnob extends StringKnob {
buildInput() {
return html`<input
type="color"
.value=${this.resolveValue(this.value)}
@input=${this.onChange}
/>`;
}
resolveValue(value: string) {
if (value && value.startsWith("--")) {
const style = getComputedStyle(document.body);
const resolved = style.getPropertyValue(value);
return resolved;
}
return value;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: ColorKnob;
}
}
List Knob
Create src/knobs/list.ts and update with the following:
import { KnobValue } from "./base";
import { html } from "lit";
import { customElement, property } from "lit/decorators.js";
export const tagName = "knob-list";
@customElement(tagName)
export class ListKnob extends KnobValue<string> {
constructor(name: string, val: string) {
super(name, val);
}
static styles = KnobValue.styles;
@property({
type: String,
attribute: "value",
})
_value = "";
buildInput() {
const options = this.getOptions();
return html`<select @change=${this.onChange}>
${Array.from(options).map(
(option) =>
html`<option
value=${option.value}
.selected=${this.value === option.value}
>
${option.textContent}
</option>`
)}
</select>`;
}
getOptions() {
const options = this.querySelectorAll(
"option"
) as NodeListOf<HTMLOptionElement>;
return Array.from(options);
}
onChange(e: Event) {
const target = e.target as HTMLSelectElement;
this.value = target.value;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: ListKnob;
}
}
Group Knob
Create src/knobs/group.ts and update with the following:
import { css, html } from "lit";
import { customElement, property } from "lit/decorators.js";
import { Knob } from "./base";
export const tagName = "knob-group";
@customElement(tagName)
export class GroupKnob extends Knob {
constructor(name: string, knobs: Knob[] = []) {
super(name);
this.knobs = knobs;
}
static styles = css`
details {
display: flex;
flex-direction: column;
align-items: flex-start;
}
details summary {
padding: 0.5rem;
}
`;
knobs: Knob[];
@property({ type: Boolean }) expanded = false;
render() {
return html`<details ?open=${this.expanded}>
<summary>${this.name}</summary>
<div class="collection">
<slot></slot>
${this.knobs.map((knob) => html`${knob}`)}
</div>
</details>`;
}
}
declare global {
interface HTMLElementTagNameMap {
[tagName]: GroupKnob;
}
}
Conclusion
If everything worked as expected, you should see the following:
If you want to learn more about building with Lit you can read the docs here.
The source for this example can be found here.
