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General Assembly Logo

Express Multer Upload API

In this lesson, we will learn how to perform a file upload to a remote server using node, express, and Amazon Web Services (AWS).



By the end of this lesson, students should be able to:

  • Send multi-part requests from client to a node api
  • Use aws-sdk to send files from our api to s3
  • Persist information about uploaded files
  • Create path names with low chance of duplication
  • Understand why files need be sent as multiple parts


  1. Fork and clone this repository. FAQ
  2. Create a new branch, training, for your work.
  3. Checkout to the training branch.
  4. Install dependencies with npm install.

Discussion: Uploading Files


  • What are the different steps of a file upload?
  • What are the issues to guard against?
  • How do we deal with a partial upload or a slow connection?

AWS s3 Buckets

Why Store Files In An AWS Bucket

The AWS s3 (storage cubed) service allows us to store files “the cloud” which is a single source of truth accessible via the internet. This means that files uploaded to s3 can be accessible to all of your app users. Changes to your files can easily be seen by everyone.

The bucket abstraction specifically lets us store files in a folder in the AWS cloud, as well as allow a specific and restrictive way of implementing access control to that folder (a policy).

In the ga-wdi-boston/aws-s3-setup-guide prerequisite, you went through some steps to allow us access to read and write to a bucket in the AWS s3 service. The CSV credentials that you downloaded at the end of that prereq is what will allow out command line and server applications permission to upload files to your AWS s3 bucket.

We'll build a command line script to upload a file to your AWS bucket. We'll use AWS.S3, specifically the upload method, to send files to AWS s3.

s3.upload Abstracts Away HTTP Requests

One important concept that we should understand is that the upload method abstracts away communicating with s3 using basic HTTP requests and responses, but in fact, the upload method does use HTTP requests to communicate with the server, and receives and returns HTTP response data to us.

s3.upload Requires A Stream Of File Data In It's params.Body

We have to consider the problem of how to send a large amount of data (a file) via a request to a server. Requests that do not contain large amounts of data can send in once chunk of information. However, requests that contains large amounts of data, such as file data, needs to communicate their data in multiple chunks. Part of the reason for this is because communication through the internet happens in pre-defined discrete chunks of data called packets that are of a set size, so a file that is larger than the size of a packet has to be split up into multiple chunks in order to be sent across the internet.

In order to send a file via s3.upload we will take advantage of fs.createReadStream which will load a file in chunks and provide them consecutively to some source. This way of providing file data is is called a stream. In this case, we will specify a stream as the Body parameter for the s3.upload function so it can send our files in chunks to s3.

Code-Along: Uploading Files To AWS From node

We will:

  • Install the aws-sdk and another tool called mime-types for this repo by running:

    npm install --save aws-sdk mime-types
  • Set up a .env file in the root of our repo to hold some environmental variables:

    BUCKET_NAME=<your AWS bucket name>
    ACCESS_KEY_ID=<access key id value from your credentials csv>
    SECRET_ACCESS_KEY=<secret access key value from your credentials csv>
  • Make a file in /lib called aws-s3-upload.js

  • Allow lib/aws-s3-upload.js to take two command line arguments:

    • The path to your file
    • The name the file will have once uploaded to the s3 bucket
  • Upload a file to s3 using the upload method from the AWS.S3 object, using a Promise to handle the asynchronous action of uploading using AWS.S3.upload

Uploading A File To AWS With Our Script

Once we are done coding, we can run the script using this format:

node lib/aws-s3-upload.js <file path> [file name]

For example, we could upload the image at /data/images/padawan.png by running:

node lib/aws-s3-upload.js ./data/images/padawan.png uploadedImage

Once the script runs successfully, you can go to your Amazon s3 Bucket and see that the file has been uploaded!

Uploading Files To AWS Through A Server

Even A Server Can Be A Client Sometimes

One of the great utilities of being able to upload files to AWS from the command line is that we can use this functionality within our servers to upload a user's files to the cloud.


  • What might this look like?
  • How would we keep track of which files were uploaded by each user to the cloud, as well as their urls in the cloud?

Let's take a minute to draw it on the board.

server client database s3 diagram

Building Our Server

This repo is already set up to run a standard express-api server as you've seen in previous lessons.

In order to set up this server to upload files to s3, we will need to:

  • Refactor our code from the last step so it can be imported as a module for our server
  • Keep track of uploaded file information in a mongo database by:
    • Write a model for Upload data
    • Write a route for Upload data

multer Makes It Easy To Send Files To An express Server

multer is a middleware for express that allows us to upload multi-part data to our server, such as file data. Just like we used fs.createReadStream to send files from a filesystem to s3, we can use multer to receive multi-part data (such as files) on an express server and save them in a folder on the server. Read more about multer usage here.

Code-Along: Refactoring lib/aws-s3-upload.js

We will:

  • Remove the parts of /lib/aws-s3-upload.js where the Promise is being run with .then and .catch methods
  • Refactor the inputs for the s3Upload function to take:
    • file - the path to the file
    • originalName - the name of the file being uploaded
    • newName - the name the file will have once uploaded
  • Export this function using module.exports so our server can use it

Code-Along: Adding An Upload Model

We will:

  • Make a file in /app/models called upload.js
  • Describe our Upload schema in /app/models/upload.js:
    • Required keys:
      • title - the original name of the file
      • url - the URL returned from a successful s3 upload
      • owner (reference key to User) - the user who the upload belongs to

Code-Along: Adding An Upload Route

We will:

  • Make a copy of the contents of /app/routes/example_routes.js in a new file called upload_routes.js in app/routes
  • Add in multer functionality to allow data sent in requests to be stored on the server, then uploaded to s3:
    • run npm install --save multer
    • require multer and set an upload folder to store files uploaded to the server
  • Modify the appropriate imports and variable names to work with Upload request
  • Use our s3Upload function in the POST route:
    • Require the function from /lib/aws-s3-upload.js
    • Add the logic to upload a file to s3 in the POST router
    • If the file is successfully uploaded, make a mongo Upload document with the data, then return that data in a JSON response to the client
    • If an error occurs at any point, handle the error using console.error
  • Add our route to server.js

Lab: Uploading Files To s3 Using curl Requests

Try and upload the image at /data/images/padawan.png to the server using the curl scripts in scripts/uploads. Remember, our upload routes are authenticated, so you'll need a token first.

NOTE: use /scripts/uploads/ to upload files. In order to send a file using a curl request, you must specify the path to the image with a @ in front of it. For example:

IMAGE_PATH=@./data/images/padawan.png ...

BONUS Lab: Uploading Files Via A Browser Client

So far we have uploaded files directly to s3 using node scripts, as well as via curl scripts sent to our custom server. In real world usage, we would most likely be sending our data to the server via a browser client. You will build a client that will be able to send an image to our server for uploading to s3 using html forms.

Break into your squads and work together for this lab. Choose one person's computer to write code on and work solely on that machine.

Using FormData Instead of getFormFields

getFormFields is not capable of grabbing file data from a form. For sending form data, including the title and file data, you will need to learn how to use the built-in FormData constructor instead. This data can be sent to your server using an jQuery.ajax but you must add the following keys to your jQuery.ajax settings:

contentType: false,
processData: false

You will:

  1. Download and set up browser-template
  2. Create sign-in and sign-up forms that allow you to sign up and save a token (feel free to poach code from your old projects)
  3. Integrate this form exactly as it is written into your HTML:
<form id="image-upload" enctype="multipart/form-data">  
    <legend>Image Upload Form</legend>
    <label> Title
      <input type="text" name="title">
    <label> Image
      <input type="file" name="image">
    <input type="submit" name="submit" value="Upload">
  1. Write event handlers to get the form data and send it to our server. Use the events.js and api.js pattern for doing this
  2. Submit an image via the form, which should upload it to the /uploads folder on your server, then send it to s3, log the transaction in the database, then return a response to the client

Bonus Questions

  • What does enctype="multipart/form-data" do in the form html?
  • How can we see the values being stored in a FormData object? It's not as easy as just logging it!
  • What do the contentType and processData keys do in the jQuery.ajax settings? What are their default values, and how could these prevent our upload from working?

Additional Resources


  1. All content is licensed under a CC­BY­NC­SA 4.0 license.
  2. All software code is licensed under GNU GPLv3. For commercial use or alternative licensing, please contact