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Getting Started

This guide walks through building a simple "ticking clock" component. More detailed information for each topic can be found in the dedicated pages under the Guide menu.

:information_desk_person: You don't have to use ES2015 to use Preact... but you should. This 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-cli 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 */
Run in REPL

Default:

import preact from 'preact';

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

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.

Using via a CDN:

<script src="https://cdn.jsdelivr.net/npm/preact/dist/preact.min.js"></script>

<!-- To load Preact as a JS Module: -->
<script src="https://cdn.jsdelivr.net/npm/preact/dist/preact.mjs" type="module"></script>

Global pragma

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

Named:

For Babel 5 and prior:

{ "jsxPragma": "h" }

For Babel 6:

{
  "plugins": [
    ["transform-react-jsx", { "pragma":"h" }]
  ]
}

For Babel 7:

{
  "plugins": [
    ["@babel/plugin-transform-react-jsx", { "pragma":"h" }]
  ]
}

Default:

For Babel 5 and prior:

{ "jsxPragma": "preact.h" }

For Babel 6:

{
  "plugins": [
    ["transform-react-jsx", { "pragma":"preact.h" }]
  ]
}

For Babel 7:

{
  "plugins": [
    ["@babel/plugin-transform-react-jsx", { "pragma":"preact.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);
Run in REPL

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);
Run in REPL

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

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

The Component Lifecycle

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 methodWhen it gets called
componentWillMountbefore the component gets mounted to the DOM
componentDidMountafter the component gets mounted to the DOM
componentWillUnmountprior to removal from the DOM
componentWillReceivePropsbefore new props get accepted
shouldComponentUpdatebefore render(). Return false to skip render
componentWillUpdatebefore render()
componentDidUpdateafter 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);
Run in REPL

Now we have a ticking clock!

Built by a bunch of lovely people like @valotas.