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Uploading Files Using JavaScript and AJAX


File uploading is a crucial functionality in many web applications. It allows users to share documents, images, videos, and other files with each other or with the application itself. Whether it's submitting a resume for a job application, uploading a profile picture, or sharing a document for collaboration, file uploading is a common requirement in modern web development.

Traditionally, file uploads were done synchronously, meaning that the user had to wait for the entire file to be uploaded before they could continue using the application. This could be time-consuming and frustrating, especially for larger files. Asynchronous file uploading, on the other hand, allows the user to continue interacting with the application while the file is being uploaded in the background. This significantly improves the user experience by making the process more seamless and responsive.

By using JavaScript and AJAX, we can take advantage of asynchronous file uploading and provide a more efficient and user-friendly experience. In this article, we will explore the various techniques and best practices for uploading files using JavaScript and AJAX.

Handling File Selection

When it comes to handling file selection in web applications, JavaScript provides a convenient way to capture user-selected files. This allows us to access and manipulate the files before uploading them to the server.

To capture user-selected files, we can make use of the HTML input element with the type attribute set to "file". This element provides a file picker dialog that allows users to browse their local file system and select one or multiple files.

Once a file is selected, JavaScript provides the File object, which represents the selected file. This object contains various properties that provide information about the file, such as its name, size, and type. These properties can be accessed and used to perform validations or display file information to the user.

To read the contents of the selected file, we can utilize the FileReader API provided by JavaScript. This API allows us to asynchronously read the contents of a file using methods such as readAsText(), readAsDataURL(), or readAsArrayBuffer(). The readAsText() method, for example, reads the file as plain text, while readAsDataURL() returns the file's contents as a data URL, which can be used to display an image or preview the file.

By utilizing the File object and the FileReader API, we can easily handle file selection and retrieve information or read the contents of the selected file in JavaScript before proceeding with the file upload process.

Tracking Upload Progress

When uploading files using JavaScript and AJAX, it is important to provide feedback to the user on the progress of the upload. This helps improve the user experience and provides transparency during the upload process.

To track the progress of file uploads, we can use the XMLHttpRequest object for making AJAX requests. The XMLHttpRequest object provides a way to send HTTP requests and receive responses from a server asynchronously.

To track the progress of an upload, we can listen for the progress event on the XMLHttpRequest object. This event is fired periodically during the upload, allowing us to retrieve information about the progress, such as the total bytes to be sent and the number of bytes that have been sent.

var xhr = new XMLHttpRequest();'POST', '/upload', true);
xhr.upload.addEventListener('progress', function(event) {
  var percent = (event.loaded / * 100;
  // Update the progress bar or display the percentage completed
}, false);

In the above code snippet, we create a new instance of XMLHttpRequest and open a POST request to the /upload endpoint. We then attach an event listener to the upload property of the XMLHttpRequest object and listen for the progress event. Inside the event listener, we can calculate the percentage of the upload completed and update the progress bar or display the percentage to the user.

To visually display the upload progress to the user, we can use various techniques, such as updating a progress bar or showing a percentage completed. This can be done by manipulating the DOM or using a library that provides pre-built components for displaying progress indicators.

By tracking the upload progress and providing visual feedback to the user, we can enhance the overall user experience and make the file upload process more intuitive and transparent.

Server-side Handling

When it comes to handling file uploads on the server-side, there are various approaches you can take depending on your technology stack. Here is an overview of how you can handle file uploads on the server-side:

Server-side Technologies and Frameworks

There are several server-side technologies and frameworks that you can use to handle file uploads, such as:

  • Node.js: Node.js is a popular JavaScript runtime that allows you to build scalable and high-performance server-side applications. You can use packages like Multer or Formidable to handle file uploads in Node.js.

  • PHP: PHP is a widely-used server-side scripting language that is specially designed for web development. PHP provides built-in functions like move_uploaded_file() and libraries like Symfony's HttpFoundation to handle file uploads.

  • ASP.NET: ASP.NET is a web application framework developed by Microsoft. You can use ASP.NET's built-in classes like HttpPostedFile or third-party libraries like FileHelpers to handle file uploads in ASP.NET.

Receiving and Processing File Uploads

To receive and process file uploads on the server-side, you need to follow these general steps:

  1. Handle the incoming request: Receive the file upload request on the server-side using the appropriate route or endpoint.

  2. Retrieve the uploaded file: Access the uploaded file from the request object using the appropriate property or method provided by your server-side technology.

  3. Validate the file: Perform any necessary validation checks on the uploaded file, such as checking file size, file type, or any other specific requirements.

  4. Store the file: Save the uploaded file to a permanent location on the server or in a database, depending on your requirements. Make sure to handle any naming conflicts or security considerations when storing the file.

  5. Respond to the client: Send a response back to the client-side indicating the success or failure of the file upload operation.

The specific implementation details may vary depending on the server-side technology or framework you are using. It is important to refer to the official documentation or resources specific to your chosen technology for more detailed instructions.

By following these steps, you will be able to successfully handle file uploads on the server-side and process the uploaded files according to your application's needs.


In this article, we have explored the process of uploading files using JavaScript and AJAX. Let's recap the key points we have covered:

  • We discussed the importance of file uploading in web applications and why uploading files asynchronously is beneficial. Asynchronous file uploading allows users to continue interacting with the web application while the file is being uploaded in the background.

  • We learned how to handle file selection using JavaScript. By using the File object and the FileReader API, we can capture user-selected files and read their contents.

  • We explored how to track the progress of file uploads using the XMLHttpRequest object and the progress event. This allows us to display the upload progress to the user in a visually appealing manner, providing feedback on the status of the upload.

  • We briefly discussed the server-side handling of file uploads. Different server-side technologies and frameworks like Node.js, PHP, and ASP.NET can be used to receive and process file uploads on the server.

In conclusion, providing a seamless file upload experience for users is crucial for a positive user experience. By implementing JavaScript and AJAX techniques, we can create a smooth and efficient file uploading process. I encourage you to further explore advanced file uploading techniques and best practices to enhance your web applications.

For more information on JavaScript, AJAX, and file upload techniques, refer to the relevant documentation and resources available online.