index.js

const fs = require("fs");
const PNG = require("pngjs").PNG;
const pixelmatch = require("pixelmatch");
const path = require("path");

const sourceImages = path.join(__dirname, "srcImages");
const destImages = path.join(__dirname, "destImages");
const diffImages = "./diff";

fs.readdir(sourceImages, function(err, files) {
  if (err) {
    return console.log("Unable to scan directory: " + err);
  }
  files.forEach(function(file) {
    if (path.extname(file) === ".png") {
      const img1 = PNG.sync.read(fs.readFileSync(`${sourceImages}/${file}`));
      const img2 = PNG.sync.read(fs.readFileSync(`${destImages}/${file}`));
      const { width, height } = img1;
      const diff = new PNG({ width, height });

      const difference = pixelmatch(
        img1.data,
        img2.data,
        diff.data,
        width,
        height,
        {
          threshold: 0
        }
      );

      if (difference) {
        if (!fs.existsSync(diffImages)) {
          fs.mkdirSync(diffImages);
        }
        fs.writeFileSync(`${diffImages}/${file}`, PNG.sync.write(diff));
      }

      console.log(`${file}: ${difference} pixels different`);
    }
  });
});