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Custom JSX in TypeScript

by Lea Rosema

I'm in a love-hate relationship with React. In my day-to-day work, I work a lot with React. In my personal projects, I prefer working with more lightweight stacks, keeping the client-side JavaScript load in the browser as lean and tiny as possible.

Still, there are certain use-cases where I need to generate markup on the client-side from time to time, usually everytime where I need JavaScript anyway for a certain functionality.

Code Example


An example where I recently used JSX was for modal dialogs in my Boulder Dash clone.

In that project, I used JSX together with web components. This way, web components start looking very similar to React class-level components, see my game menu component.

A shortened version of this component is below:

import { h, fragment, renderTree } from '../utils/jsx-factory';

export class GameMenu extends HTMLElement {

  static register() {
    customElements.define('game-menu', GameMenu);

  gameMenu: HTMLDialogElement|null = null;
  menuButton: HTMLButtonElement|null = null;

  render() {
    const currentURL = document.location.href;
        <button class="burger" aria-controls="gameMenu" aria-label="open menu"> 
          <svg viewBox="0 0 16 16" fill="currentColor" aria-hidden="true">
            <rect x="1" y="1" width="14" height="3" />
            <rect x="1" y="6" width="14" height="3" />
            <rect x="1" y="11" width="14" height="3" />
        <dialog class="game-menu flow" id="gameMenu">
          <form method="dialog">
            <button id="buttonReturnToGame" class="button">Return to game</button>
          <a href={currentURL} class="button">Restart game</a>
          <a href="/" class="button">Back to main menu</a>

  connectedCallback() {
    this.menuButton = this.querySelector<HTMLButtonElement>('.burger')!;
    this.gameMenu = this.querySelector<HTMLDialogElement>('.game-menu')!;
    this.menuButton.addEventListener('click', this.onClickButton);

  disconnectedCallback() {
    this.menuButton?.removeEventListener('click', this.onClickButton);
    this.innerHTML = '';

  onClickButton = () => this.gameMenu?.showModal();

The methods connectedCallback and disconnectedCallback are quite similar to the React lifecycle methods componentDidMount and componentWillUnmount.

One major difference to React is the fact I'm using class attributes rather than className. React chose to do so as class and for are reserved JavaScript keywords. Apparently, it works fine with class attributes so I prefer to stick to HTML as close as possible.

You could add certain transformations in your own JSX implementation to make both class and className work, but I decided to not do that.

I also kept the event handling separate and just went with using addEventListener. With some additional effort, we could also add support for adding event handlers in a declarative way, like React does.

Setting up JSX for TypeScript


You can enable JSX for TypeScript by editing the tsconfig.json by setting a couple of options.

  "jsx": "react",
  "jsxFactory": "h",
  "jsxFragmentFactory": "fragment",

There are multiple different types of JSX implementations. I'm using the legacy JSX factory implementation for now which was used in React up to version 16.

React version 17 introduced a new kind of JSX factory which I don't use yet. I will cover the differences of the old and new JSX in a follow-up article.

Providing an implementation for the JSX factory


In order to make the JSX factory work, we need to provide implementations for the h() function and also for fragment.

Im my case, h() returns an object using a recursive DOMTree interface. It describes the structure of the generated JSX element.

export interface DOMTree {
  tagName: string;
  attribs: Record<string, string>,
  children: DOMTree[]

This way, the implementation for h() and fragment is pretty straightforward:

const svgNS = 'http://www.w3.org/2000/svg';

export const fragment = 'fragment';

export function h(
  tagName: string, 
  attribs: Record<string, string>, 
  ...children: DOMTree[]): DOMTree {
  return {
    tagName, attribs, children

In a former version of my h() implementation, I created actual dom nodes directly via document.createElement instead of returning a data structure.

But in order to make inline SVG code work, it is important to switch to the XML namespace pointing to http://www.w3.org/2000/svg as soon as there is an <svg> tag, so my nodes need to know about their parent elements.

As a basic solution, I created a recursive renderTree function that takes care of that, using the element.namespaceURI property to retrieve the current XML namespace.

Additionally, support of fragments (using <></> in JSX) are made possible via the DocumentFragment-API.

const svgNS = 'http://www.w3.org/2000/svg';

 * Render a DOM structure
 * @param node the container element where the DOM tree is appended to.
 * @param tree the DOM structure to be created
export function renderTree(node: Element, tree: DOMTree) {
  const namespace = tree.tagName === 'svg' ? svgNS : node.namespaceURI;
  let el: Element|DocumentFragment;
  if (tree.tagName === fragment) {
    el = new DocumentFragment();
  } else {
    el = document.createElementNS(namespace, tree.tagName);
    for (const [attrib, value] of Object.entries(tree.attribs || {})) {
      el.setAttribute(attrib, value);
  for (const child of tree.children) {
    if (typeof child === "string") {
    renderTree(el instanceof DocumentFragment ? node : el, child);


Providing JSX type definitions


Finally, you need to declare a JSX namespace in a type declaration file, suffixed .d.ts.

Be careful with the naming of the files. When the factory is in jsx.ts, don't name the type definition file jsx.d.ts as there may be conflicts.

declare interface DOMTree {
  tagName: string;
  attribs: Record<string, string>;
  children: DOMTree[];

declare namespace JSX {
  interface Element extends DOMTree {}
  interface Attributes {
    [attrib: string]: string;

  interface IntrinsicElements {
    [elem: string]: Attributes;

Final thought


A final thought I had when I was about to finish up this article was: can we also use it server-side?

You can make use of your web component code node.js by using ts-node. This way, you can use it in an express application or in a static site generator such as Eleventy. A nice buzzword for that would be "isomorphic typescript" 🥳.

When it comes to node applications, you often still work a lot with the require() notation for importing dependencies, aka the CommonJS module system (including Eleventy stable as of current). Importing a `.tsx`` file inside node is a bit trickier:

require('ts-node').register({lazy: true, esm: false, moduleTypes: {
  'src/**/*.{ts,tsx}': 'cjs'

// Then, you can use require with your typescript files:
const { h, fragment } = require('./src/utils/jsx.ts');
const { MyComponent } = require('./src/components/my-component.tsx');

For rendering the component server-side, you will need a DOM implementation. JSDOM or LinkeDOM will do. I feel I should also write an in-depth article about this.