Jquery and HTML FormData returns "Uncaught TypeError: Illegal invocation"

Quick Cleanup: Syntax Check

Sometimes it's not about complex issues. It's about the little, noticeable things. Here are some quick common mistakes and their solutions:

Validate your selector

Hit the right target with your jQuery selector:

let formElement = $('#yourForm')[0]; // Your target practice with JQuery selectors!
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Correctly retrieve input values

Use .val() correctly. Copy-paste with caution!

let inputValue = $('#yourInputField').val(); // Everyone forgets, you don't!
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Validate FormData add-ons

Double-check your FormData key-value pairs as these are the messengers of successful data transmission:

formData.append('key', 'value'); // Just like sticking a note on the office billboard 
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Check other potential hiccups

AJAX request errors do creep in from syntactic issues unrelated to FormData. So, wear your detective hat and hunt 'em down.

When files attack: Multiple file uploads

When dealing with multiple files it's advantageous to suit up each file with its own FormData object. This gives every file its autonomous treatment:

$('input[type=file]').on('change', function() {
  const files = this.files;
  for (let i = 0; i < files.length; i++) {
    let formData = new FormData();
    formData.append('file', files[i]);
    
    // AJAX POST Request: The super express delivery
    $.ajax({
      url: 'upload.php',
      type: 'POST',
      data: formData,
      contentType: false,
      processData: false,
      success: response => console.log(`File ${i} uploaded like a moonshot!`, response),
      error: (jqXHR, textStatus, errorThrown) => console.error('Upload Error: Houston, we have a problem', textStatus, errorThrown)
    });
  }
});
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Doing this averts common file handling errors and is especially useful when uploading images or docs in bulk, or shipping heavy artillery (if you're a viking).

ProTips: Level Up!

Implement async uploads

Asynchronous uploads spruce up user experience by keeping the app's wheels spinning while the cargo is being shipped:

async function uploadFile(file) {
  // Rest of your upload logic does a tango here
}
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Cover yourself, use error handling

Use “umbrella” error handling to keep the rain of network issues, file size limits, or oddball response structures at bay. Keeps the app's health sunny!

Communicate with users

Let the users quench their curiosity with real-time updates on upload progress or success. Good relationships are built on good communication, you know!

xhr.upload.onprogress = function(event) {
  let percentComplete = (event.loaded / event.total) * 100;
  // Update the UI with the progress. It's like tracking your amazon package!
};
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Security first!

Double check that your server-side scripts run a pat-down security check on the uploaded files. We're trying to keep the malware mob at bay after all.