HTML5 Multiple File Upload AJAX Using Fetch API

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Frontend File Uploads: Mastering HTML5 Multiple Upload AJAX with Fetch API

Welcome to the start of an exciting, beginner-friendly series! Dealing with **file uploads** is a fundamental task in web development, and modern techniques make it far more powerful and user-friendly than the old-school methods.

In this tutorial, we’re going to dive deep into the **frontend** mechanics of creating a **multiple file upload** system using **HTML5**, the **Fetch API** for **AJAX**, and a touch of **Drag and Drop** functionality. This will lay the groundwork for a robust system, with the backend processing to follow in our next post!

Let’s get started on building a sleek and functional file upload interface.

The HTML5 Foundation

Every modern web application starts with solid **HTML**. For file uploads, the core is a simple <input type="file"> element with two crucial attributes:

1. multiple: This tells the browser to allow the user to select more than one file.
2. accept: This helps restrict the types of files the user can select (e.g., image/* for all image types).

Here’s the basic HTML structure we’ll use:

<div id="drop-area">
    <p>Drag and Drop files here, or click to select files</p>
    <input type="file" id="fileElem" multiple accept="image/*" style="display: none;">
    <label for="fileElem" class="button">Select Files</label>
    <progress id="progressBar" value="0" max="100" style="display:none;"></progress>
</div>
<div id="status"></div>

We’ve added a few extras:

• #drop-area: This is the main container that will receive the dropped files.
• The <input> is hidden (style="display: none;") and triggered by a user-friendly <label> acting as a button.
• #progressBar: A simple HTML5 element to show upload progress.
• #status: A space for messages (like “Upload Complete!”).

Adding Drag and Drop Functionality

To make this user-friendly, we’ll implement **Drag and Drop**. This requires listening for a few specific **JavaScript** events on our #drop-area:

• dragenter and dragleave: To change the area’s visual style when a user drags a file over it.
• dragover: To prevent the browser’s default behavior (which is to open the file).
• drop: The key event where we actually access the files.

The core code for handling the drop event looks like this:

const dropArea = document.getElementById('drop-area');
const fileInput = document.getElementById('fileElem');

// Prevent default drag behaviors
['dragenter', 'dragover', 'dragleave', 'drop'].forEach(eventName => {
  dropArea.addEventListener(eventName, preventDefaults, false);
});

function preventDefaults(e) {
  e.preventDefault();
  e.stopPropagation();
}

// Handle dropped files
dropArea.addEventListener('drop', handleDrop, false);

function handleDrop(e) {
  const dt = e.dataTransfer;
  const files = dt.files;

  // Pass the files to the upload function
  uploadFiles(files);
}

// Also trigger upload when the hidden input changes
fileInput.addEventListener('change', (e) => {
    uploadFiles(e.target.files);
});

When a file is dropped or selected, we pass the collection of files to our main uploadFiles function.

AJAX Upload using the Fetch API

The **Fetch API** is the modern, promise-based standard for making network requests, replacing older XMLHttpRequest techniques for most tasks. To send files, we must use the **FormData** object. This object easily constructs a set of key/value pairs representing form fields and their values, including the file data.

Here is the uploadFiles function, focusing purely on the frontend preparation and sending:

function uploadFiles(files) {
    const formData = new FormData();
    const statusDiv = document.getElementById('status');
    const progressBar = document.getElementById('progressBar');

    // 1. Append all selected files to the FormData object
    // Note: The key name 'uploaded_files' must match what the backend expects.
    for (let i = 0; i < files.length; i++) {
        formData.append('uploaded_files[]', files[i]);
    }

    // 2. Configure the Fetch request
    // IMPORTANT: Replace '/upload-endpoint' with your actual backend URL.
    fetch('/upload-endpoint', {
        method: 'POST',
        // Fetch automatically sets the 'Content-Type' header correctly 
        // for FormData, so we don't need to manually set it!
        body: formData 
    })
    .then(response => {
        // 3. Handle the response (This is where the next tutorial will focus!)
        if (response.ok) {
            statusDiv.innerHTML = 'Upload successful!';
            progressBar.style.display = 'none';
        } else {
            statusDiv.innerHTML = 'Upload failed!';
            progressBar.style.display = 'none';
        }
        return response.json(); // Assuming the backend returns JSON
    })
    .catch(error => {
        // 4. Handle network errors
        statusDiv.innerHTML = `Error: ${error.message}`;
        progressBar.style.display = 'none';
        console.error('Upload Error:', error);
    });

    // Display a loading message and the progress bar (though progress tracking 
    // is more complex and often handled by the XHR object, 
    // for this simple fetch, we just show the bar).
    statusDiv.innerHTML = 'Uploading...';
    progressBar.style.display = 'block';
    progressBar.value = 50; // Simple placeholder progress
}

Next Steps: The Backend!

And that’s it for the frontend! You now have a working **HTML5 Drag and Drop interface** that collects multiple files and sends them to a backend endpoint using the modern **Fetch API**.

In the **next tutorial**, we will focus on the **backend**, showing you how to correctly receive the FormData object and process (save, validate, resize, etc.) the uploaded_files on the server using a language like PHP, Node.js, or Python.

Screenshots and Screencast

Keep Learning JavaScript!

If you’re eager to deepen your understanding of the language that powers the frontend-**JavaScript**-I have resources designed just for you:

• **Book:** Check out my book, **”Learning JavaScript: A Programming Beginner’s Guide,”** available now! Grab your copy on Amazon today.
• **Course:** Prefer a guided course format? The comprehensive **”Learning JavaScript”** course covers everything from basics to advanced topics. Enroll in the course here.
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