React

Compared to Webpack, Parcel's paradigm is to use your HTML file as the entry point, not merely the main script:

<div id="app"></div>
<script src="./index.js"></script>

import React from "react";
import ReactDom from "react-dom";

const App = () => <h1>Hello!</h1>;

ReactDom.render(document.getElementById("app"), <App />);

¶ HMR (Fast Refresh)

Parcel has first-class support for React Fast Refresh (which supersedes react-hot-loader, a userland plugin that botched HMR support onto React). It is (in most cases) able to retain state between reloads (even after an error).

For further information, take a look at the official documentation.

¶ Selected Limitations

¶ State in class components is reset between reloads

With class components slowly being deprecated, their state will not be preserved.

¶ Declaring a Default Export Using a Function Expression Isn't Recognized

Editing this component would reset value because the Fast Refresh Babel plugin cannot instrument the default export declaration.

import React, { useState } from "react";

export default () => {
  const [value] = useState(Date.now());

  return <h1>Hello! {value}</h1>;
};

import React from "react";
import ReactDom from "react-dom";
import Component from "./Component.js";

const App = (
  <h1>
    <Component />
  </h1>
);

ReactDom.render(..., <App />);

See also Dan Abramov's tweet

¶ Exporting Values That Are Not Components Will Reset the State:

Editing Component would reset the value state, because of the other non-component export.

import React, { useState } from "react";

const Component = () => {
  const [value] = useState(Date.now());

  return <h1>Hello! {value}</h1>;
};
export default Component;

export function utility() {
  return Date.now();
}

import React from "react";
import ReactDom from "react-dom";
import Component, { utility } from "./Component.js";

console.log(utility());

const App = (
  <h1>
    <Component />
  </h1>
);

ReactDom.render(..., <App />);

¶ Modifying the Asset That Calls Render Will Reset All State:

Understandably, modifying App will call ReactDom.render again:

import React from "react";
import ReactDom from "react-dom";

const App = () => <h1>Hello!</h1>;

ReactDom.render(..., <App />);

(The HMR functionality is provided by @parcel/transformer-react-refresh-babel, @parcel/transformer-react-refresh-wrap and @parcel/runtime-react-refresh.)