S3

Bun provides fast, native bindings for interacting with S3-compatible object storage services.

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

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 is designed to feel similar to fetch’s Response and Blob APIs (like Bun’s local filesystem APIs).

s3.ts

import { s3, write, S3Client } from "bun";

// Bun.s3 reads environment variables for credentials
// file() returns a lazy reference to a file on S3
const metadata = s3.file("123.json");

// Download from S3 as JSON
const data = await metadata.json();

// Upload to S3
await write(metadata, JSON.stringify({ name: "John", age: 30 }));

// Presign a URL (synchronous - no network request needed)
const url = metadata.presign({
  acl: "public-read",
  expiresIn: 60 * 60 * 24, // 1 day
});

// Delete the file
await metadata.delete();

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

There are several ways to interact with Bun’s S3 API.

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

import { S3Client } from "bun";

const client = new S3Client({
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // sessionToken: "..."
  // acl: "public-read",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
  // endpoint: "https://<region>.digitaloceanspaces.com", // DigitalOcean Spaces
  // endpoint: "http://localhost:9000", // MinIO
});

// Bun.s3 is a global singleton that is equivalent to `new Bun.S3Client()`

Working with S3 Files

The file method in S3Client returns a lazy reference to a file on S3.

s3.ts

// A lazy reference to a file on S3
const s3file: S3File = client.file("123.json");

Like Bun.file(path), the S3Client’s file method is synchronous. It does zero network requests until you call a method that depends on a network request.

Reading files from S3

If you’ve used the fetch API, you’re familiar with the Response and Blob APIs. S3File extends Blob. The same methods that work on Blob also work on S3File.

s3.ts

// Read an S3File as text
const text = await s3file.text();

// Read an S3File as JSON
const json = await s3file.json();

// Read an S3File as an ArrayBuffer
const buffer = await s3file.arrayBuffer();

// Get only the first 1024 bytes
const partial = await s3file.slice(0, 1024).text();

// Stream the file
const stream = s3file.stream();
for await (const chunk of stream) {
  console.log(chunk);
}

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 directly transfers the string to JavaScriptCore (the engine) without transcoding and without duplicating the string in memory. When you use .bytes() or .arrayBuffer(), it will also avoid duplicating the bytes in memory.

These helper methods not only simplify the API, they also make it faster.

Writing & uploading files to S3

Writing to S3 works the same way.

s3.ts

// Write a string (replacing the file)
await s3file.write("Hello World!");

// Write a Buffer (replacing the file)
await s3file.write(Buffer.from("Hello World!"));

// Write a Response (replacing the file)
await s3file.write(new Response("Hello World!"));

// Write with content type
await s3file.write(JSON.stringify({ name: "John", age: 30 }), {
  type: "application/json",
});

// Write with content encoding (e.g. for pre-compressed data)
await s3file.write(compressedData, {
  type: "application/json",
  contentEncoding: "gzip",
});

// Write with content disposition
await s3file.write(pdfData, {
  type: "application/pdf",
  contentDisposition: 'attachment; filename="report.pdf"',
});

// Write using a writer (streaming)
const writer = s3file.writer({ type: "application/json" });
writer.write("Hello");
writer.write(" World!");
await writer.end();

// Write using Bun.write
await Bun.write(s3file, "Hello World!");

Working with large files (streams)

Bun automatically handles multipart uploads for large files and provides streaming capabilities. The same API that works for local files also works for S3 files.

s3.ts

// Write a large file
const bigFile = Buffer.alloc(10 * 1024 * 1024); // 10MB
const writer = s3file.writer({
  // Automatically retry on network errors up to 3 times
  retry: 3,

  // Queue up to 10 requests at a time
  queueSize: 10,

  // Upload in 5 MB chunks
  partSize: 5 * 1024 * 1024,
});
for (let i = 0; i < 10; i++) {
  writer.write(bigFile);
  await writer.flush();
}
await writer.end();

---

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 facilitate this, you can presign URLs for S3 files. This generates a URL with a signature that allows a user to securely upload that specific file to S3, without exposing your credentials or granting them unnecessary access to your bucket.

The default behaviour is to generate a GET URL that expires in 24 hours. Bun attempts to infer the content type from the file extension. If inference is not possible, it will default to application/octet-stream.

s3.ts

import { s3 } from "bun";

// Generate a presigned URL that expires in 24 hours (default)
const download = s3.presign("my-file.txt"); // GET, text/plain, expires in 24 hours

const upload = s3.presign("my-file", {
  expiresIn: 3600, // 1 hour
  method: "PUT",
  type: "application/json", // Sets response-content-type in the presigned URL
});

// Presign with content disposition (e.g. force download with a specific filename)
const downloadUrl = s3.presign("report.pdf", {
  expiresIn: 3600,
  contentDisposition: 'attachment; filename="quarterly-report.pdf"',
});

// You can call .presign() if on a file reference, but avoid doing so
// unless you already have a reference (to avoid memory usage).
const myFile = s3.file("my-file.txt");
const presignedFile = myFile.presign({
  expiresIn: 3600, // 1 hour
});

Setting ACLs

To set an ACL (access control list) on a presigned URL, pass the acl option:

s3.ts

const url = s3file.presign({
  acl: "public-read",
  expiresIn: 3600,
});

You can pass any of the following ACLs:

ACL	Explanation
"public-read"	The object is readable by the public.
"private"	The object is readable only by the bucket owner.
"public-read-write"	The object is readable and writable by the public.
"authenticated-read"	The object is readable by the bucket owner and authenticated users.
"aws-exec-read"	The object is readable by the AWS account that made the request.
"bucket-owner-read"	The object is readable by the bucket owner.
"bucket-owner-full-control"	The object is readable and writable by the bucket owner.
"log-delivery-write"	The object is writable by AWS services used for log delivery.

Expiring URLs

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

s3.ts

const url = s3file.presign({
  // Seconds
  expiresIn: 3600, // 1 hour

  // access control list
  acl: "public-read",

  // HTTP method
  method: "PUT",
});

method

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

s3.ts

const url = s3file.presign({
  method: "PUT",
  // method: "DELETE",
  // method: "GET",
  // method: "HEAD",
  // method: "POST",
  // method: "PUT",
});

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.

This will automatically redirect the user to the 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

const response = new Response(s3file);
console.log(response);

Response (0 KB) {
  ok: false,
  url: "",
  status: 302,
  statusText: "",
  headers: Headers {
    "location": "https://<account-id>.r2.cloudflarestorage.com/...",
  },
  redirected: true,
  bodyUsed: false
}

---

Support for S3-Compatible Services

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

Using Bun’s S3Client with AWS S3

AWS S3 is the default. You can also pass a region option instead of an endpoint option for AWS S3.

s3.ts

import { S3Client } from "bun";

// AWS S3
const s3 = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // region: "us-east-1",
});

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

import { S3Client } from "bun";

// Google Cloud Storage
const gcs = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  endpoint: "https://storage.googleapis.com",
});

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

import { S3Client } from "bun";

// CloudFlare R2
const r2 = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  endpoint: "https://<account-id>.r2.cloudflarestorage.com",
});

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

import { S3Client } from "bun";

const spaces = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  // region: "nyc3",
  endpoint: "https://<region>.digitaloceanspaces.com",
});

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

import { S3Client } from "bun";

const minio = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",

  // Make sure to use the correct endpoint URL
  // It might not be localhost in production!
  endpoint: "http://localhost:9000",
});

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 /storage/v1/s3 path. Make sure to set Enable connection via S3 protocol on in the supabase dashboard in https://supabase.com/dashboard/project/<account-id>/settings/storage and to set the region informed in the same section.

s3.ts

import { S3Client } from "bun";

const supabase = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  region: "us-west-1",
  endpoint: "https://<account-id>.supabase.co/storage/v1/s3/storage",
});

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

When using a S3 Virtual Hosted-Style endpoint, you need to set the virtualHostedStyle option to true.

• If you don’t specify an endpoint, Bun will automatically determine the AWS S3 endpoint using 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

import { S3Client } from "bun";

// AWS S3 endpoint inferred from region and bucket
const s3 = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  bucket: "my-bucket",
  virtualHostedStyle: true, 
  // endpoint: "https://my-bucket.s3.us-east-1.amazonaws.com",
  // region: "us-east-1",
});

// AWS S3
const s3WithEndpoint = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  endpoint: "https://<bucket-name>.s3.<region>.amazonaws.com",
  virtualHostedStyle: true, 
});

// Cloudflare R2
const r2WithEndpoint = new S3Client({
  accessKeyId: "access-key",
  secretAccessKey: "secret-key",
  endpoint: "https://<bucket-name>.<account-id>.r2.cloudflarestorage.com",
  virtualHostedStyle: true, 
});

---

Credentials

Credentials are one of the hardest parts of using S3. By default, Bun reads the following environment variables for credentials.

Option name	Environment variable
accessKeyId	S3_ACCESS_KEY_ID
secretAccessKey	S3_SECRET_ACCESS_KEY
region	S3_REGION
endpoint	S3_ENDPOINT
bucket	S3_BUCKET
sessionToken	S3_SESSION_TOKEN

If the S3_* environment variable is not set, Bun will also check for the AWS_* environment variable, for each of the above options.

Option name	Fallback environment variable
accessKeyId	AWS_ACCESS_KEY_ID
secretAccessKey	AWS_SECRET_ACCESS_KEY
region	AWS_REGION
endpoint	AWS_ENDPOINT
bucket	AWS_BUCKET
sessionToken	AWS_SESSION_TOKEN

These environment variables are read from .env files or from the process environment at initialization time (process.env is not used for this).

These defaults are overridden by the options you pass to s3.file(credentials), new Bun.S3Client(credentials), or any of the methods that accept credentials. So if, for example, you use the same credentials for different buckets, you can set the credentials once in your .env file and then pass bucket: "my-bucket" to the s3.file() function without having to specify all the credentials again.

S3Client objects

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

s3.ts

import { S3Client } from "bun";

const client = new S3Client({
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // sessionToken: "..."
  endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
  // endpoint: "http://localhost:9000", // MinIO
});

// Write using a Response
await file.write(new Response("Hello World!"));

// Presign a URL
const url = file.presign({
  expiresIn: 60 * 60 * 24, // 1 day
  acl: "public-read",
});

// Delete the file
await file.delete();

S3Client.prototype.write

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

s3.ts

const client = new Bun.S3Client({
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  endpoint: "https://s3.us-east-1.amazonaws.com",
  bucket: "my-bucket",
});

await client.write("my-file.txt", "Hello World!");
await client.write("my-file.txt", new Response("Hello World!"));

// equivalent to
// await client.file("my-file.txt").write("Hello World!");

S3Client.prototype.delete

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

s3.ts

const client = new Bun.S3Client({
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
});

await client.delete("my-file.txt");
// equivalent to
// await client.file("my-file.txt").delete();

S3Client.prototype.exists

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

s3.ts

const client = new Bun.S3Client({
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
});

const exists = await client.exists("my-file.txt");
// equivalent to
// const exists = await client.file("my-file.txt").exists();

S3File

S3File instances are created by calling the S3Client instance method or the s3.file() function. 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

interface S3File extends Blob {
  slice(start: number, end?: number): S3File;
  exists(): Promise<boolean>;
  unlink(): Promise<void>;
  presign(options: S3Options): string;
  text(): Promise<string>;
  json(): Promise<any>;
  bytes(): Promise<Uint8Array>;
  arrayBuffer(): Promise<ArrayBuffer>;
  stream(options: S3Options): ReadableStream;
  write(
    data: string | Uint8Array | ArrayBuffer | Blob | ReadableStream | Response | Request,
    options?: BlobPropertyBag,
  ): Promise<number>;

  exists(options?: S3Options): Promise<boolean>;
  unlink(options?: S3Options): Promise<void>;
  delete(options?: S3Options): Promise<void>;
  presign(options?: S3Options): string;

  stat(options?: S3Options): Promise<S3Stat>;
  /**
   * Size is not synchronously available because it requires a network request.
   *
   * @deprecated Use `stat()` instead.
   */
  size: NaN;

  // ... more omitted for brevity
}

Like Bun.file(), S3File extends Blob, so all the methods that are available on Blob are also available on S3File. The same API for reading data from a local file is also available for reading data from S3.

Method	Output
await s3File.text()	string
await s3File.bytes()	Uint8Array
await s3File.json()	JSON
await s3File.stream()	ReadableStream
await s3File.arrayBuffer()	ArrayBuffer

That means using S3File instances with fetch(), Response, and other web APIs that accept Blob instances works out of the box.

Partial reads with slice

To read a partial range of a file, you can use the slice method.

s3.ts

const partial = s3file.slice(0, 1024);

// Read the partial range as a Uint8Array
const bytes = await partial.bytes();

// Read the partial range as a string
const text = await partial.text();

Internally, this works by using 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, you can use the delete method.

s3.ts

await s3file.delete();
// await s3File.unlink();

delete is the same as unlink.

Error codes

When Bun’s S3 API throws an error, it will have a code property that matches 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 Object Storage service returns an error (that is, not Bun), it will be 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, you can use the S3Client.write static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

// Write string
await S3Client.write("my-file.txt", "Hello World");

// Write JSON with type
await S3Client.write("data.json", JSON.stringify({ hello: "world" }), {
  ...credentials,
  type: "application/json",
});

// Write from fetch
const res = await fetch("https://example.com/data");
await S3Client.write("data.bin", res, credentials);

// Write with ACL
await S3Client.write("public.html", html, {
  ...credentials,
  acl: "public-read",
  type: "text/html",
});

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, you can use the S3Client.presign static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

const url = S3Client.presign("my-file.txt", {
  ...credentials,
  expiresIn: 3600,
});

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, you can use the S3Client.list static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

// List (up to) 1000 objects in the bucket
const allObjects = await S3Client.list(null, credentials);

// List (up to) 500 objects under `uploads/` prefix, with owner field for each object
const uploads = await S3Client.list({
  prefix: 'uploads/',
  maxKeys: 500,
  fetchOwner: true,
}, credentials);

// Check if more results are available
if (uploads.isTruncated) {
  // List next batch of objects under `uploads/` prefix
  const moreUploads = await S3Client.list({
    prefix: 'uploads/',
    maxKeys: 500,
    startAfter: uploads.contents!.at(-1).key
    fetchOwner: true,
  }, credentials);
}

This is equivalent to calling new S3Client(credentials).list().

S3Client.exists (static)

To check if an S3 file exists, you can use the S3Client.exists static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

const exists = await S3Client.exists("my-file.txt", credentials);

The same method also works on S3File instances.

s3.ts

import { s3 } from "bun";

const s3file = s3.file("my-file.txt", {
  // ...credentials,
});

const exists = await s3file.exists();

S3Client.size (static)

To check the size of an S3 file without downloading it, you can use the S3Client.size static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

const bytes = await S3Client.size("my-file.txt", credentials);

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, you can use the S3Client.stat static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
  // endpoint: "https://<account-id>.r2.cloudflarestorage.com", // Cloudflare R2
};

const stat = await S3Client.stat("my-file.txt", credentials);

{
  etag: "\"7a30b741503c0b461cc14157e2df4ad8\"",
  lastModified: 2025-01-07T00:19:10.000Z,
  size: 1024,
  type: "text/plain;charset=utf-8",
}

S3Client.delete (static)

To delete an S3 file, you can use the S3Client.delete static method.

s3.ts

import { S3Client } from "bun";

const credentials = {
  accessKeyId: "your-access-key",
  secretAccessKey: "your-secret-key",
  bucket: "my-bucket",
  // endpoint: "https://s3.us-east-1.amazonaws.com",
};

await S3Client.delete("my-file.txt", credentials);
// equivalent to
// await new S3Client(credentials).delete("my-file.txt");

// S3Client.unlink is alias of S3Client.delete
await S3Client.unlink("my-file.txt", credentials);

s3:// protocol

To make it easier to use the same code for local files and S3 files, the s3:// protocol is supported in fetch and Bun.file().

s3.ts

const response = await fetch("s3://my-bucket/my-file.txt");
const file = Bun.file("s3://my-bucket/my-file.txt");

You can additionally pass s3 options to the fetch and Bun.file functions.

s3.ts

const response = await fetch("s3://my-bucket/my-file.txt", {
  s3: {
    accessKeyId: "your-access-key",
    secretAccessKey: "your-secret-key",
    endpoint: "https://s3.us-east-1.amazonaws.com",
  },
  headers: {
    range: "bytes=0-1023",
  },
});

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

Like Response and Blob, S3File assumes UTF-8 encoding by default.

When calling one of the text() or json() methods on an S3File:

• When a UTF-16 byte order mark (BOM) is detected, it will be treated as UTF-16. JavaScriptCore natively supports UTF-16, so it skips the UTF-8 transcoding process (and strips the BOM). This is mostly good, but it does mean if you have invalid surrogate pairs characters in your UTF-16 string, they will be passed through to JavaScriptCore (same as source code).
• When a UTF-8 BOM is detected, it gets stripped before the string is passed to JavaScriptCore and invalid UTF-8 codepoints are replaced with the Unicode replacement character (\uFFFD).
• UTF-32 is not supported.