Skip to main content
Production servers often read, upload, and write files to S3-compatible object storage services instead of the local filesystem. Historically, that means local filesystem APIs you use in development can’t be used in production. When you use Bun, things are different.

Bun’s S3 API is fast

Bun's S3 API is fast

Left: Bun v1.1.44. Right: Node.js v23.6.0

Bun provides fast, native bindings for interacting with S3-compatible object storage services. Its S3 API resembles fetch’s Response and Blob APIs (like Bun’s local filesystem APIs).
s3.ts
S3 is the de facto standard internet filesystem. Bun’s S3 API works with S3-compatible storage services like:
  • AWS S3
  • Cloudflare R2
  • DigitalOcean Spaces
  • MinIO
  • Backblaze B2
  • …and any other S3-compatible storage service

Basic Usage

Bun.S3Client & Bun.s3

Bun.s3 is equivalent to new Bun.S3Client(), relying on environment variables for credentials. To explicitly set credentials, pass them to the Bun.S3Client constructor.
s3.ts

Working with S3 Files

The file method in S3Client returns a lazy reference to a file on S3.
s3.ts
Like Bun.file(path), the S3Client’s file method is synchronous. It makes no network requests until you call a method that needs one.

Reading files from S3

S3File extends Blob, so the same methods that work on Blob also work on S3File.
s3.ts

Memory optimization

Methods like text(), json(), bytes(), or arrayBuffer() avoid duplicating the string or bytes in memory when possible. If the text happens to be ASCII, Bun transfers the string directly to JavaScriptCore (the engine) without transcoding and without duplicating it in memory. .bytes() and .arrayBuffer() also avoid duplicating the bytes in memory.

Writing & uploading files to S3

Writing to S3 works the same way.
s3.ts

Working with large files (streams)

Bun automatically handles multipart uploads for large files and supports streaming. The same API that works for local files also works for S3 files.
s3.ts

Presigning URLs

When your production service needs to let users upload files to your server, it’s often more reliable for the user to upload directly to S3 instead of your server acting as an intermediary. To do this, presign URLs for S3 files. Presigning generates a URL with a signature that lets a user upload that specific file to S3, without exposing your credentials or granting them unnecessary access to your bucket. By default, Bun generates a GET URL that expires in 24 hours.
s3.ts

Setting ACLs

To set an ACL (access control list) on a presigned URL, pass the acl option:
s3.ts
You can pass any of the following ACLs:

Expiring URLs

To set an expiration time for a presigned URL, pass the expiresIn option.
s3.ts

method

To set the HTTP method for a presigned URL, pass the method option.
s3.ts

new Response(S3File)

To redirect users to a presigned URL for an S3 file, pass an S3File instance to a Response object as the body. The response redirects the user to a presigned URL for the S3 file, saving you the memory, time, and bandwidth cost of downloading the file to your server and sending it back to the user.
s3.ts

Support for S3-Compatible Services

Bun’s S3 implementation works with any S3-compatible storage service. Specify the appropriate endpoint:

Using Bun’s S3Client with AWS S3

AWS S3 is the default. With AWS S3, you can pass a region option instead of an endpoint option.
s3.ts

Using Bun’s S3Client with Google Cloud Storage

To use Bun’s S3 client with Google Cloud Storage, set endpoint to "https://storage.googleapis.com" in the S3Client constructor.
s3.ts

Using Bun’s S3Client with Cloudflare R2

To use Bun’s S3 client with Cloudflare R2, set endpoint to the R2 endpoint in the S3Client constructor. The R2 endpoint includes your account ID.
s3.ts

Using Bun’s S3Client with DigitalOcean Spaces

To use Bun’s S3 client with DigitalOcean Spaces, set endpoint to the DigitalOcean Spaces endpoint in the S3Client constructor.
s3.ts

Using Bun’s S3Client with MinIO

To use Bun’s S3 client with MinIO, set endpoint to the URL that MinIO is running on in the S3Client constructor.
s3.ts

Using Bun’s S3Client with supabase

To use Bun’s S3 client with Supabase, set endpoint to the Supabase endpoint in the S3Client constructor. The Supabase endpoint includes your account ID and the /storage/v1/s3 path. In the Supabase dashboard at https://supabase.com/dashboard/project/<account-id>/settings/storage, turn on Enable connection via S3 protocol and use the region shown in that section.
s3.ts

Using Bun’s S3Client with S3 Virtual Hosted-Style endpoints

When using a virtual hosted-style endpoint, set the virtualHostedStyle option to true.
  • If you don’t specify an endpoint, Bun determines the AWS S3 endpoint from the provided region and bucket. - If no region is specified, Bun defaults to us-east-1. - If you explicitly provide an endpoint, you don’t need to specify a bucket name.
s3.ts

Credentials

By default, Bun reads the following environment variables for credentials. For each option, if the S3_* environment variable is not set, Bun falls back to the matching AWS_* environment variable. Bun reads these environment variables from .env files or from the process environment at initialization time (process.env is not used for this). Options you pass to s3.file(credentials), new Bun.S3Client(credentials), or any of the methods that accept credentials override these defaults. So if you use the same credentials for different buckets, you can set the credentials once in your .env file and pass only bucket: "my-bucket" to s3.file().

S3Client objects

When you’re not using environment variables, or you’re using multiple buckets, create an S3Client object to set credentials explicitly.
s3.ts

S3Client.prototype.write

To upload or write a file to S3, call write on the S3Client instance.
s3.ts

S3Client.prototype.delete

To delete a file from S3, call delete on the S3Client instance.
s3.ts

S3Client.prototype.exists

To check if a file exists in S3, call exists on the S3Client instance.
s3.ts

S3File

Calling file() on an S3Client instance, or s3.file(), returns an S3File. Like Bun.file(), S3File instances are lazy: they don’t refer to something that necessarily exists at the time of creation. That’s why all the methods that don’t involve network requests are fully synchronous.
Type Reference
Like Bun.file(), S3File extends Blob, so all the methods available on Blob are also available on S3File. The same API that reads data from a local file also reads data from S3. That means S3File instances work with fetch(), Response, and other web APIs that accept Blob instances.

Partial reads with slice

To read a partial range of a file, use the slice method.
s3.ts
Internally, Bun uses the HTTP Range header to request only the bytes you want. This slice method is the same as Blob.prototype.slice.

Deleting files from S3

To delete a file from S3, use the delete method.
s3.ts
delete is the same as unlink.

Error codes

When Bun’s S3 API throws an error, the error has a code property with one of the following values:
  • ERR_S3_MISSING_CREDENTIALS
  • ERR_S3_INVALID_METHOD
  • ERR_S3_INVALID_PATH
  • ERR_S3_INVALID_ENDPOINT
  • ERR_S3_INVALID_SIGNATURE
  • ERR_S3_INVALID_SESSION_TOKEN
When the S3 service itself returns an error (that is, not Bun), it is an S3Error instance (an Error instance with the name "S3Error").

S3Client static methods

The S3Client class provides several static methods for interacting with S3.

S3Client.write (static)

To write data directly to a path in the bucket, use the S3Client.write static method.
s3.ts
This is equivalent to calling new S3Client(credentials).write("my-file.txt", "Hello World").

S3Client.presign (static)

To generate a presigned URL for an S3 file, use the S3Client.presign static method.
s3.ts
This is equivalent to calling new S3Client(credentials).presign("my-file.txt", { expiresIn: 3600 }).

S3Client.list (static)

To list some or all (up to 1,000) objects in a bucket, use the S3Client.list static method.
s3.ts
This is equivalent to calling new S3Client(credentials).list().

S3Client.exists (static)

To check if an S3 file exists, use the S3Client.exists static method.
s3.ts
The same method also works on S3File instances.
s3.ts

S3Client.size (static)

To check the size of an S3 file without downloading it, use the S3Client.size static method.
s3.ts
This is equivalent to calling new S3Client(credentials).size("my-file.txt").

S3Client.stat (static)

To get the size, etag, and other metadata of an S3 file, use the S3Client.stat static method.
s3.ts

S3Client.delete (static)

To delete an S3 file, use the S3Client.delete static method.
s3.ts

s3:// protocol

fetch and Bun.file() support the s3:// protocol, so the same code works for local files and S3 files.
s3.ts
You can also pass s3 options to fetch and Bun.file.
s3.ts

UTF-8, UTF-16, and BOM (byte order mark)

Like Response and Blob, S3File assumes UTF-8 encoding by default. When calling text() or json() on an S3File:
  • When Bun detects a UTF-16 byte order mark (BOM), it treats the data as UTF-16. JavaScriptCore natively supports UTF-16, so Bun skips the UTF-8 transcoding step (and strips the BOM). One consequence: invalid surrogate pairs in your UTF-16 string pass through to JavaScriptCore (same as source code).
  • When Bun detects a UTF-8 BOM, it strips the BOM and replaces invalid UTF-8 codepoints with the Unicode replacement character (\uFFFD) before passing the string to JavaScriptCore.
  • UTF-32 is not supported.