Uploading files from frontend in Supabase

Introduction

Uploading files from the frontend lets users send pictures, documents, or other files to your app. This helps store and use user data easily.

When users need to upload profile pictures.

When users submit documents or reports.

When users share images or videos in a chat app.

When you want to save user-generated content like drawings or notes.

Syntax

Supabase

const { data, error } = await supabase.storage
  .from('bucket-name')
  .upload('path/to/file.ext', fileObject);

supabase.storage.from('bucket-name') selects where to store the file.

.upload('path/to/file.ext', fileObject) uploads the file to that path.

Examples

Uploads a profile picture named profile.png for user 123 into the avatars bucket.

Supabase

const { data, error } = await supabase.storage
  .from('avatars')
  .upload('user123/profile.png', file);

Uploads a PDF report named report1.pdf into the documents bucket.

Supabase

const { data, error } = await supabase.storage
  .from('documents')
  .upload('reports/report1.pdf', file);

Sample Program

This example shows how to upload a file selected by the user from an HTML file input. It uploads the file to the user-files bucket under the uploads/ folder. If successful, it logs and alerts the file path.

Supabase

import { createClient } from '@supabase/supabase-js';

const supabaseUrl = 'https://xyzcompany.supabase.co';
const supabaseKey = 'public-anonymous-key';
const supabase = createClient(supabaseUrl, supabaseKey);

async function uploadFile(file) {
  const filePath = `uploads/${file.name}`;
  const { data, error } = await supabase.storage
    .from('user-files')
    .upload(filePath, file);

  if (error) {
    console.error('Upload error:', error.message);
    return null;
  }

  console.log('File uploaded at:', data.path);
  return data.path;
}

// Example usage with a file input element
const fileInput = document.querySelector('#fileInput');
fileInput.addEventListener('change', async (event) => {
  const file = event.target.files[0];
  if (file) {
    const path = await uploadFile(file);
    alert(`File uploaded to: ${path}`);
  }
});

Important Notes

Make sure the bucket (user-files in the example) exists in your Supabase storage.

File names should be unique or you risk overwriting existing files.

Check for errors to handle upload failures gracefully.

Summary

Uploading files from frontend lets users send files to your app storage.

Use supabase.storage.from(bucket).upload(path, file) to upload.

Always handle errors and confirm uploads to improve user experience.

Practice

1. What does the supabase.storage.from(bucket).upload(path, file) method do in a frontend app?

easy

A. Downloads a file from the storage bucket

B. Uploads a file to the specified storage bucket at the given path

C. Deletes a file from the storage bucket

D. Lists all files in the storage bucket

Solution

1. Step 1: Understand the method purpose

   The method upload(path, file) is used to send a file from the frontend to the backend storage.

2. Step 2: Identify the action on storage

   Since it uses upload, it means adding or saving a file, not deleting or downloading.

3. Final Answer:

   Uploads a file to the specified storage bucket at the given path -> Option B

4. Quick Check:

   Upload method = Upload file [OK]

Hint: Upload method means sending file to storage [OK]

Common Mistakes:

• Confusing upload with download or delete
• Thinking it lists files instead of uploading
• Ignoring the bucket and path parameters

2. Which of the following is the correct syntax to upload a file named myFile to the bucket avatars at path user1/profile.png using Supabase storage in frontend JavaScript?

easy

A. supabase.storage.from('avatars').upload('user1/profile.png', myFile)

B. supabase.storage.upload('avatars', 'user1/profile.png', myFile)

C. supabase.storage.from('avatars').upload(myFile, 'user1/profile.png')

D. supabase.upload.storage.from('avatars').file('user1/profile.png', myFile)

Solution

1. Step 1: Check method chaining and parameters

   The correct method chain is supabase.storage.from(bucket).upload(path, file).

2. Step 2: Match parameters order

   The first argument to upload is the path string, second is the file object.

3. Final Answer:

   supabase.storage.from('avatars').upload('user1/profile.png', myFile) -> Option A

4. Quick Check:

   Correct method and parameter order = supabase.storage.from('avatars').upload('user1/profile.png', myFile) [OK]

Hint: Remember: from(bucket).upload(path, file) [OK]

Common Mistakes:

• Swapping file and path arguments
• Using upload directly on storage without from()
• Incorrect method chaining order

3. Given this code snippet, what will be logged if the upload succeeds?

const { data, error } = await supabase.storage.from('docs').upload('file.txt', fileInput.files[0]);
if (error) {
  console.log('Upload failed');
} else {
  console.log('Upload succeeded:', data);
}

medium

A. Upload succeeded: { path: 'file.txt', ... }

B. SyntaxError due to await usage

C. Upload succeeded: undefined

D. Upload failed

Solution

1. Step 1: Understand the upload response

   On success, data contains info about the uploaded file, including its path.

2. Step 2: Check error handling

   If error is null, the else block runs and logs success with data.

3. Final Answer:

   Upload succeeded: { path: 'file.txt', ... } -> Option A

4. Quick Check:

   Success logs data object = Upload succeeded: { path: 'file.txt', ... } [OK]

Hint: Success logs data object, error logs failure [OK]

Common Mistakes:

• Assuming data is undefined on success
• Confusing error presence with success
• Thinking await causes syntax error outside async

4. You wrote this code to upload a file but it always logs 'Upload succeeded' even though the file wasn't uploaded. What is the likely mistake?

const { data, error } = supabase.storage.from('images').upload('pic.png', file);
if (error) {
  console.log('Upload failed');
} else {
  console.log('Upload succeeded');
}

medium

A. Wrong bucket name 'images'

B. File variable 'file' is not defined

C. Missing await before supabase.storage.from(...).upload(...)

D. Upload method does not exist on storage

Solution

1. Step 1: Check async usage

   The upload method returns a promise and must be awaited to get data and error.

2. Step 2: Identify missing await effect

   Without await, { data, error } destructures the unresolved Promise, leaving both undefined. Thus if (error) is false, incorrectly logging success.

3. Final Answer:

   Missing await before supabase.storage.from(...).upload(...) -> Option C

4. Quick Check:

   Async calls need await to get results [OK]

Hint: Always await async upload calls [OK]

Common Mistakes:

• Not awaiting the upload promise
• Assuming upload is synchronous
• Ignoring file variable existence

5. You want to upload multiple files selected by the user to the bucket uploads and keep their original filenames. Which approach correctly uploads all files and handles errors properly?

hard

A. Use upload with a fixed filename for all files to overwrite previous uploads

B. Upload all files at once by passing the array to upload method

C. Upload files without awaiting and ignore errors for faster UI

D. Use a loop with await inside to upload each file and check errors after each upload

Solution

1. Step 1: Understand multiple file uploads

   Supabase storage upload handles one file at a time, so loop over files.

2. Step 2: Handle async and errors properly

   Use await inside the loop to wait for each upload and check for errors to inform the user.

3. Final Answer:

   Use a loop with await inside to upload each file and check errors after each upload -> Option D

4. Quick Check:

   Upload files one by one with await and error check [OK]

Hint: Upload files one by one with await and check errors [OK]

Common Mistakes:

• Passing array directly to upload method
• Not awaiting uploads causing race conditions
• Overwriting files by using same filename