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March 03, 2025 00:27
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preact
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Preact Cheatsheet

A concise reference for Preact, a fast 3kB alternative to React, with the same modern API. This cheat sheet covers the essentials for building performant web applications with Preact.

Core Concepts

Components

Components are the core building blocks of Preact applications. They manage state and render UI.

Functional Components (Hooks):

import { useState } from 'preact/hooks';

function MyComponent(props) {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

Class Components:

import { Component } from 'preact';

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }

  increment = () => {
    this.setState({ count: this.state.count + 1 });
  }

  render() {
    return (
      <div>
        <p>Count: {this.state.count}</p>
        <button onClick={this.increment}>Increment</button>
      </div>
    );
  }
}

JSX & Rendering

Preact uses JSX to describe UI. The render function transforms JSX into DOM nodes.

JSX Example:

<div className="container">
  <h1>Hello, Preact!</h1>
  <p>This is a simple example.</p>
</div>

Rendering:

import { render } from 'preact';
import MyComponent from './MyComponent';

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

Virtual DOM

Preact uses a virtual DOM to efficiently update the actual DOM. It compares the previous and current virtual DOM trees and only applies the necessary changes.

Hooks

useState

Declares a state variable.

import { useState } from 'preact/hooks';

const [count, setCount] = useState(0);

count: The current state value.
setCount: A function to update the state.

Updating the state triggers a re-render.

setCount(count + 1);

Functional updates:

setCount(prevCount => prevCount + 1);

useEffect

Performs side effects in functional components.

import { useEffect } from 'preact/hooks';

useEffect(() => {
  // Code to run after render
  document.title = `Count: ${count}`;

  return () => {
    // Cleanup function (optional)
  };
}, [count]); // Dependency array

The effect runs after every render. The dependency array controls when the effect runs. If the array is empty [], the effect runs only on mount and unmount.

useRef

Creates a mutable ref object.

import { useRef } from 'preact/hooks';

const inputRef = useRef(null);

The ref object persists across renders. Useful for accessing DOM nodes or storing mutable values without causing re-renders.

Accessing the current value:

inputRef.current.focus();

Component Lifecycle

Lifecycle Methods (Class Components)

componentDidMount(): Invoked immediately after a component is mounted.

componentDidMount() {
  // Perform initialization tasks here
}

componentWillUnmount(): Invoked immediately before a component is unmounted and destroyed.

componentWillUnmount() {
  // Perform cleanup tasks here
}

shouldComponentUpdate(nextProps, nextState): Determines if the component should re-render.

shouldComponentUpdate(nextProps, nextState) {
  // Return true to update, false to prevent update
  return nextProps.value !== this.props.value;
}

componentDidUpdate(prevProps, prevState): Invoked immediately after an update occurs.

componentDidUpdate(prevProps, prevState) {
  // Perform side effects based on the update
}

Preact CLI

Creating a New Project

Preact CLI is the recommended way to start a new Preact project.

npx preact-cli create my-app
cd my-app
npm install
npm run dev

Common Commands

npm run dev

Starts the development server with hot reloading.

npm run build

Builds the project for production.

npm run serve

Serves the production build locally.

Configuration

Preact CLI uses a preact.config.js file to configure the build process.

// preact.config.js
export default (config, env, helpers) => {
  // Customize webpack config here
};