Streaming file from S3 with Express including information on length and filetype

One approach is listening the httpHeaders event and creating a stream within it.

s3.getObject(params)    .on('httpHeaders', function (statusCode, headers) {        res.set('Content-Length', headers['content-length']);        res.set('Content-Type', headers['content-type']);        this.response.httpResponse.createUnbufferedStream()            .pipe(res);    })    .send();

For my project, I simply do a headObject in order to retrieve the object metadata only (it's really fast and avoid to download the object). Then I add in the response all the headers I need to propagate for the piping:

    var s3 = new AWS.S3();    var params = {        Bucket: bucket,        Key: key    };    s3.headObject(params, function (err, data) {        if (err) {            // an error occurred            console.error(err);            return next();        }        var stream = s3.getObject(params).createReadStream();        // forward errors        stream.on('error', function error(err) {            //continue to the next middlewares            return next();        });        //Add the content type to the response (it's not propagated from the S3 SDK)        res.set('Content-Type', mime.lookup(key));        res.set('Content-Length', data.ContentLength);        res.set('Last-Modified', data.LastModified);        res.set('ETag', data.ETag);        stream.on('end', () => {            console.log('Served by Amazon S3: ' + key);        });        //Pipe the s3 object to the response        stream.pipe(res);    });

Building on André Werlang's answer, we have done the following to augment AWS Request objects with a forwardToExpress method:

const _ = require('lodash');const AWS = require('aws-sdk');AWS.Request.prototype.forwardToExpress = function forwardToExpress(res, next) {    this    .on('httpHeaders', function (code, headers) {        if (code < 300) {            res.set(_.pick(headers, 'content-type', 'content-length', 'last-modified'));        }                                })    .createReadStream()    .on('error', next)    .pipe(res);};    

Then, in our route handlers, we can do something like this:

s3.getObject({Bucket: myBucket, Key: myFile}).forwardToExpress(res, next);