A string containing the version of the bun CLI that is currently running.

// => "0.6.4"


The git commit of Bun that was compiled to create the current bun CLI.

// => "f02561530fda1ee9396f51c8bc99b38716e38296"


An alias for process.env.


An absolute path to the entrypoint of the current program (the file that was executed with bun run).

// /path/to/script.ts

This is particular useful for determining whether a script is being directly executed, as opposed to being imported by another script.

if (import.meta.path === Bun.main) {
  // this script is being directly executed
} else {
  // this file is being imported from another script

This is analogous to the require.main = module trick in Node.js.


Bun.sleep(ms: number)

Returns a Promise that resolves after the given number of milliseconds.

await Bun.sleep(1000);
console.log("hello one second later!");

Alternatively, pass a Date object to receive a Promise that resolves at that point in time.

const oneSecondInFuture = new Date(Date.now() + 1000);

await Bun.sleep(oneSecondInFuture);
console.log("hello one second later!");


Bun.sleepSync(ms: number)

A blocking synchronous version of Bun.sleep.

Bun.sleepSync(1000); // blocks thread for one second
console.log("hello one second later!");


Bun.which(bin: string)

Returns the path to an executable, similar to typing which in your terminal.

const ls = Bun.which("ls");
console.log(ls); // "/usr/bin/ls"

By default Bun looks at the current PATH environment variable to determine the path. To configure PATH:

const ls = Bun.which("ls", {
  PATH: "/usr/local/bin:/usr/bin:/bin",
console.log(ls); // "/usr/bin/ls"

Pass a cwd option to resolve for executable from within a specific directory.

const ls = Bun.which("ls", {
  cwd: "/tmp",
  PATH: "",

console.log(ls); // null


Bun.peek(prom: Promise)

Reads a promise's result without await or .then, but only if the promise has already fulfilled or rejected.

import { peek } from "bun";

const promise = Promise.resolve("hi");

// no await!
const result = peek(promise);
console.log(result); // "hi"

This is important when attempting to reduce number of extraneous microticks in performance-sensitive code. It's an advanced API and you probably shouldn't use it unless you know what you're doing.

import { peek } from "bun";
import { expect, test } from "bun:test";

test("peek", () => {
  const promise = Promise.resolve(true);

  // no await necessary!

  // if we peek again, it returns the same value
  const again = peek(promise);

  // if we peek a non-promise, it returns the value
  const value = peek(42);

  // if we peek a pending promise, it returns the promise again
  const pending = new Promise(() => {});

  // If we peek a rejected promise, it:
  // - returns the error
  // - does not mark the promise as handled
  const rejected = Promise.reject(
    new Error("Successfully tested promise rejection"),
  expect(peek(rejected).message).toBe("Successfully tested promise rejection");

The peek.status function lets you read the status of a promise without resolving it.

import { peek } from "bun";
import { expect, test } from "bun:test";

test("peek.status", () => {
  const promise = Promise.resolve(true);

  const pending = new Promise(() => {});

  const rejected = Promise.reject(new Error("oh nooo"));


Opens a file in your default editor. Bun auto-detects your editor via the $VISUAL or $EDITOR environment variables.

const currentFile = import.meta.url;

You can override this via the debug.editor setting in your bunfig.toml

editor = "code"

Or specify an editor with the editor param. You can also specify a line and column number.

Bun.openInEditor(import.meta.url, {
  editor: "vscode", // or "subl"
  line: 10,
  column: 5,



Recursively checks if two objects are equivalent. This is used internally by expect().toEqual() in bun:test.

const foo = { a: 1, b: 2, c: { d: 3 } };

// true
Bun.deepEquals(foo, { a: 1, b: 2, c: { d: 3 } });

// false
Bun.deepEquals(foo, { a: 1, b: 2, c: { d: 4 } });

A third boolean parameter can be used to enable "strict" mode. This is used by expect().toStrictEqual() in the test runner.

const a = { entries: [1, 2] };
const b = { entries: [1, 2], extra: undefined };

Bun.deepEquals(a, b); // => true
Bun.deepEquals(a, b, true); // => false

In strict mode, the following are considered unequal:

// undefined values
Bun.deepEquals({}, { a: undefined }, true); // false

// undefined in arrays
Bun.deepEquals(["asdf"], ["asdf", undefined], true); // false

// sparse arrays
Bun.deepEquals([, 1], [undefined, 1], true); // false

// object literals vs instances w/ same properties
class Foo {
  a = 1;
Bun.deepEquals(new Foo(), { a: 1 }, true); // false


Bun.escapeHTML(value: string | object | number | boolean): string

Escapes the following characters from an input string:

  • " becomes """
  • & becomes "&"
  • ' becomes "'"
  • < becomes "&lt;"
  • > becomes "&gt;"

This function is optimized for large input. On an M1X, it processes 480 MB/s - 20 GB/s, depending on how much data is being escaped and whether there is non-ascii text. Non-string types will be converted to a string before escaping.

Bun.stringWidth() ~6,756x faster string-width alternative

Get the column count of a string as it would be displayed in a terminal. Supports ANSI escape codes, emoji, and wide characters.

Example usage:

Bun.stringWidth("hello"); // => 5
Bun.stringWidth("\u001b[31mhello\u001b[0m"); // => 5
Bun.stringWidth("\u001b[31mhello\u001b[0m", { countAnsiEscapeCodes: true }); // => 12

This is useful for:

  • Aligning text in a terminal
  • Quickly checking if a string contains ANSI escape codes
  • Measuring the width of a string in a terminal

This API is designed to match the popular "string-width" package, so that existing code can be easily ported to Bun and vice versa.

In this benchmark, Bun.stringWidth is a ~6,756x faster than the string-width npm package for input larger than about 500 characters. Big thanks to sindresorhus for their work on string-width!

❯ bun string-width.mjs
cpu: 13th Gen Intel(R) Core(TM) i9-13900
runtime: bun 1.0.29 (x64-linux)

benchmark                                          time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------------------------------- -----------------------------
Bun.stringWidth     500 chars ascii              37.09 ns/iter   (36.77 ns … 41.11 ns)  37.07 ns  38.84 ns  38.99 ns

❯ node string-width.mjs

benchmark                                          time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------------------------------- -----------------------------
npm/string-width    500 chars ascii             249,710 ns/iter (239,970 ns … 293,180 ns) 250,930 ns  276,700 ns 281,450 ns

To make Bun.stringWidth fast, we've implemented it in Zig using optimized SIMD instructions, accounting for Latin1, UTF-16, and UTF-8 encodings. It passes string-width's tests.

View full benchmark

TypeScript definition:

namespace Bun {
  export function stringWidth(
     * The string to measure
    input: string,
    options?: {
       * If `true`, count ANSI escape codes as part of the string width. If `false`, ANSI escape codes are ignored when calculating the string width.
       * @default false
      countAnsiEscapeCodes?: boolean;
       * When it's ambiugous and `true`, count emoji as 1 characters wide. If `false`, emoji are counted as 2 character wide.
       * @default true
      ambiguousIsNarrow?: boolean;
  ): number;


Converts a file:// URL to an absolute path.

const path = Bun.fileURLToPath(new URL("file:///foo/bar.txt"));
console.log(path); // "/foo/bar.txt"


Converts an absolute path to a file:// URL.

const url = Bun.pathToFileURL("/foo/bar.txt");
console.log(url); // "file:///foo/bar.txt"


Compresses a Uint8Array using zlib's GZIP algorithm.

const buf = Buffer.from("hello".repeat(100)); // Buffer extends Uint8Array
const compressed = Bun.gzipSync(buf);

buf; // => Uint8Array(500)
compressed; // => Uint8Array(30)

Optionally, pass a parameters object as the second argument:

zlib compression options


Decompresses a Uint8Array using zlib's GUNZIP algorithm.

const buf = Buffer.from("hello".repeat(100)); // Buffer extends Uint8Array
const compressed = Bun.gzipSync(buf);

const dec = new TextDecoder();
const uncompressed = Bun.gunzipSync(compressed);
// => "hellohellohello..."


Compresses a Uint8Array using zlib's DEFLATE algorithm.

const buf = Buffer.from("hello".repeat(100));
const compressed = Bun.deflateSync(buf);

buf; // => Uint8Array(25)
compressed; // => Uint8Array(10)

The second argument supports the same set of configuration options as Bun.gzipSync.


Decompresses a Uint8Array using zlib's INFLATE algorithm.

const buf = Buffer.from("hello".repeat(100));
const compressed = Bun.deflateSync(buf);

const dec = new TextDecoder();
const decompressed = Bun.inflateSync(compressed);
// => "hellohellohello..."


Serializes an object to a string exactly as it would be printed by console.log.

const obj = { foo: "bar" };
const str = Bun.inspect(obj);
// => '{\nfoo: "bar" \n}'

const arr = new Uint8Array([1, 2, 3]);
const str = Bun.inspect(arr);
// => "Uint8Array(3) [ 1, 2, 3 ]"


This is the symbol that Bun uses to implement Bun.inspect. You can override this to customize how your objects are printed. It is identical to util.inspect.custom in Node.js.

class Foo {
  [Bun.inspect.custom]() {
    return "foo";

const foo = new Foo();
console.log(foo); // => "foo"


Returns the number of nanoseconds since the current bun process started, as a number. Useful for high-precision timing and benchmarking.

// => 7288958


Bun implements a set of convenience functions for asynchronously consuming the body of a ReadableStream and converting it to various binary formats.

const stream = (await fetch("https://bun.sh")).body;
stream; // => ReadableStream

await Bun.readableStreamToArrayBuffer(stream);
// => ArrayBuffer

await Bun.readableStreamToBlob(stream);
// => Blob

await Bun.readableStreamToJSON(stream);
// => object

await Bun.readableStreamToText(stream);
// => string

// returns all chunks as an array
await Bun.readableStreamToArray(stream);
// => unknown[]

// returns all chunks as a FormData object (encoded as x-www-form-urlencoded)
await Bun.readableStreamToFormData(stream);

// returns all chunks as a FormData object (encoded as multipart/form-data)
await Bun.readableStreamToFormData(stream, multipartFormBoundary);


Resolves a file path or module specifier using Bun's internal module resolution algorithm. The first argument is the path to resolve, and the second argument is the "root". If no match is found, an Error is thrown.

Bun.resolveSync("./foo.ts", "/path/to/project");
// => "/path/to/project/foo.ts"

Bun.resolveSync("zod", "/path/to/project");
// => "/path/to/project/node_modules/zod/index.ts"

To resolve relative to the current working directory, pass process.cwd() or "." as the root.

Bun.resolveSync("./foo.ts", process.cwd());
Bun.resolveSync("./foo.ts", "/path/to/project");

To resolve relative to the directory containing the current file, pass import.meta.dir.

Bun.resolveSync("./foo.ts", import.meta.dir);

serialize & deserialize in bun:jsc

To save a JavaScript value into an ArrayBuffer & back, use serialize and deserialize from the "bun:jsc" module.

import { serialize, deserialize } from "bun:jsc";

const buf = serialize({ foo: "bar" });
const obj = deserialize(buf);
console.log(obj); // => { foo: "bar" }

Internally, structuredClone and postMessage serialize and deserialize the same way. This exposes the underlying HTML Structured Clone Algorithm to JavaScript as an ArrayBuffer.