Image-flipping server

image-flipper.js

"use strict";
const http = require("http");
const url = require("url");
const request = require("request");
const jsdom = require("jsdom");
const Canvas = require("canvas");

http.createServer(function (req, res) {
  console.log(req.url);
  const pageUrl = url.parse(req.url, true).query["proxy-me"];
  if (!pageUrl) {
    res.end();
    return;
  }

  jsdom.env(pageUrl, function (err, window) {
    fixStylesheetUrls(window, pageUrl);

    const images = window.document.querySelectorAll("img");

    Promise.all(Array.prototype.map.call(images, function (imgEl) {
      return getImageContents(imgEl.src)
        .then(rotateImage)
        .then(function (rotatedCanvas) {
          if (rotatedCanvas) {
            imgEl.src = rotatedCanvas.toDataURL();
          }
        });
    }))
    .then(function () {
      res.end(jsdom.serializeDocument(window.document));
    })
    .catch(function (err) {
      res.end(err.stack);
    });
  });
}).listen(5002);

function fixStylesheetUrls(window, baseUrl) {
  Array.prototype.forEach.call(window.document.querySelectorAll("link"), function (linkEl) {
    linkEl.href = url.resolve(baseUrl, linkEl.href);
  });
}

function getImageContents(imgSrc) {
  return new Promise(function (resolve, reject) {
    request(imgSrc, { encoding: null }, function (error, response, buffer) {
      if (error) {
        reject(error);
      }
      resolve(buffer);
    });
  });
}

function rotateImage(imgBuffer) {
    // Turn the image buffer into something the canvas can work with.
    const img = new Canvas.Image();
    img.src = imgBuffer;

    if (img.width === 0 || img.height === 0) {
        // Image is invalid; bail out.
        return;
    }

    // Create a canvas
    const canvas = new Canvas(img.width, img.height);
    const ctx = canvas.getContext("2d");

    // Do the funky translate-then-rotate-then-translate-back dance.
    ctx.translate(img.width / 2, img.height / 2);
    ctx.rotate(Math.PI);
    ctx.translate(-img.width / 2, -img.height / 2);

    // Draw the image onto our rotated canvas.
    ctx.drawImage(img, 0, 0, img.width, img.height);

    return canvas;
}