Скачать или смотреть How to Ensure Proper File Upload Extensions in JavaScript: A Guide to Handling PDF Files

Discover how to validate file uploads in JavaScript by checking for the desired extensions. Learn best practices to ensure uploads meet your requirements, focusing on PDF file formats.
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This video is based on the question https://stackoverflow.com/q/71588709/ asked by the user 'marun' ( https://stackoverflow.com/u/14558787/ ) and on the answer https://stackoverflow.com/a/71588832/ provided by the user 'Hira Kumar Maharjan' ( https://stackoverflow.com/u/9821615/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Ensure Proper File Upload Extensions in JavaScript

When it comes to uploading files through web applications, one common challenge developers face is validating the file types. For instance, you might want to ensure that users can only upload PDF files. In this post, we’ll dive into how to check for file extensions during the upload process using JavaScript’s File API.

The Problem

Imagine you have a form for users to upload documents. The requirement is that only .pdf files should be uploaded. If a user tries to upload a different file type (like a .docx, .jpg, or any other), you want to alert them with a clear message. Here’s a simplified version of what the user's HTML form might look like:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

To effectively validate the file upload, we need to check the MIME type of the files, rather than relying solely on the file extension. The correct MIME type for PDF files is application/pdf. Below is how you can implement the validation logic using JavaScript.

Step-by-Step JavaScript Code Explanation

Here is the code you need:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of Key Segments

Element Selection:

We first select the file input element using document.getElementById() method.

Event Listener:

We add an event listener that triggers when the user selects a file. This is done using the "change" event.

File Size Check:

Inside the event listener, we loop through all selected files and check if any of them exceed the maximum file size (1MB or 1048576 bytes) and alert the user if they do.

MIME Type Verification:

We then check if the file’s MIME type does not match application/pdf. If it doesn't, an alert is shown to inform the user that the file type is not allowed.

Conclusion

By implementing the checks outlined above, you can effectively ensure that only PDF files are uploaded through your web forms. Validating file types not only enhances your application's functionality but also improves user experience by providing immediate feedback when an invalid file is attempted to be uploaded.

This method addresses both file size and type validation, making it a comprehensive solution for handling file uploads in your JavaScript applications.

Remember, clear and precise validation messages are essential in guiding your users through a smooth file upload experience. Happy coding!