preact

What is preact?

Preact is a fast, 3kB alternative to React with the same modern API. It provides the thinnest possible Virtual DOM abstraction on top of the DOM. Its goal is to provide the same rich and robust ecosystem that React has, while being leaner and more efficient, often used for performance-critical applications and situations where bundle size is a factor.

What are preact's main functionalities?

Creating Components

This code sample demonstrates how to create a simple Preact component and render it to the DOM. It's similar to React but with a smaller footprint.

import { h, render, Component } from 'preact';

class MyComponent extends Component {
  render() {
    return <div>Hello, Preact!</div>;
  }
}

render(<MyComponent />, document.body);

Using Hooks

This code sample shows how to use hooks in Preact, specifically the useState hook to create a simple counter component.

import { h, render, useState } from 'preact/hooks';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

render(<Counter />, document.body);

Preact/Compat

This code sample illustrates how to use Preact/Compat to achieve React compatibility, allowing developers to use Preact as a drop-in replacement for React.

import React from 'preact/compat';
import ReactDOM from 'preact/compat';

function App() {
  return <h1>Hello from Preact/Compat</h1>;
}

ReactDOM.render(<App />, document.getElementById('app'));

Other packages similar to preact

react

React is a declarative, efficient, and flexible JavaScript library for building user interfaces. It's larger in size compared to Preact but has a larger ecosystem and more built-in features.

inferno

Inferno is an extremely fast, React-like library for building high-performance user interfaces on both the client and server. It's similar to Preact in terms of performance goals but has its own set of APIs and optimizations.

vue

Vue.js is a progressive framework for building user interfaces. Unlike Preact, Vue has a different API and embraces a different philosophy, focusing on declarative rendering and component composition with a more opinionated set of tools.

svelte

Svelte is a radical new approach to building user interfaces. Instead of doing the bulk of its work in the browser, Svelte shifts that work into a compile step that happens when you build your app. It's different from Preact in that it doesn't use a virtual DOM.

README

Preact

Preact is an attempt to recreate the core value proposition of React (or similar libraries like Mithril) using as little code as possible, with first-class support for ES2015. Currently the library is around 3kb (minified & gzipped).

It retains a large amount of compatibility with React, but only the stateless functional components and ES6 Classes interface. As one would expect coming from React, Components are simple building blocks for composing a User Interface.

:information_desk_person: Documentation is available in the Preact Wiki.

Demos

• ESBench is built using Preact.
• Nectarine.rocks (Github Project) :peach:
• Documentation Viewer (Github Project)
• TodoMVC (Github Project)
• Hacker News Minimal (Github Project)
• Preact Boilerplate (Github Project) :zap:
• Flickr Browser (@ CodePen)
• Animating Text (@ CodePen)
• 60FPS Rainbow Spiral (@ CodePen)
• Simple Clock (@ JSFiddle)
• 3D + ThreeJS (@ CodePen)
• Create your Own! (@ JSFiddle)

Libraries & Add-ons

• :earth_americas: preact-router: URL routing for your components.
• :tulip: preact-svg: Use inline <svg> in Preact.
• :page_facing_up: preact-render-to-string: Universal rendering.
• :raised_hands: preact-compat: use any React library with Preact. (full example)
• :rocket: preact-photon: build beautiful desktop UI with photon.
• :microscope: preact-jsx-chai: JSX assertion testing (no DOM or phantomjs!)
• :bookmark_tabs: preact-markup: Render HTML & Custom Elements as JSX & Components
• :pencil: preact-richtextarea: Simple HTML editor component

Getting Started

:information_desk_person: You don't have to use ES2015 to use Preact... but you should.

The following guide assumes you have some sort of ES2015 build set up using babel and/or webpack/browserify/gulp/grunt/etc. If you don't, start with preact-boilerplate or a CodePen Template.

Import what you need

The preact module provides both named and default exports, so you can either import everything under a namespace of your choosing, or just what you need as locals:

Named:

import { h, render, Component } from 'preact';

// Tell Babel to transform JSX into h() calls:
/** @jsx h */

Default:

import preact from 'preact';

// Tell Babel to transform JSX into preact.h() calls:
/** @jsx preact.h */

Named imports work well for highly structured applications, whereas the default import is quick and never needs to be updated when using different parts of the library.

Instead of declaring the @jsx pragma in your code, it's best to configure it globally in a .babelrc:

{ "jsxPragma": "h" }

Rendering JSX

Out of the box, Preact provides an h() function that turns your JSX into Virtual DOM elements (here's how). It also provides a render() function that creates a DOM tree from that Virtual DOM.

To render some JSX, just import those two functions and use them like so:

import { h, render } from 'preact';

render((
	<div id="foo">
		<span>Hello, world!</span>
		<button onClick={ e => alert("hi!"); }>Click Me</button>
	</div>
), document.body);

This should seem pretty straightforward if you've used hyperscript or one of its many friends.

Rendering hyperscript with a virtual DOM is pointless, though. We want to render components and have them updated when data changes - that's where the power of virtual DOM diffing shines. :star2:

Components

Preact exports a generic Component class, which can be extended to build encapsulated, self-updating pieces of a User Interface. Components support all of the standard React lifecycle methods, like shouldComponentUpdate() and componentWillReceiveProps(). Providing specific implementations of these methods is the preferred mechanism for controlling when and how components update.

Components also have a render() method, but unlike React this method is passed (props, state) as arguments. This provides an ergonomic means to destructure props and state into local variables to be referenced from JSX.

Let's take a look at a very simple Clock component, which shows the current time.

import { h, render, Component } from 'preact';

class Clock extends Component {
	render() {
		let time = new Date().toLocaleTimeString();
		return <span>{ time }</span>;
	}
}

// render an instance of Clock into <body>:
render(<Clock />, document.body);

That's great. Running this produces the following HTML DOM structure:

<span>10:28:57 PM</span>

In order to have the clock's time update every second, we need to know when <Clock> gets mounted to the DOM. If you've used HTML5 Custom Elements, this is similar to the attachedCallback and detachedCallback lifecycle methods. Preact invokes the following lifecycle methods if they are defined for a Component:

Lifecycle method	When it gets called
componentWillMount	before the component gets mounted to the DOM
componentDidMount	after the component gets mounted to the DOM
componentWillUnmount	prior to removal from the DOM
componentDidUnmount	after removal from the DOM
componentWillReceiveProps	before new props get accepted
shouldComponentUpdate	before render(). Return false to skip render
componentWillUpdate	before render()
componentDidUpdate	after render()

So, we want to have a 1-second timer start once the Component gets added to the DOM, and stop if it is removed. We'll create the timer and store a reference to it in componentDidMount, and stop the timer in componentWillUnmount. On each timer tick, we'll update the component's state object with a new time value. Doing this will automatically re-render the component.

import { h, render, Component } from 'preact';

class Clock extends Component {
	constructor() {
		super();
		// set initial time:
		this.state.time = Date.now();
	}

	componentDidMount() {
		// update time every second
		this.timer = setInterval(() => {
			this.setState({ time: Date.now() });
		}, 1000);
	}

	componentWillUnmount() {
		// stop when not renderable
		clearInterval(this.timer);
	}

	render(props, state) {
		let time = new Date(state.time).toLocaleTimeString();
		return <span>{ time }</span>;
	}
}

// render an instance of Clock into <body>:
render(<Clock />, document.body);

Now we have a ticking clock!

Props & State

The concept (and nomenclature) for props and state is the same as in React. props are passed to a component by defining attributes in JSX, state is internal state. Changing either triggers a re-render, though by default Preact re-renders Components asynchronously for state changes and synchronously for props changes. You can tell Preact to render prop changes asynchronously by setting options.syncComponentUpdates to false.

---

Linked State

One area Preact takes a little further than React is in optimizing state changes. A common pattern in ES2015 React code is to use Arrow functions within a render() method in order to update state in response to events. Creating functions enclosed in a scope on every render is inefficient and forces the garbage collector to do more work than is necessary.

One solution to this is to bind component methods declaratively. Here is an example using decko:

class Foo extends Component {
	@bind
	updateText(e) {
		this.setState({ text: e.target.value });
	}
	render({ }, { text }) {
		return <input value={text} onInput={this.updateText} />;
	}
}

While this achieves much better runtime performance, it's still a lot of unnecessary code to wire up state to UI.

Fortunately there is a solution, in the form of linkState(). Calling component.linkState('text') returns a function that accepts an Event and uses it's associated value to update the given property in your component's state. Calls to linkState() with the same state property are cached, so there is no performance penalty. Here is the previous example rewritten using Linked State:

class Foo extends Component {
	render({ }, { text }) {
		return <input value={text} onInput={this.linkState('text')} />;
	}
}

Simple and effective. It handles linking state from any input type, or an optional second parameter can be used to explicitly provide a keypath to the new state value.

Examples

Here is a somewhat verbose Preact <Link> component:

class Link extends Component {
	render(props, state) {
		return <a href={ props.href }>{ props.children }</a>;
	}
}

Since this is ES6/ES2015, we can further simplify:

class Link extends Component {
	render({ href, children }) {
		return <a {...{ href, children }} />;
	}
}

// or, for wide-open props support:
class Link extends Component {
	render(props) {
		return <a {...props} />;
	}
}

// or, as a stateless functional component:
const Link = ({ children, ...props }) => (
	<a {...props}>{ children }</a>
);

Extensions

It is likely that some projects based on Preact would wish to extend Component with great new functionality.

Perhaps automatic connection to stores for a Flux-like architecture, or mixed-in context bindings to make it feel more like React.createClass(). Just use ES2015 inheritance:

class BoundComponent extends Component {
	constructor(props) {
		super(props);
		this.bind();
	}
	bind() {
		this.binds = {};
		for (let i in this) {
			this.binds[i] = this[i].bind(this);
		}
	}
}

// example usage
class Link extends BoundComponent {
	click() {
		open(this.href);
	}
	render() {
		let { click } = this.binds;
		return <span onclick={ click }>{ children }</span>;
	}
}

The possibilities are pretty endless here. You could even add support for rudimentary mixins:

class MixedComponent extends Component {
	constructor() {
		super();
		(this.mixins || []).forEach( m => Object.assign(this, m) );
	}
}

License

MIT