Generate map tiles from a large image with nodejs

generate.js

// Given a large input image, this script generates map tiles of 256*256px
// at power-of-2 zoom levels. It's not super efficient or anything, just a
// quickly hacked together script.

//Use the tiles in leaflet.js like this:
/*
  <div id='map' style='height:100%;'></div>
  <script>
    var map = L.map('map', {
        attributionControl: false
    }).setView([0, 0], 1);
    var layer_url = './images/{z}-{x}-{y}.png';
    L.tileLayer(layer_url, {
        maxZoom: 20, // <-- change to suit
        noWrap: true
    }).addTo(map);
  </script>
*/


// Instructions:
// 1. create a new directory
// 2. run `npm init -y`
// 3. run `npm install sharp --save`
// 4. copy your image into the directory and call it `input.png`
// 5. make a folder called `output`
// 6. run this script with `node generate.js`

(async function() {

  let input = 'input.png';  // <-- this is your input image
  let sharp = require('sharp');

  let metadata = await sharp(input).metadata();
  let fullPixelSize = Math.max(metadata.width, metadata.height);

  await sharp(input)
    .resize(fullPixelSize, fullPixelSize)
    .background({r: 0, g: 0, b: 0, alpha: 0})
    .embed()
    .toFile("squarified.png");

  console.log("squarified ✓");

  //sharp.cache(false);

  let zoom = 0;
  while(true) {


    // double size for each consecutive zoom level, starting at 256px:
    let pixelSize = 256 * 2**zoom;

    if(pixelSize > fullPixelSize) {
      zoom--;
      break;
    }
    
    console.log(`Processing zoom level ${zoom}...`);

    await sharp("squarified.png")
      .resize(pixelSize, pixelSize)
      .toFile(`resized-${zoom}.png`);


    let rows = 2**zoom;
    let cols = 2**zoom;

    for(let row = 0; row < rows; row++) {
      for(let col = 0; col < cols; col++) {

        await sharp(`resized-${zoom}.png`)
          .extract({ left: col*256, top: row*256, width: 256, height: 256 })
          .toFile(`./output/${zoom}-${col}-${row}.png`);

      }
    }

    zoom++;
  }

  if(zoom < 0) {
    console.error(`Seems that you've input an image that's less than 256 pixels`);
  } else {
    console.log(`Finished at zoom=${zoom}`);
  }

})();