lambda-image-resizer.js

exports.handler = function(event, context) {
  // We'd get this from the Lambda event in reality
  var sampleInput = {
    from: 1,
    to: 39,
    bucketName: 'test-bucket',
    imageSizes: [{
      width: 600,
      height: 400,
      description: 'A medium thumbnail',
      prefix: 'thumb-',
      suffix: '-600x400'
    }, {
      width: 200,
      height: 100,
      description: 'A mini thumbnail',
      prefix: 'thumb-',
      suffix: '-200x100'
    }]
  };

  sampleInput.contentType = sampleInput.contentType || '.jpg';
  
  // Init the S3 service wrapper
  S3Wrapper.init({
    bucketName: input.bucketName
  });

  // toUrl is now a function that will return a full S3 url for an image index
  var toUrl = UrlTransformer.toImageUrl(sampleInput.contentType);

  // Image.transformImage is curried, so transformImageToSizes is now a function
  // that will transform an image to each thumb size from sampleInput.imageSizes
  var transformImageToSizes = Image.transformImage(sampleInput.imageSizes);

  var imageIndexes = _.range(input.from, input.to + 1);
  
  // Paths where the images should be stored
  var urls = _.map(imageIndexes, toUrl);
  
  when
    .all(_.map(urls, transformImageToSizes))
    .then(function() {
      console.log('complete!');
      context.done(null, true);
    }, function(reason) {
      console.log('failed!', reason);
      context.fail(reason, null);
    });

};

/**** UrlTransformer ****/

var UrlTransformer = {
  toImageUrl: function(contentType, imageIndex) { return imageIndex + contentType; },
  getResizedImageKey: function(imageSize, key) {
    var keyParts = key.split('.');
    var suffix = imageSize.suffix || imageSize.height + 'x' + imageSize.width;
    var prefix = imageSize.prefix || '';
    return prefix + keyParts[0] + suffix + '.' + keyParts[1];
  }
}

/**** Image ****/

var Image = {
  transformImage: transformImage,
  resizeImage: resizeImage,
  getImageThumbs: getImageThumbs
}

// Gets an image from S3, resizes it, and saves the thumbs back to S3
var transformImage = _.curry(function(imageSizes, path) {
  return S3.getImage(path)
    .then(getImageThumbs(imageSizes, null))
    .catch(function(err) {
      console.log('error in transformImage', err);
    });
});

// Recursively resize the image to each image size.
// We assume that imageSizes is a sorted array so that it goes from bigger to smaller
var getImageThumbs = _.curry(function(imageSizes, originalImageKey, image) {
  var currentSize = _.head(imageSizes);

  if(!currentSize || !image) {
    return when(true);
  }

  originalImageKey = originalImageKey || image.Key;

  return resizeImage(currentSize, image)
    .then(S3.saveImage(UrlTransformer.getResizedImageKey(currentSize, originalImageKey)))
    .then(getImageThumbs(_.tail(imageSizes), originalImageKey))
    .catch(function(err) {
      console.log('error in getImageThumbs', err);
    });
});

// Does the actual resizing, using sharp
// https://www.npmjs.com/package/sharp
function resizeImage(currentSize, image) {
  return when.promise(function(resolve, reject) {
    var resizedImageString = currentSize.height + 'x' + currentSize.width;
    
    sharp(image.Body)
      .resize(currentSize.width, currentSize.height)
      .toBuffer(function(err,resizedImage, info) {
        image.Body = resizedImage;
        resolve(image);
      });
  });
}