File I/O

Note — The Bun.file and Bun.write APIs documented on this page are heavily optimized and represent the recommended way to perform file-system tasks using Bun. For operations that are not yet available with Bun.file, such as mkdir or readdir, you can use Bun's nearly complete implementation of the node:fs module.

Bun provides a set of optimized APIs for reading and writing files.

Reading files (Bun.file())

Bun.file(path): BunFile

Create a BunFile instance with the Bun.file(path) function. A BunFile represents a lazily-loaded file; initializing it does not actually read the file from disk.

const foo = Bun.file("foo.txt"); // relative to cwd
foo.size; // number of bytes
foo.type; // MIME type

The reference conforms to the Blob interface, so the contents can be read in various formats.

const foo = Bun.file("foo.txt");

await foo.text(); // contents as a string
await foo.stream(); // contents as ReadableStream
await foo.arrayBuffer(); // contents as ArrayBuffer
await foo.bytes(); // contents as Uint8Array

File references can also be created using numerical file descriptors or file:// URLs.

Bun.file(new URL(import.meta.url)); // reference to the current file

A BunFile can point to a location on disk where a file does not exist.

const notreal = Bun.file("notreal.txt");
notreal.size; // 0
notreal.type; // "text/plain;charset=utf-8"
const exists = await notreal.exists(); // false

The default MIME type is text/plain;charset=utf-8, but it can be overridden by passing a second argument to Bun.file.

const notreal = Bun.file("notreal.json", { type: "application/json" });
notreal.type; // => "application/json;charset=utf-8"

For convenience, Bun exposes stdin, stdout and stderr as instances of BunFile.

Bun.stdin; // readonly

Writing files (Bun.write())

Bun.write(destination, data): Promise<number>

The Bun.write function is a multi-tool for writing payloads of all kinds to disk.

The first argument is the destination which can have any of the following types:

  • string: A path to a location on the file system. Use the "path" module to manipulate paths.
  • URL: A file:// descriptor.
  • BunFile: A file reference.

The second argument is the data to be written. It can be any of the following:

  • string
  • Blob (including BunFile)
  • ArrayBuffer or SharedArrayBuffer
  • TypedArray (Uint8Array, et. al.)
  • Response

All possible permutations are handled using the fastest available system calls on the current platform.

See syscalls

To write a string to disk:

const data = `It was the best of times, it was the worst of times.`;
await Bun.write("output.txt", data);

To copy a file to another location on disk:

const input = Bun.file("input.txt");
const output = Bun.file("output.txt"); // doesn't exist yet!
await Bun.write(output, input);

To write a byte array to disk:

const encoder = new TextEncoder();
const data = encoder.encode("datadatadata"); // Uint8Array
await Bun.write("output.txt", data);

To write a file to stdout:

const input = Bun.file("input.txt");
await Bun.write(Bun.stdout, input);

To write the body of an HTTP response to disk:

const response = await fetch("https://bun.sh");
await Bun.write("index.html", response);

Incremental writing with FileSink

Bun provides a native incremental file writing API called FileSink. To retrieve a FileSink instance from a BunFile:

const file = Bun.file("output.txt");
const writer = file.writer();

To incrementally write to the file, call .write().

const file = Bun.file("output.txt");
const writer = file.writer();

writer.write("it was the best of times\n");
writer.write("it was the worst of times\n");

These chunks will be buffered internally. To flush the buffer to disk, use .flush(). This returns the number of flushed bytes.

writer.flush(); // write buffer to disk

The buffer will also auto-flush when the FileSink's high water mark is reached; that is, when its internal buffer is full. This value can be configured.

const file = Bun.file("output.txt");
const writer = file.writer({ highWaterMark: 1024 * 1024 }); // 1MB

To flush the buffer and close the file:


Note that, by default, the bun process will stay alive until this FileSink is explicitly closed with .end(). To opt out of this behavior, you can "unref" the instance.


// to "re-ref" it later


Bun's implementation of node:fs is fast, and we haven't implemented a Bun-specific API for reading directories just yet. For now, you should use node:fs for working with directories in Bun.

Reading directories (readdir)

To read a directory in Bun, use readdir from node:fs.

import { readdir } from "node:fs/promises";

// read all the files in the current directory
const files = await readdir(import.meta.dir);

Reading directories recursively

To recursively read a directory in Bun, use readdir with recursive: true.

import { readdir } from "node:fs/promises";

// read all the files in the current directory, recursively
const files = await readdir("../", { recursive: true });

Creating directories (mkdir)

To recursively create a directory, use mkdir in node:fs:

import { mkdir } from "node:fs/promises";

await mkdir("path/to/dir", { recursive: true });


The following is a 3-line implementation of the Linux cat command.

// Usage
// $ bun ./cat.ts ./path-to-file

import { resolve } from "path";

const path = resolve(process.argv.at(-1));
await Bun.write(Bun.stdout, Bun.file(path));

To run the file:

bun ./cat.ts ./path-to-file

It runs 2x faster than GNU cat for large files on Linux.


interface Bun {
  stdin: BunFile;
  stdout: BunFile;
  stderr: BunFile;

  file(path: string | number | URL, options?: { type?: string }): BunFile;

    destination: string | number | BunFile | URL,
      | string
      | Blob
      | ArrayBuffer
      | SharedArrayBuffer
      | TypedArray
      | Response,
  ): Promise<number>;

interface BunFile {
  readonly size: number;
  readonly type: string;

  text(): Promise<string>;
  stream(): ReadableStream;
  arrayBuffer(): Promise<ArrayBuffer>;
  json(): Promise<any>;
  writer(params: { highWaterMark?: number }): FileSink;
  exists(): Promise<boolean>;

export interface FileSink {
    chunk: string | ArrayBufferView | ArrayBuffer | SharedArrayBuffer,
  ): number;
  flush(): number | Promise<number>;
  end(error?: Error): number | Promise<number>;
  start(options?: { highWaterMark?: number }): void;
  ref(): void;
  unref(): void;