Client Checking file size using HTML5?

Confirming file selection before verification

Guarantee that a file has been chosen prior to validating its size:

document.querySelector('#fileInput').addEventListener('change', function() {
  if (this.files.length === 0) {
    alert('No file selected! Yet, here you are, asking for its size? 🙃');
    return;
  }
  // go ahead with size verification...
});


Attaching this event listener ensures error-free runtime in absence of a file selection.

Catering to file information

Reporting auxiliary details, specifically the file name and type could be relevant:

const fileInput = document.getElementById('fileInput');
fileInput.addEventListener('change', function () {
  const file = this.files[0];
  const fileSize = file.size; // the anticipated file size
  const fileName = file.name; // doesn't matter if it's "super_secret_identity.docx"
  const fileType = file.type; // could be PDF, TXT, or crypto asset. We never know!

  // Start processing the file details...
});


This comprehensive approach keeps users informed about their file's properties providing a more thorough user experience.

A leap to HTML5 and JavaScript

Transition from Flash and implement HTML5 paired with JavaScript libraries for more modern and efficient file handling and validation. Tools like jQuery simplify this:

$('#fileInput').on('change', function(e) {
  const fileSize = e.target.files[0].size; // Fetching the file size - easy, right?
  // process the file size here...
});


Such an approach gives a more unified, user-friendly experience that's contemporary and easy to manage.

User-friendly approach with changeable file size units

Adding a feature that allows users to choose the unit of file size for display, i.e., Bytes, KB, MB, GB enhances usability:

function formatFileSize(size, unit) {
  // Convert the file size to the unit chosen (Bytes, KB, MB, GB)
  // code to convert the size goes here...
}

// UI element for unit selection goes here...


This added feature showcases versatility and consideration for a broad user base, hitting the right notes of functionality.

Taking into account data transfer security

Server-side security checks should supplement the client-side validation, to get a robust data transfer and increased security.

// Server-side script example
if ($_FILES['file']['size'] > 1048576) {
  // manage the error - maybe, send a message to the client
}


Deploying both these checks, i.e., server-side and client-side, supplies a safe and dependable file handling system.

Keep updated with HTML5 improvements

Stay updated with the latest HTML5 features, investing time to understand and put to use these methods to deliver cutting-edge answers.

Try to avoid cross-browser issues

Remember to test your solution across browsers and consider implementing polyfills for features not consistently supported across platforms.