Upload Files Using Pre-signed URLs Slack

Using pre-signed URLs, a client can upload files directly to an S3-compatible cloud storage server (S3) without exposing the S3 credentials to the user.

This guide describes how to use the presignedPutObject API from the MinIO JavaScript Library to generate a pre-signed URL. This is demonstrated through a JavaScript example in which an Express Node.js server exposes an endpoint to generate a pre-signed URL and a client-side web application uploads a file to MinIO Server using that URL.

  1. Create the Server

  2. Create the Client-side Web Application

1. Create the Server

The server consists of an Express Node.js server that exposes an endpoint called /presignedUrl. This endpoint uses a Minio.Client object to generate a short-lived, pre-signed URL that can be used to upload a file to Mino Server.

// In order to use the MinIO JavaScript API to generate the pre-signed URL, begin by instantiating
// a `Minio.Client` object and pass in the values for your server.
// The example below uses values for

const Minio = require('minio')

var client = new Minio.Client({
    endPoint: '',
    port: 9000,
    useSSL: true,
    accessKey: 'Q3AM3UQ867SPQQA43P2F',
    secretKey: 'zuf+tfteSlswRu7BJ86wekitnifILbZam1KYY3TG'

// Instantiate an `express` server and expose an endpoint called `/presignedUrl` as a `GET` request that
// accepts a filename through a query parameter called `name`. For the implementation of this endpoint,
// invoke [`presignedPutObject`]( 
// on the `Minio.Client` instance to generate a pre-signed URL, and return that URL in the response:

// express is a small HTTP server wrapper, but this works with any HTTP server
const server = require('express')()

server.get('/presignedUrl', (req, res) => {
    client.presignedPutObject('uploads',, (err, url) => {
        if (err) throw err

server.get('/', (req, res) => {
    res.sendFile(__dirname + '/index.html');


2. Create the Client-side Web Application

The client-side web application’s user interface contains a selector field that allows the user to select files for upload, as well as a button that invokes an onclick handler called upload:

<input type="file" id="selector" multiple>
<button onclick="upload()">Upload</button>

<div id="status">No uploads</div>

<script type="text/javascript">
  // `upload` iterates through all files selected and invokes a helper function called `retrieveNewURL`.
  function upload() {
        // Get selected files from the input element.
        var files = document.querySelector("#selector").files;
        for (var i = 0; i < files.length; i++) {
            var file = files[i];
            // Retrieve a URL from our server.
            retrieveNewURL(file, (file, url) => {
                // Upload the file to the server.
                uploadFile(file, url);

    // `retrieveNewURL` accepts the name of the current file and invokes the `/presignedUrl` endpoint to
    // generate a pre-signed URL for use in uploading that file: 
    function retrieveNewURL(file, cb) {
        fetch(`/presignedUrl?name=${}`).then((response) => {
            response.text().then((url) => {
                cb(file, url);
        }).catch((e) => {

    // ``uploadFile` accepts the current filename and the pre-signed URL. It then uses `Fetch API`
    // to upload this file to S3 at `` using the URL:
    function uploadFile(file, url) {
        if (document.querySelector('#status').innerText === 'No uploads') {
            document.querySelector('#status').innerHTML = '';
        fetch(url, {
            method: 'PUT',
            body: file
        }).then(() => {
            // If multiple files are uploaded, append upload status on the next line.
            document.querySelector('#status').innerHTML += `<br>Uploaded ${}.`;
        }).catch((e) => {

Note: This uses the File API, QuerySelector API, fetch API & Promise API.