exports.js

// dependencies
var async = require('async')
var AWS = require('aws-sdk')
var gm = require('gm').subClass({ imageMagick: true }) // Enable ImageMagick integration.

// constants
var MAX_WIDTH = 100
var MAX_HEIGHT = 100

// get reference to S3 client
var s3 = new AWS.S3()

exports.handler = function (event, context) {
  // Read options from the event.
  var srcBucket = event.Records[0].s3.bucket.name
  var srcKey = event.Records[0].s3.object.key
  var dstBucket = srcBucket + '-thumb'
  var dstKey = srcKey

  // Sanity check: validate that source and destination are different buckets.
  if (srcBucket === dstBucket) {
    console.error('Destination bucket must not match source bucket.')
    return
  }

  // Infer the image type.
  var typeMatch = srcKey.match(/\.([^.]*)$/)
  if (!typeMatch) {
    console.error('unable to infer image type for key ' + srcKey)
    return
  }

  var validImageTypes = ['png', 'jpg', 'jpeg', 'gif']
  var imageType = typeMatch[1]
  if (validImageTypes.indexOf(imageType.toLowerCase()) < 0) {
    return
  }

  // Download the image from S3, transform, and upload to a different S3 bucket.
  async.waterfall([
    function download (next) {
      // Download the image from S3 into a buffer.
      s3.getObject({
        Bucket: srcBucket,
        Key: srcKey
      }, next)
    },
    function tranform (response, next) {
      gm(response.Body).size(function (err, size) {
        // Infer the scaling factor to avoid stretching the image unnaturally.
        var scalingFactor = Math.min(
          MAX_WIDTH / size.width,
          MAX_HEIGHT / size.height
        )
        var width = scalingFactor * size.width
        var height = scalingFactor * size.height

        // Transform the image buffer in memory.
        this.resize(width, height)
          .toBuffer(imageType, function (err, buffer) {
            if (err) {
              next(err)
            } else {
              next(null, response.ContentType, buffer)
            }
          })
      })
    },
    function upload (contentType, data, next) {
      // Stream the transformed image to a different S3 bucket.
      s3.putObject({
        Bucket: dstBucket,
        Key: dstKey,
        Body: data,
        ContentType: contentType,
        ACL: 'public-read'
      }, next)
    }],
    function (err) {
      if (err) {
        console.error(
          'Unable to resize ' + srcBucket + '/' + srcKey +
          ' and upload to ' + dstBucket + '/' + dstKey +
          ' due to an error: ' + err
        )
        context.done()
      } else {
        context.done()
      }
    }
  )
}