To extract the code into an npm library that can be easily implemented in a new Preact app, you will need to structure your code as a reusable package. Here’s a step-by-step guide to creating an npm package for lazy loading in Preact:

Step-by-Step Guide

1. Set Up the NPM Package

1. Create a new directory for your library:

   mkdir preact-lazy-load
   cd preact-lazy-load

2. Initialize the npm package:

   npm init -y

3. Install Preact as a dependency:

   npm install preact

4. Create the required directory structure:

   mkdir src
   touch src/index.js src/LazyImage.js src/LazyIframe.js src/LazyBackgroundImage.js

2. Write the Library Code

src/index.js

This file will export all the components.

export { default as LazyImage } from './LazyImage';
export { default as LazyIframe } from './LazyIframe';
export { default as LazyBackgroundImage } from './LazyBackgroundImage';

src/LazyImage.js

import { h } from 'preact';
import { useRef, useEffect } from 'preact/hooks';

const LazyImage = ({ src, srcset, sizes, alt }) => {
  const imgRef = useRef(null);

  useEffect(() => {
    if ('loading' in HTMLImageElement.prototype) {
      imgRef.current.src = src;
      imgRef.current.srcset = srcset;
    } else {
      setupLazyLoadingFallback();
    }

    function setupLazyLoadingFallback() {
      if ('IntersectionObserver' in window) {
        const observer = new IntersectionObserver((entries, observer) => {
          entries.forEach(entry => {
            if (entry.isIntersecting) {
              imgRef.current.src = src;
              imgRef.current.srcset = srcset;
              observer.unobserve(imgRef.current);
            }
          });
        }, { rootMargin: '0px 0px 50px 0px' });
        observer.observe(imgRef.current);
      } else {
        imgRef.current.src = src;
        imgRef.current.srcset = srcset;
      }
    }
  }, [src, srcset]);

  return (
    <img
      ref={imgRef}
      sizes={sizes}
      loading="lazy"
      alt={alt}
    />
  );
};

export default LazyImage;

src/LazyIframe.js

import { h } from 'preact';
import { useRef, useEffect } from 'preact/hooks';

const LazyIframe = ({ src, title }) => {
  const iframeRef = useRef(null);

  useEffect(() => {
    if ('loading' in HTMLIFrameElement.prototype) {
      iframeRef.current.src = src;
    } else {
      setupLazyLoadingFallback();
    }

    function setupLazyLoadingFallback() {
      if ('IntersectionObserver' in window) {
        const observer = new IntersectionObserver((entries, observer) => {
          entries.forEach(entry => {
            if (entry.isIntersecting) {
              iframeRef.current.src = src;
              observer.unobserve(iframeRef.current);
            }
          });
        }, { rootMargin: '0px 0px 50px 0px' });
        observer.observe(iframeRef.current);
      } else {
        iframeRef.current.src = src;
      }
    }
  }, [src]);

  return (
    <iframe
      ref={iframeRef}
      loading="lazy"
      title={title}
    ></iframe>
  );
};

export default LazyIframe;

src/LazyBackgroundImage.js

import { h } from 'preact';
import { useRef, useEffect } from 'preact/hooks';

const LazyBackgroundImage = ({ src, children }) => {
  const divRef = useRef(null);

  useEffect(() => {
    if ('IntersectionObserver' in window) {
      const observer = new IntersectionObserver((entries) => {
        entries.forEach(entry => {
          if (entry.isIntersecting) {
            divRef.current.style.backgroundImage = `url(${src})`;
            divRef.current.setAttribute('data-loaded', 'true');
            observer.disconnect();
          }
        });
      }, { rootMargin: '0px 0px 50px 0px' });
      observer.observe(divRef.current);
    } else {
      // Fallback for older browsers
      divRef.current.style.backgroundImage = `url(${src})`;
      divRef.current.setAttribute('data-loaded', 'true');
    }
  }, [src]);

  return (
    <div ref={divRef} className="lazy-background">
      {children}
    </div>
  );
};

export default LazyBackgroundImage;

3. Add Styles

Since styles need to be included for the library to work correctly, create a style.css file and include it in the package.

Create src/style.css:

.lazy-background {
  opacity: 0;
  transition: opacity 0.3s;
  background-size: cover;
  background-repeat: no-repeat;
  background-position: center;
}

.lazy-background[data-loaded] {
  opacity: 1;
}

Modify src/index.js to include the styles:

import './style.css';

export { default as LazyImage } from './LazyImage';
export { default as LazyIframe } from './LazyIframe';
export { default as LazyBackgroundImage } from './LazyBackgroundImage';

4. Bundle the Library

You will need to set up a bundler like Rollup to bundle your library. Here is an example configuration using Rollup.

1. Install Rollup and necessary plugins:

   npm install --save-dev rollup @rollup/plugin-node-resolve @rollup/plugin-commonjs @rollup/plugin-babel @rollup/plugin-terser

2. Create rollup.config.js:

   import resolve from '@rollup/plugin-node-resolve';
   import commonjs from '@rollup/plugin-commonjs';
   import babel from '@rollup/plugin-babel';
   import { terser } from '@rollup/plugin-terser';
   
   export default {
     input: 'src/index.js',
     output: {
       file: 'dist/index.js',
       format: 'esm',
       sourcemap: true,
     },
     plugins: [
       resolve(),
       commonjs(),
       babel({
         exclude: 'node_modules/**',
         presets: ['@babel/preset-env', '@babel/preset-react']
       }),
       terser(),
     ],
     external: ['preact']
   };

3. Add build scripts to package.json:

   "scripts": {
     "build": "rollup -c"
   },

5. Build and Publish the Library

1. Build the library:

   npm run build

2. Publish the package:

   npm publish

6. Use the Library in a New Preact App

1. Create a new Preact app:

   npx preact-cli create default my-new-preact-app
   cd my-new-preact-app

2. Install the library:

   npm install preact-lazy-load

3. Use the library in your Preact app:

src/index.js

import { h, render } from 'preact';
import App from './components/App';
import './style.css';

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

src/components/App.js

import { h } from 'preact';
import { LazyImage, LazyIframe, LazyBackgroundImage } from 'preact-lazy-load';

const App = () => (
  <div>
    <h1>Native Lazy Loading in Preact</h1>
    
    <h2>Lazy Load Image</h2>
    <LazyImage 
      src="path/to/image.jpg" 
      srcset="path/to/image-320w.jpg 320w, path/to/image-480w.jpg 480w" 
      sizes="(max-width: 600px) 320px, 480px" 
      alt="Description of image"
    />
    
    <h2>Lazy Load Iframe</h2>
    <LazyIframe 
      src="https://www.youtube.com/embed/dQw4w9WgXcQ" 
      title="Sample Video"
    />
    
    <h2>Lazy Load Background Image</h2>
    <LazyBackgroundImage 
      src="path/to/background-image.jpg"
    >
      <p>Content overlaying the background image</p>
    </LazyBackgroundImage>
  </div>
);

export default App;

With this setup, you have created an npm library for lazy loading components in Preact that can be easily integrated into any Preact project.