How to Validate File Size Before Uploading with JavaScript?

How to Validate File Size before Uploading Using JavaScript

In web development, it's often necessary to validate user-submitted files before uploading them to a server. One important validation check is ensuring the file size does not exceed specified limits. JavaScript provides the File API, which allows you to inspect file properties, including size.

To validate file size using JavaScript, you can use the following steps:

1. Check Browser Support: Verify that the File API is supported by the browser.
2. Get File Object: Retrieve the file object using the element.
3. Read File Size: Use the File.size property to obtain the size of the file in bytes.
4. Compare Size: Check if the file size exceeds the specified threshold. If it does, display an error message or prevent the file from being uploaded.
5. Display Result: Notify the user with an appropriate message, either indicating successful validation or an error.

Here's an example JavaScript code that demonstrates file size validation:

// Get the input element
const input = document.getElementById('fileinput');

// Add event listener for selecting a file
input.addEventListener('change', function() {
  // Verify browser support
  if (!window.FileReader) {
    console.log("File API not supported");
    return;
  }

  // Get the first selected file
  const file = input.files[0];

  // Validate file size
  const maxSize = 5 * 1024 * 1024; // 5MB
  if (file.size > maxSize) {
    alert("File exceeds the maximum size of 5MB");
  } else {
    alert("File size is valid");
  }
});

By implementing this validation, you can prevent users from uploading excessively large files, ensuring server resources are not used excessively.

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