Simple example of sending files (images in my case) with jQuery (or HTML form) to a NodeJS Express backend using Multer

index.html

// Code for client
// Should be put inside a folder called 'public', so Express serves it (see line 12 in index.js)

<input id="hiddenFileInput" style="display:none;" type="file" multiple>
<button id="button">Upload files</button>

<script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
<script type="text/javascript">    
    $('#button').on('pointerdown', () => {
        $('#hiddenFileInput').click(); //Used so we can use a custom button for file upload
    })

    $('input[type=file]').change(function () {
        var files = this.files;
        var fieldNameImportantForMulter = 'media';

        var formData = new FormData();
        for (var i = 0; i < files.length; i++) {
            var file = files[i];
            formData.append(fieldNameImportantForMulter, file, file.name);
        }

        // Via https://stackoverflow.com/a/5976031/3763660
        $.ajax({
            cache: false,
            contentType: false,
            processData: false,
            method: 'POST',
            url : '/api',
            data : formData,
            success : function(response){
                console.log('Success', response);
            },

            error : function(error){
                console.log('Error', error);
            }
        })
    });
</script>

index.js

// NodeJS code – you have to npm install Express and Multer before it works
const express = require('express');
// BodyParser does not parse "multipart/form-data" which multiple files are (https://stackoverflow.com/a/23599340/3763660)
var multer  = require('multer'); // Multer is a wrapper for express built on top of busboy for "multipart/form-data"
var upload = multer();

const app = express()
const port = 3000
const fieldNameSpecifiedInForm = 'media';
const maxNumberOfFiles = 40;

app.use('/', express.static('public')); //from https://expressjs.com/en/starter/static-files.html

app.post('/api', upload.array(fieldNameSpecifiedInForm, maxNumberOfFiles), function (req, res, next) {
  // req.files is array of `media` files
  // req.body will contain the text fields, if there were any
  console.log(req.files, req.body);
  let files = req.files;
  for(let index in files){
  	let file = files[index];
  	console.log(file.originalname)
  }
  res.send("Data received");
})

app.listen(port, () => {
  console.log(`Example app listening at http://localhost:${port}`)
})

index_without_javascript.html

// Code for client (alternative)
// Does the same thing as index.html but with HTML only
<form action="/api" enctype="multipart/form-data" method="post">
    <input type="file" name="media" multiple>
    <input type="submit" value="Send data">
</form>