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Discover how to easily `upload files` from your frontend to backend using React, TypeScript, and Express with a step-by-step guide.
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How to Upload Files from Frontend to Backend in a Full-Stack Web App

If you're developing a full-stack web application, one functionality that you will likely need is the capability to upload files from the frontend to the backend. From allowing users to send documents to processing data files, file uploads can be a fundamental aspect of your app. In this guide, we'll explore how to implement file uploads using React on the frontend and Express on the backend, specifically with TypeScript and Axios.

The Problem

As part of a thesis project, you might find yourself needing to send CSV files from the frontend to your backend application. In this instance, we have a scenario where a function is set up but encounters a file validation error when attempting to upload the file to the server. This results in a 400 Bad Request response. Let's examine what could be going wrong.

Analyzing the Frontend Functionality

The function designed to send the CSV file operates as follows:

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

This function uses Axios to post the form data to the backend. The important part here is ensuring that the form data is set up correctly. Now let's look at how the onSubmit function calls the upload function.

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

The Issue

The issue arises because dataset.file is actually a FileList, which means it contains a list of files. To upload a file correctly, you need to access the first file in the list, rather than trying to send the whole collection. This discrepancy is what causes the file validation to fail on the backend.

The Solution

To solve the issue, you have to modify the onSubmit function so that it correctly extracts the file from the FileList. Here’s the revised code:

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

Key Points to Remember

FileList Handling: Always check if the uploaded file is a FileList. If it is, access the specific file using index [0] to avoid errors in file validation on the backend.

Error Handling: Ensure that your backend correctly handles the received file data and validate that it is processed as required.

Backend Implementation

Lastly, let's take a brief overview of the backend Express code handling the file upload:

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

Make sure the middleware is in place to validate the uploaded file and that the controller is properly processing it. This complete chain of function should be coherent and correctly configured to enable smooth file uploads.

Conclusion

Implementing file uploads in a full-stack application can involve navigating details and handling various types of data. This guide has covered the necessary steps and provided solutions for common mistakes when uploading files from the frontend to the backend. Always ensure you validate file types and manage FileLists appropriately to prevent errors.



Now you have a clearer path for uploading files in your upcoming web applications. Happy coding!