Serverless coding environments for the web

Usage no npm install needed!

<script type="module">
  import playgroundElements from '';



Serverless coding environments for the web.

OverviewGetting StartedProject filesModule resolutionTypeScriptHiding & FoldingCustom layoutsBundlingSandbox securityComponentsStylingSyntax highlightingContributingFAQ


Playground Elements are a set of components for creating interactive editable coding environments on the web, with live updating previews. You can use Playground Elements to:

  • Embed editable code examples in your documentation.
  • Build interactive tutorials and example galleries.
  • Build full-featured coding sandboxes (think Glitch or JSBin).

🤯 No backend required

Unlike other coding environments, Playground never sends code to a backend server. Instead, Playground uses a Service Worker to create a virtual URL-space that runs 100% within the browser. If you can host static files, you can host a Playground.

TypeScript support

Playground automatically compiles .ts files using TypeScript, and automatically fetches typings for your imports in order to display errors. Compilation happens in a Web Worker on a separate thread, so your page stays responsive.

🧩 Web Components

Playground uses Web Components, so it doesn't require a framework. But it will play nicely with any framework you're already using, like React, Vue, and Angular.

🍱 Mix-and-match for flexible layout

Playground is broken up into small components like an editor, file picker, and preview. Mix-and-match components to create any layout you want, or just use <playground-ide> for an easy out-of-the-box experience.

🎨 Themable

Playground is fully themeable with CSS Custom Properties, down to the color of each kind of syntax-highlighted token. You can import themes from VSCode using the configurator, and it comes with a number of presets too.

Try the demo and configurator!

Getting Started


Install from NPM:

npm i playground-elements

Hello World

Create a <playground-ide> element in your HTML, and specify your project files inline:

<playground-ide editable-file-system line-numbers resizable>
  <script type="sample/html" filename="index.html">
    <!doctype html>
      <script type="module" src="./index.js">&lt;/script>

  <script type="sample/ts" filename="index.ts">


Use a server like @web/dev-server to handle bare module import resolution automatically:

npm i -D @web/dev-server
npx web-dev-server --node-resolve --watch

Or, use a tool like Rollup to resolve bare module imports to paths at build time. If you need more help with building and serving, check out the Modern Web Guides.


Playground is supported by all modern browsers. It requires support for custom elements, JS modules, service workers, and web workers.

    Supported:   Supported Chrome     Supported Firefox     Supported Safari     Supported Edge

Unsupported:   Unsupported Internet Explorer

Project files

There are 3 ways to specify the files of a playground project:

Option 1: Inline scripts

Add one or more <script> tags as children of your <playground-ide> or <playground-project>, using the following attributes:

Attribute          Description
type Required filetype. Options: sample/html, sample/css, sample/js, sample/ts, sample/json, sample/importmap.
filename Required filename.
label Optional label for display in playground-tab-bar. If omitted, the filename is displayed.
hidden If present, the file won't be visible in playground-tab-bar.
preserve-whitespace Disable the default behavior where leading whitespace that is common to all lines is removed.

Be sure to escape closing </script> tags within your source as <&lt;script>.

  <script type="sample/html" filename="index.html">
    <!doctype html>
      <script type="module" src="javascript.js">&lt;/script>
      <script type="module" src="typescript.js">&lt;/script>
      <link rel="stylesheet" href="styles.css">
      <p>Hello World!</p>

  <script type="sample/js" filename="javascript.js">
    console.log('hello from javascript');

  <script type="sample/ts" filename="typescript.ts">
    console.log('hello from typescript');

  <script type="sample/css" filename="styles.css">
    body { color: blue; }

Option 2: JSON configuration

Set the project-src attribute or projectSrc property to a JSON file with format:

Property          Description
files An object mapping filenames to file data.
files.content Optional text content of the file. If omitted, a fetch is made to retrieve the file by filename, relative to the manifest URL.
files.contentType Optional MIME type of the file. If omitted, type is taken from either the fetch response Content-Type header, or inferred from the filename extension when content is set.
files.label Optional label for display in playground-tab-bar. If omitted, the filename is displayed.
files.hidden If true, the file won't be visible in playground-tab-bar.
extends Optional URL to another JSON config file to extend from. Configs are deeply merged. URLs are interpreted relative to the URL of each extendee config.
<playground-ide project-src="/path/to/my/project.json"> </playground-ide>
  "files": {
    "index.html": {},
    "typescript.ts": {
      "content": "console.log('hello');"
    "javascript.js": {
      "contentType": "text/javascript"
    "styles.css": {
      "label": "Style"

Option 3: Config property

In JavaScript, directly set the config property to an object. The format is identical to the JSON config file.

const ide = document.querySelector('playground-ide');
ide.config = {
  files: {
    'index.html': {},
    'typescript.ts': {
      content: "console.log('hello');",

If both project-src and config are set, then the one set most recently has precedence. When either are set, inline scripts are ignored.

Module resolution

By default, bare module specifiers in JavaScript and TypeScript files are transformed to special ./node_modules/ URLs, and fetched behind-the-scenes from at the latest version.

// What you write:
import {html} from 'lit';

// What playground serves:
import {html} from './node_modules/lit@2.0.2/index.js';

// What playground fetches behind-the-scenes:


To customize the version of a module you import, create a file called package.json in your project containing a dependencies map. This works exactly like it does when using NPM locally.

  "dependencies": {
    "lit": "^2.0.2"

TIP: Use the hidden attribute or property to hide the package.json file from being displayed in the list of project files, if you don't want the end-user to be able to see or modify it.

Export conditions

Playground supports Node-style export conditions when resolving modules in dependencies, and sets the following conditions: module, import, development.

Import maps

For full control over module resolution, you can configure an import map. You may want to do this to change CDNs or point to a locally served copy of a module:

  "files": { ... },
  "importMap": {
    "imports": {
      "lit": "^2.0.2",
      "lit/": "^2.0.2/"

When using inline project files, you can specify your import map like so:

  <script type="sample/importmap">
      "imports": {
        "lit": "^2.0.2",
        "lit/": "^2.0.2/"

If an import map is defined, but does not contain an entry for a bare module, then playground defaults to the URL.


Playground automatically compiles .ts files using TypeScript.

The following compiler settings are used:

Name         Value
target ES2017
module ESNext
moduleResolution node
experimentalDecorators true

Note that when you import another project module from a .ts file, your import statement should use the .js extension (the same as you would do when running tsc locally):

import './my-other-module.js';

Hiding & folding

If a region of code in a Playground project file is surrounded by playground-hide and playground-hide-end comments, then that region won't be visible or editable by the user, but it will still be compiled and served.

Similarly, if a region is surrounded by playground-fold and playground-fold-end comments, then the region will be replaced with a that expands to reveal the original editable code when clicked.

Use these special regions to help users focus on a particular part of a file, by de-emphasizing boilerplate or unrelated code.

JavaScript fold example

Note that JavaScript // style comments are not supported.

/* playground-fold */
import {html, LitElement} from 'lit';
/* playground-fold-end */

class MyElement extends LitElement {
  render() {
    return html`Hello <slot></slot>!`;
/* playground-fold */

customElements.define('my-element', MyElement);


HTML hide example

<!-- playground-hide -->
  <title>Boring stuff</title>
  <script type="module" src="./my-element.js"></script>
<!-- playground-hide-end -->
<!-- playground-hide -->
<!-- playground-hide-end -->



Hiding and folding is enabled by default, but can be disabled by setting the pragmas property to "off" (disabled with comments hidden) or "off-visible" (disabled with comments visible). The pragmas property is available on ide, file-editor, and code-editor.

Custom layouts

<playground-ide> provides a complete out-of-the-box experience that's a good start for many use-cases, but you can mix-and-match the various Playground sub-components to make your custom layouts.

For example, say we need a layout with an editor above, a preview below, and only one particular file from the project visible — like this:

To do this, first import just the components you need. The main playground-elements import loads all Playground elements, but when making a custom layout it's a good idea to only load the sub-components you're actually using. This will make your JavaScript bundle smaller.

<script type="module">
  import 'playground-elements/playground-project.js';
  import 'playground-elements/playground-file-editor.js';
  import 'playground-elements/playground-preview.js';

Next create a <playground-project>, with some inline project files. We could also write our project files separately, and specify them in a JSON manifest. This project element manages the virtual file system, and coordinates with the Playground workers. We give it a unique id, which we'll use to connect up the editor and preview.

<playground-project id="project1">
  <script type="sample/ts" filename="index.ts">
    window.addEventListener('DOMContentLoaded', () => {
      const world = document.createTextNode(' World!');

  <script type="sample/html" filename="index.html">
    <!doctype html>
      <script type="module" src="./index.js">&lt;/script>

Next create an editor and preview. Connect both to the project by setting the property attribute to the project element's id. We could also directly set the project property to the project element, if we were using JavaScript.

By setting the filename attribute on the editor, we've pinned it to one particular file. Since we didn't include a <playground-tab-bar>, there's no way for the user to see or switch to other files in the project.

<div class="example">

  <playground-preview project="project1"> </playground-preview>

Finally, add a little style:

  .example {
    width: 500px;
    border: 1px solid #ccc;
  .example > playground-file-editor {
    height: 110px;
  .example > playground-preview {
    height: 100px;
    border-top: 1px solid #ccc;
    --playground-preview-toolbar-background: #eaeaea;


Playground uses a Web Worker to perform TypeScript compilation. If you are bundling or otherwise modifying the layout of the playground-elements NPM package, you may need to add special handling for this file.


Use the Rollup @web/rollup-plugin-import-meta-assets plugin to automatically copy the worker script into the correct location. See examples/rollup for an example configuration.


Webpack 5+ automatically supports loading Web Workers with no additional plugins. See examples/webpack for an example configuration.


If you are bundling in another way, you'll need to configure your build so that the file node_modules/playground-elements/playground-typescript-worker.js is copied into the same directory as your bundle.

For example, if you bundled playground elements into ./js/app.js, then you should copy the worker module to ./js/playground-typescript-worker.js.

Sandbox security

⚠️ Changing the sandbox base URL from the default can create a security vulnerability for your site if not done carefully. Do not change the default unless you have a specific reason to, and please read this entire section carefully.

The sandboxBaseUrl property and sandbox-base-url attribute can be used to override the origin where untrusted code will execute when displaying Playground previews. The default origin is, which is secure because it is unprivileged and cannot modify the host window.

You may wish to override this default sandbox base URL if you do not want a dependency on, e.g. for isolation from outages, or because your network does not have access to it. Note that Playground currently also uses to retrieve imported bare modules that are not otherwise handled by an import map, so the dependency cannot currently be completely eliminated.


Playground previews work by using a service worker. This service worker takes control over requests to a particular URL space, allowing it to respond to HTTP requests using the files from your local project, instead of from a remote server. The playground preview component contains an <iframe> pointing to the index.html within that URL space.

When JavaScript in this preview <iframe> executes, it does so with the full privileges of whichever origin the service worker was registered on. This means it can access cookies on that origin, make HTTP requests to sensitive URLs on that origin, and directly access the <iframe> parent window if it has the same origin.

The JavaScript in Playground project files should always be considered untrusted and potentially malicious. This is particularly the case if you implement a share feature, because a user can be tricked into executing malicious code simply by visiting a URL.

By default, the sandbox base URL is<version>/playground-projects/. This is a secure default because is unprivileged and cannot modify the host window.


If you change the sandbox base URL from the default, ensure that the new URL meets all of the following requirements:

  1. Must be a different origin to the origin hosting the Playground components. This prevents untrusted code from modifying the parent window using window.parent, e.g. to change your sign-in link to a malicious URL.

    NOTE: It is highly recommended to furthermore use either an entirely different site, or to use the Origin-Agent-Cluster header, to improve performance and prevent lockups. See Process isolation for more information.

  2. Must not have access to any sensitive cookies. This prevents untrusted code from e.g. reading and forwarding your user's authentication token.

  3. Must not have access to any sensitive resources or APIs, either through the same-origin policy, or through CORS headers that grant the origin access to resources on other origins. This prevents untrusted code from e.g. making a request to your get_credit_card or change_password APIs.

  4. Must serve the following two pre-minified files from the playground-elements NPM package at the same version as your imported components:

    • playground-service-worker.js
    • playground-service-worker-proxy.html

Process isolation

Some browsers such as Chrome are sometimes able to allocate a separate process or thread for iframes. This is highly desirable for Playground, because it improves responsiveness and prevents full lockups (resulting from e.g. an infinite loop accidentally written by a user).

By default, this iframe process isolation can only occur if the iframe and the parent window are different sites. While an origin is defined by (protocol + subdomain + top-level domain + port), a site is defined only by (protocol + top-level domain). For example, and are different-origin but same-site, whereas and are different-origin and different-site.

Alternatively, if the Origin-Agent-Cluster: ?1 header is set on all server responses from one or the other origins, then iframe process isolation can also occur with different-origin but same-site configurations. Note that this header must truly be set on all responses from the origin, because the browser will remember the setting based on the first response it gets from that origin. See "Requesting performance isolation with the Origin-Agent-Cluster header" for more information about this header.




All-in-one project, editor, file switcher, and preview with a horizontal side-by-side layout.


Name          Type Default   Description
projectSrc string undefined URL of the project manifest to load
config ProjectManifest undefined Get or set the project configuration and files, (details).
lineNumbers boolean false   Render a gutter with line numbers in the editor
editableFileSystem boolean false   Allow adding, removing, and renaming files
resizable boolean false   Allow dragging the line between editor and preview to change relative sizes
sandboxBaseUrl string module parent directory Base URL for untrusted JavaScript execution (⚠️ use with caution, see sandbox security). Resolved relative to the module containing the definition of <playground-project>.
pragmas "on" \| "off" \| "off-visible" "on" How to handle playground-hide and playground-fold comments (details).
modified boolean false Whether the user has modified, added, or removed any project files. Resets whenever a new project is loaded.


Name          Description
default Inline files (details)


Invisible element that coordinates the filesystem, build worker, and service worker. Unless you're using <playground-ide>, all Playground layouts need a project element.


Name          Type Default   Description
projectSrc string undefined URL of a project files manifest to load.
config ProjectManifest undefined Get or set the project configuration and files, (details).
sandboxScope string "playground-elements" The service worker scope to register on.
sandboxBaseUrl string module parent directory Base URL for untrusted JavaScript execution (⚠️ use with caution, see sandbox security). Resolved relative to the module containing the definition of <playground-project>.
diagnostics Map<string, lsp.Diagnostic> undefined Map from filename to array of Language Server Protocol diagnostics resulting from the latest compilation.
modified boolean false Whether the user has modified, added, or removed any project files. Resets whenever a new project is loaded.


Method         Description
addFile(filename: string) Create a new file. Type is inferred from filename extension.
deleteFile(filename: string) Delete a file.
renameFile(oldName: string, newName: string) Rename a file.


Name          Description
default Inline files (details)


Event          Description
filesChanged A file was added, removed, or renamed.
urlChanged The preview URL has changed
compileStart A build has started.
compileEnd A build has completed.



Property        Type Default   Description
project string \| PlaygroundProject undefined The project this bar is associated with. Either the <playground-project> itself, or its id in the host scope.
editor string \| PlaygroundFileEditor undefined The editor this bar controls. Either the <playground-file-editor> itself, or its id in the host scope.
editableFileSystem boolean false   Allow adding, removing, and renaming files



Name        Type Default   Description
project string \| PlaygroundProject undefined The project that this editor is associated with. Either the <playground-project> node itself, or its id in the host scope.
filename string undefined The name of the project file that is currently being displayed. Set when changing tabs. Does not reflect to attribute.
type "js" \| "ts" \| "html" \| "css" undefined File type.
lineNumbers boolean false   Render a gutter with line numbers in the editor
pragmas "on" \| "off" \| "off-visible" "on" How to handle playground-hide and playground-fold comments (details).
readonly boolean false   Do not allow edits


A pure text editor based on CodeMirror with syntax highlighting for HTML, CSS, JavaScript, and TypeScript.


Name        Type Default   Description
value string ""   Code as string
type "js" \| "ts" \| "html" \| "css" undefined   Language of the file to syntax highlight
readonly boolean false   Do not allow edits
lineNumbers boolean false   Render a gutter with line numbers in the editor
pragmas "on" \| "off" \| "off-visible" "on" How to handle playground-hide and playground-fold comments (details).


Event          Description
changed User made an edit to the active file



Name        Type Default   Description
project string PlaygroundProject undefined The project that this editor is associated with. Either the <playground-project> node itself, or its id in the host scope.
location string ""


Floating controls for adding, deleting, and renaming files.


Name        Type Default   Description
state "closed" \| "menu" \| "rename" \| "newfile" "closed"   The kind of control to display.

closed: Hidden.
menu: Menu with "Rename" and "Delete" items.
rename: Control for renaming an existing file.
newfile: Control for creating a new file.
filename string undefined When state is menu or newfile, the name of the relevant file.
anchorElement HTMLElement undefined   Absolutely position these controls at the bottom-left corner of this element.


Event         Detail  Description
newFile {filename: string} The specified new file was created through these controls.


TIP: Use the configurator to quickly experiment with themes and other customizations.

Custom Properties

Name                                 Default    Description
--playground-bar-height   40px               height of the tab bar and the preview bar
--playground-code-font-family       monospace            font-family of code in the editor
--playground-code-font-size         14px          font-size of code in the editor
--playground-code-line-height         1.4em          line-height of code in the editor
--playground-code-TOKEN-color   various              Color of each kind of TOKEN in syntax highlighted-code. See the syntax highlighting section for details.
--playground-highlight-color   #6200EE               Color of the active file-picker tab label and indicator, and the preview loading bar
--playground-code-background   #FFFFFF               background of the code editor
--playground-code-gutter-background   var(--playground-code-background, #FFFFFF)               background of the line-numbers gutter
--playground-code-gutter-box-shadow   none               box-shadow of the line-numbers gutter
--playground-code-gutter-border-right   none               border-right of the line-numbers gutter
--playground-code-linenumber-color   #767676               color of line-numbers
--playground-code-cursor-color   var(--playground-code-default-color, #000000)               color of the cursor
--playground-code-selection-background   #D7D4F0               background of selected text
--playground-code-padding   0               padding around the editor code block
--playground-code-line-padding   0 4px              padding around each line of code
--playground-tab-bar-background   #EAEAEA               background of the file-picker tab bar
--playground-tab-bar-active-background   transparent               background of the active file-picker tab
--playground-tab-bar-foreground-color   #000000               Text color of inactive file-picker tabs
--playground-tab-bar-active-color   var(--playground-highlight-color, #6200EE)               Text color of active file-picker tab
--playground-tab-bar-indicator-color   var(--playground-highlight-color, #6200EE)               color of active file-picker tab indicator (use transparent to hide)
--playground-tab-bar-font-size   14px               font-size of tab titles in the file-picker tab bar
--playground-preview-toolbar-background   #FFFFFF               background of the preview toolbar
--playground-preview-toolbar-foreground-color   #444444               Text color of the preview toolbar
--playground-border   1px solid #DDDDDD               Outer and inner border
--playground-floating-controls-highlight-color   var(--playground-highlight-color, #6200EE)               Highlight color of popup controls buttons and inputs

Shadow Parts

The following CSS shadow parts are exported, which you can style with additional rules not covered by the above CSS custom properties.

Part name                     Exported by Description
tab-bar                 ide Tab bar file switcher
editor                 ide Editor
preview                 ide Preview
preview-toolbar                 ide, preview Preview top bar
preview-location                 ide, preview Preview top bar "Result" heading
preview-reload-button                 ide, preview Preview top bar reload button
preview-loading-indicator                 ide, preview Preview top bar horizontal loading indicator
diagnostic-tooltip ide, file-editor, code-editor The tooltip that appears when hovering over a code span that has an error
dialog ide, file-editor, code-editor Dialogs appearing on top of a component (e.g. the editor keyboard help modal that shows on keyboard focus)

Syntax highlighting


The playground-elements package includes a directory of pre-configured syntax-highlighting themes. To load a theme, import its stylesheet, and apply the corresponding class name to the playground element or one of its ancestors:

  <playground-ide class="playground-theme-ayu-mirage"></playground-ide

A .js file is also provided for each theme, which exports a Lit CSSResult. You can include this directly in the static styles of your own Lit components, or get a CSSStyleSheet or string representation for other use cases:

import ayuMirageTheme from 'playground-elements/themes/ayu-mirage.css.js';

ayuMirageTheme; // Lit CSSResult
ayuMirageTheme.styleSheet; // CSSStyleSheet
ayuMirageTheme.cssText; // string

Custom syntax highlighting

Each kind of language token is controlled by a CSS custom property with the name --playground-code-TOKEN-color. For example, the keyword token is controlled by --playground-code-keyword-color.

Token Default JS/TS HTML CSS
default #000000 {}[]; <p>foo</p> {}:;
atom #221199 true &amp; bold
attribute #0000CC <foo bar> @media screen { }
builtin #3300AA #id { }
callee #000000 func() calc()
comment #AA5500 // foo <!-- foo --> /* foo */
def #0000FF let foo = bar
/**@param {string} foo*/
keyword #770088 class blue
meta #555555 <!doctype html>
number #116644 4 4px
operator #000000 =
property #000000 class foo { bar; } color:
qualifier #555555 .class { }
string #AA1111 "foo" <a b="c"> content: "foo"
string-2 #FF5500 `foo`
zoom: 50% 1
tag #117700 /**@param {string} foo*/ <foo> div { }
type #008855 let foo: string
/**@param {string} foo*/
variable #000000 let foo = bar @keyframes spin { }
variable-2 #0055AA (arg) => { arg } 2
variable-3 #008855 ::hover
local #0000FF (arg) => { }


  1. In CSS, string-2 is used for "non-standard" properties, but the list is outdated.
  2. In JS/TS, variable-2 is used for function-local variables.


Playground uses the google_modes CodeMirror syntax highlighting modes for TS/JS/HTML, because they support highlighting of HTML and CSS within JavaScript tagged template literals.


Contributions are very welcome.

For substantial changes, please file an issue first to discuss the changes. For small changes, sending a PR immediately is fine.

Initialize the repo:

git clone
cd playground-elements
npm i
npm run build

Build continuously:

npm run watch

Launch the configurator/demo locally:

npm run serve


How do I save and share a project?

Use the config property of a <playground-ide> or <playground-project> to get or set the current state of the project (details).

How you persist and retrieve serialized project state is up to you. Here are a few ideas:

  • JSON + base64url encode the config, and save it to the URL hash.

    Note that built-in btoa function is not safe for this purpose because it cannot encode non-latin code points, and the + character has a special meaning in URLs. See here for an example safe implementation, and #102 to track adding this implementation to Playground itself.

  • Integrate with a third-party API like GitHub gists.

  • Write to your own datastore.

How do I run custom build steps like JSX or SASS?

Support for build plugins like JSX, SASS, and CSS modules are on the roadmap, but are not yet available. Follow and comment on #66.

Why isn't module resolution working?

There are currently some missing features in module resolution that you might be hitting. Please comment on the issue if it affects you:

  • Imports in HTML files are not transformed (#93)
  • The import map scopes field is not supported (#103)