Neat Little Tricks by Anup Bishnoi

React implemented in a Tweet!

Let’s implement the basic React API in a tweet, with React DOM & State management as a separate plugin in another tweet.

What we want, and shall get

  • Top-down, one-way, state-driven rendering.
  • Compose HTML primitives into higher-level components.
  • Ability to write JSX to define components.
  • Custom props for higher-order components.
  • Event handlers that can change application state and trigger re-renders.

What we won’t get

  • No virtual DOM diffing between old and new renders for efficient DOM updates, so re-renders will replace HTML content.
  • setState won’t be asynchronous, but synchronous.
  • DOM event handlers won’t have React-ified names (like onClick), but default DOM names (like onclick).
  • Only necessary API surface area implemented, so yes React.createElement, and no React.cloneElement.
  • No React Hooks API.


Here’s React defined as a global

React={Component:function(e){this.props=e},[C="createElement"]:(t,s,...a)=>t.bind?new t(s):(e=document[C](t),(()=>{for(n in s)e[n]=s[n]})(),g(a).map(t=>e[A](t.props?t.render():t.part?t:new Text(t))),e)},A="appendChild",g=(e=>(e=e||[],[e]))

What does the above do?

  • Defines a global React.
  • Defines React.Component, the component base class.
  • Defines React.createElement which can create both HTML primitives and custom components.
  • React.createElement also accepts props, and children which get appended to the created DOM element.
  • JSX essentially transpiles to React.createElement calls, with contents passed in as children argument. These children can be custom components, primitive HTML elements, or plain text. Code handles them all correctly.


Read the annotated, prettified code below for line-by-line explanation.

For now, moving on to more features!

React DOM + State management

You can bring in React DOM and React’s State management API come as a separate plugin, implemented in another tweet!


What does the above define?


Let’s build React’s own Tic-tac-toe tutorial with React-in-a-tweet™.

You can fiddle with the following over at JSFiddle, or try it out right here!

Video of working example:

Usage code

Here’s how that sweet sweet demo is implemented.

Throw in a root HTML element:

<div id="root"></div>

Sprinkle some CSS to make things look right (from React tutorial):

body { font: 14px "Century Gothic", Futura, sans-serif; margin: 20px; }
ol, ul { padding-left: 30px; }
.board-row:after { clear: both; content: ""; display: table; }
.status { margin-bottom: 10px; } 
.square {
  background: #fff; border: 1px solid #999; float: left;
  font-size: 24px; font-weight: bold; line-height: 34px;
  height: 34px; margin-right: -1px; margin-top: -1px;
  padding: 0; text-align: center; width: 34px;
.square:focus { outline: none; }
.kbd-navigation .square:focus { background: #ddd; }
.game { display: flex; flex-direction: row; }
.game-info { margin-left: 20px; }

And now the work horse, the JSX-enabled usage of React-in-a-tweet™ (also lifted from React tutorial):

function Square(props) {
  return (
    <button className="square" onclick={props.onClick}>

Returning JSX!

The way that works is: Babel transpiles this JSX to React.createElement calls, which are handled by React-in-a-tweet just fine.

Remember, JSX transpiles to React.createElement() calls, and passes in the component type as the first argument, props defined in JSX as the second argument, and any children provided as the third argument.

For example:

<button className="square" onclick={props.onClick}>
  Square value: {props.value}

transpiles to:

React.createElement("button", {
  className: "square",
  onclick: props.onClick
  "Square value: ", props.value,
  React.createElement("div", null, "nested")

So, transpiled code includes React.createElement() calls that contain 3 or more arguments: type, props, and ...children.

Also notice that the click handler on button above has the DOM-defined name onclick, instead of the name that React gives it - onClick.

This is because React-in-a-tweet does not implement React’s synthetic events. But, just like React, you can still assign functions to onclick.

Wait, what about props.onClick though? Why isn’t it it props.onclick? Well, that’s just a prop name, which can be anything you like :)

Moving on to more definitions:

class Board extends React.Component {
  renderSquare(i) {
    return (
        onClick={() => this.props.onClick(i)}

  render() {
    return (
        <div className="board-row">
        <div className="board-row">
        <div className="board-row">

Notice how Board’s render function can call this.renderSquare() to fill in parts of the rendered output. Works just fine.

renderSquare() returns a custom component, Square, not an HTML primitive. Works fine.

Also notice how the onClick handler defined in renderSquare() is just an arrow function. Works great.

But wait, what’s className doing there? Isn’t that a React-ified HTML attribute name? Well, no siree. className is pure DOM API. Works as is.

Now let’s define the root game component itself:

class Game extends React.Component {
  constructor(props) {
    this.state = {
      history: [
          squares: Array(9).fill(null)
      stepNumber: 0,
      xIsNext: true

  handleClick(i) {
    console.log('handleClick', i);
    const history = this.state.history.slice(0, this.state.stepNumber + 1);
    const current = history[history.length - 1];
    const squares = current.squares.slice();
    if (calculateWinner(squares) || squares[i]) {
    squares[i] = this.state.xIsNext ? "X" : "O";
      history: history.concat([
          squares: squares
      stepNumber: history.length,
      xIsNext: !this.state.xIsNext

  jumpTo(step) {
      stepNumber: step,
      xIsNext: (step % 2) === 0

  render() {
    const history = this.state.history;
    const current = history[this.state.stepNumber];
    const winner = calculateWinner(current.squares);

    const moves =, move) => {
      const desc = move ?
        'Go to move #' + move :
        'Go to game start';
      return (
        <li key={move}>
          <button onclick={() => this.jumpTo(move)}>{desc}</button>

    let status;
    if (winner) {
      status = "Winner: " + winner;
    } else {
      status = "Next player: " + (this.state.xIsNext ? "X" : "O");

    return (
      <div className="game">
        <div className="game-board">
            onClick={i => this.handleClick(i)}
        <div className="game-info">

So much code! Well, as far as React API is concerned, there are only a few interesting things going on in there.

  • Game’s constructor sets initial state (this.state), storing history and other Tic-tac-toe-specific data.
  • handleClick click handler uses this.setState() and passes in new state.
  • render does conditional rendering based on whether the game has been won yet, and returns JSX for the entire game board.
  • Since game state is defined entirely as Game’s React state, the entire game board will re-render on every game state change in this example. Regardless of this example, React-in-a-tweet does allow re-rendering specific parts of a UI as long as those specific UI sections hold their own state, instead of depending on a single global state.
ReactDOM.render(<Game />, document.getElementById("root"));

React DOM API that triggers the first-render, y’all. Passing in JSX for <Game /> works. It’s just the React you know and love!

Finally, a helper function that calculates winner after every move:

function calculateWinner(squares) {
  const lines = [[0,1,2], [3,4,5], [6,7,8], [0,3,6], [1,4,7], [2,5,8], [0,4,8], [2,4,6]];
  for (let i = 0; i < lines.length; i++) {
    const [a, b, c] = lines[i];
    if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) return squares[a];
  return null;

Wait, how does React-in-a-tweet itself work?

Glad you asked!

Here’s a line-by-line explanation of the tweet-sized source code of React-in-a-tweet:

Define React global:

React = {

Define and implement the following API:

  • React.Component, just a simple base class which stores the props passed to it.
    Component: function(p) {
      this.props = p
  • React.createElement, the work horse of rendering, which accepts:
    • t, HTML type name or React component class,
    • p, props
    • ...c, children to be appended to the dom element created.
    [C = 'createElement']: (t, p, ...c) => (

In the case of t being a React-in-a-tweet component class, ie, a constructor function derived from React.Component, we can just instantiate that function with passed props and return the resulting React component instance.

        ? new t(p)

Otherwise, t is an HTML primitive element name, like button, or span. In this case, create a new HTML element using document.createElement(t)

        : (
          e = document[C](t),

and assign all the props passed to it as direct properties of the HTML element. This is how onclicks and other event handlers get registered.

If you want to use React-like CSS-in-JS, you can modify the part below to loop over a passed style object, and assign its properties to e[n].style.

          (() => {
            for (n in p)
              e[n] = p[n]

We must also append the children passed in as arguments into the created DOM element.

Looping over a flattened list of children, handle the following cases for each child element (e):

  • If a.props exists, then it’s a React.Component instance. Call a.render() to get the corresponding instantiated HTML element.
  • If a.part exists, then it’s a regular HTML element instance already. Do nothing.
  • Else, a must be plain text. Initialize a new HTML Text element to hold the string.

In all cases, after instantiating, append the resulting DOM element as a child to the parent DOM element e.

Note: A is just shorthand for appendChild, as defined below.

Finally, return e, the element created and filled in by this execution of React.createElement().

          g(c).map(a => (
                ? a.render()
                : a.part
                  ? a
                  : new Text(a)

Lastly, define g, just a helper function that flattens elements, lists, and list of lists into a single list, to support React.createElement’s children argument semantics.

g = a => (
  a = a || [], ? a.flatMap(g) : [a]
A = 'appendChild',

And that’s it!

Understanding the React DOM + Component.setState tweet is left as an exercise to the reader.