Binary Data

Bun implements several data types and utilities for working with binary data in JavaScript, most of which are Web-standard. Bun-specific APIs are noted as such. This cheat sheet doubles as a table of contents; click a class in the left column to jump to its section.

Class	Description
`TypedArray`	A family of classes that provide an `Array`-like interface for interacting with binary data. Includes `Uint8Array`, `Uint16Array`, `Int8Array`, and more.
`Buffer`	A subclass of `Uint8Array` that implements a wide range of convenience methods. Unlike the others in this table, it’s a Node.js API (which Bun implements). It isn’t available in the browser.
`DataView`	A class that provides a `get/set` API for writing some number of bytes to an `ArrayBuffer` at a particular byte offset. Often used for reading or writing binary protocols.
`Blob`	A readonly blob of binary data usually representing a file. Has a MIME `type`, a `size`, and methods for converting to `ArrayBuffer`, `ReadableStream`, and string.
`File`	A subclass of `Blob` that represents a file. Has a `name` and `lastModified` timestamp. Node.js v20 has experimental support.
`BunFile`	Bun only. A subclass of `Blob` that represents a lazily-loaded file on disk. Created with `Bun.file(path)`.

`ArrayBuffer` and views

JavaScript had no language-native way to store and manipulate binary data until ECMAScript v5 (2009) introduced a range of mechanisms for it. The most fundamental building block is ArrayBuffer, a data structure that represents a sequence of bytes in memory.

// this buffer can store 8 bytes
const buf = new ArrayBuffer(8);

Despite the name, it isn’t an array and supports none of the array methods and operators you might expect. You can’t read or write values from an ArrayBuffer directly; all you can do is check its size and create “slices” from it.

const buf = new ArrayBuffer(8);
buf.byteLength; // => 8

const slice = buf.slice(0, 4); // returns new ArrayBuffer
slice.byteLength; // => 4

To read or write the data, you need a “view”: a class that wraps an ArrayBuffer instance and lets you read and manipulate the underlying data. There are two types of views: typed arrays and DataView.

`DataView`

The DataView class is a lower-level interface for reading and manipulating the data in an ArrayBuffer. The following creates a DataView and sets the first byte to 3.

const buf = new ArrayBuffer(4);
// [0b00000000, 0b00000000, 0b00000000, 0b00000000]

const dv = new DataView(buf);
dv.setUint8(0, 3); // write value 3 at byte offset 0
dv.getUint8(0); // => 3
// [0b00000011, 0b00000000, 0b00000000, 0b00000000]

Next, write a Uint16 at byte offset 1. This requires two bytes. The value 513 is 2 * 256 + 1; in bytes, that’s 00000010 00000001.

dv.setUint16(1, 513);
// [0b00000011, 0b00000010, 0b00000001, 0b00000000]

console.log(dv.getUint16(1)); // => 513

The first three bytes of the underlying ArrayBuffer now have values. Even though the second and third bytes were written with setUint16(), you can still read each component byte with getUint8().

console.log(dv.getUint8(1)); // => 2
console.log(dv.getUint8(2)); // => 1

Writing a value that needs more space than the underlying ArrayBuffer has throws an error. The following writes a Float64 (which requires 8 bytes) at byte offset 0, but the buffer is only four bytes long.

dv.setFloat64(0, 3.1415);
// ^ RangeError: Out of bounds access

The following methods are available on DataView:

Getters	Setters
`getBigInt64()`	`setBigInt64()`
`getBigUint64()`	`setBigUint64()`
`getFloat32()`	`setFloat32()`
`getFloat64()`	`setFloat64()`
`getInt16()`	`setInt16()`
`getInt32()`	`setInt32()`
`getInt8()`	`setInt8()`
`getUint16()`	`setUint16()`
`getUint32()`	`setUint32()`
`getUint8()`	`setUint8()`

`TypedArray`

Typed arrays are a family of classes that provide an Array-like interface for interacting with data in an ArrayBuffer. Whereas a DataView lets you write numbers of varying size at a particular offset, a TypedArray interprets the underlying bytes as an array of numbers, each of a fixed size.

It’s common to refer to this family of classes collectively by their shared superclass TypedArray. This class is internal to JavaScript; you can’t directly create instances of it, and TypedArray is not defined in the global scope. Think of it as an interface or an abstract class.

const buffer = new ArrayBuffer(3);
const arr = new Uint8Array(buffer);

// contents are initialized to zero
console.log(arr); // Uint8Array(3) [0, 0, 0]

// assign values like an array
arr[0] = 0;
arr[1] = 10;
arr[2] = 255;
arr[3] = 255; // no-op, out of bounds

The typed array classes, and how each interprets the bytes in an ArrayBuffer:

Class	Description
`Uint8Array`	Every one (1) byte is interpreted as an unsigned 8-bit integer. Range 0 to 255.
`Uint16Array`	Every two (2) bytes are interpreted as an unsigned 16-bit integer. Range 0 to 65535.
`Uint32Array`	Every four (4) bytes are interpreted as an unsigned 32-bit integer. Range 0 to 4294967295.
`Int8Array`	Every one (1) byte is interpreted as a signed 8-bit integer. Range -128 to 127.
`Int16Array`	Every two (2) bytes are interpreted as a signed 16-bit integer. Range -32768 to 32767.
`Int32Array`	Every four (4) bytes are interpreted as a signed 32-bit integer. Range -2147483648 to 2147483647.
`Float16Array`	Every two (2) bytes are interpreted as a 16-bit floating point number. Range -6.104e5 to 6.55e4.
`Float32Array`	Every four (4) bytes are interpreted as a 32-bit floating point number. Range -3.4e38 to 3.4e38.
`Float64Array`	Every eight (8) bytes are interpreted as a 64-bit floating point number. Range -1.7e308 to 1.7e308.
`BigInt64Array`	Every eight (8) bytes are interpreted as a signed `BigInt`. Range -9223372036854775808 to 9223372036854775807 (though `BigInt` is capable of representing larger numbers).
`BigUint64Array`	Every eight (8) bytes are interpreted as an unsigned `BigInt`. Range 0 to 18446744073709551615 (though `BigInt` is capable of representing larger numbers).
`Uint8ClampedArray`	Same as `Uint8Array`, but automatically “clamps” to the range 0-255 when assigning a value to an element.

The following table shows how the same bytes in an ArrayBuffer are interpreted by different typed array classes.

	Byte 0	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5	Byte 6	Byte 7
`ArrayBuffer`	`00000000`	`00000001`	`00000010`	`00000011`	`00000100`	`00000101`	`00000110`	`00000111`
`Uint8Array`	0	1	2	3	4	5	6	7
`Uint16Array`	256 (`1 * 256 + 0`)		770 (`3 * 256 + 2`)		1284 (`5 * 256 + 4`)		1798 (`7 * 256 + 6`)
`Uint32Array`	50462976				117835012
`BigUint64Array`	506097522914230528n

To create a typed array from a pre-defined ArrayBuffer:

// create typed array from ArrayBuffer
const buf = new ArrayBuffer(10);
const arr = new Uint8Array(buf);

arr[0] = 30;
arr[1] = 60;

// all elements are initialized to zero
console.log(arr); // => Uint8Array(10) [ 30, 60, 0, 0, 0, 0, 0, 0, 0, 0 ];

Instantiating a Uint32Array from this same ArrayBuffer throws an error.

const buf = new ArrayBuffer(10);
const arr = new Uint32Array(buf);
//          ^  RangeError: ArrayBuffer length minus the byteOffset
//             is not a multiple of the element size

A Uint32 value requires four bytes (32 bits). Because the ArrayBuffer is 10 bytes long, there’s no way to cleanly divide its contents into 4-byte chunks. To fix this, create a typed array over a particular “slice” of the ArrayBuffer. The following Uint32Array only “views” the first 8 bytes of the underlying ArrayBuffer: a byteOffset of 0 and a length of 2, the number of Uint32 values the array holds.

// create typed array from ArrayBuffer slice
const buf = new ArrayBuffer(10);
const arr = new Uint32Array(buf, 0, 2);

/*
  buf    _ _ _ _ _ _ _ _ _ _    10 bytes
  arr   [_______,_______]       2 4-byte elements
*/

arr.byteOffset; // 0
arr.length; // 2

You don’t need to create an ArrayBuffer instance first; pass a length to the typed array constructor instead:

const arr2 = new Uint8Array(5);

// all elements are initialized to zero
// => Uint8Array(5) [0, 0, 0, 0, 0]

Typed arrays can also be instantiated directly from an array of numbers, or another typed array:

// from an array of numbers
const arr1 = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7]);
arr1[0]; // => 0;
arr1[7]; // => 7;

// from another typed array
const arr2 = new Uint8Array(arr);

Typed arrays provide the same methods as regular arrays, with a few exceptions. For example, push and pop are not available, because they would require resizing the underlying ArrayBuffer.

const arr = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7]);

// supports common array methods
arr.filter(n => n > 128); // Uint8Array(1) [255]
arr.map(n => n * 2); // Uint8Array(8) [0, 2, 4, 6, 8, 10, 12, 14]
arr.reduce((acc, n) => acc + n, 0); // 28
arr.forEach(n => console.log(n)); // 0 1 2 3 4 5 6 7
arr.every(n => n < 10); // true
arr.find(n => n > 5); // 6
arr.includes(5); // true
arr.indexOf(5); // 5

See the MDN documentation for more on typed array properties and methods.

`Uint8Array`

Uint8Array is the most common typed array in JavaScript. It represents a classic “byte array”: a sequence of 8-bit unsigned integers between 0 and 255. In Bun, it has methods for converting between byte arrays and their base64 or hex string representations.

new Uint8Array([1, 2, 3, 4, 5]).toBase64(); // "AQIDBA=="
Uint8Array.fromBase64("AQIDBA=="); // Uint8Array(4) [1, 2, 3, 4, 5]

new Uint8Array([255, 254, 253, 252, 251]).toHex(); // "fffefdfcfb=="
Uint8Array.fromHex("fffefdfcfb"); // Uint8Array(5) [255, 254, 253, 252, 251]

It is the return value of TextEncoder#encode, and the input type of TextDecoder#decode, two utility classes that translate between strings and various binary encodings, most notably "utf-8".

const encoder = new TextEncoder();
const bytes = encoder.encode("hello world");
// => Uint8Array(11) [ 104, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100 ]

const decoder = new TextDecoder();
const text = decoder.decode(bytes);
// => hello world

`Buffer`

Bun implements Buffer, a Node.js API for working with binary data that pre-dates the introduction of typed arrays in the JavaScript spec. It has since been re-implemented as a subclass of Uint8Array. It provides a wide range of methods, including several Array-like and DataView-like methods.

const buf = Buffer.from("hello world");
// => Buffer(11) [ 104, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100 ]

buf.length; // => 11
buf[0]; // => 104, ascii for 'h'
buf.writeUInt8(72, 0); // => ascii for 'H'

console.log(buf.toString());
// => Hello world

See the Node.js documentation.

`Blob`

Blob is a Web API commonly used for representing files. It originated in browsers (unlike ArrayBuffer, which is part of JavaScript itself), but Node.js and Bun support it too. You rarely create Blob instances directly; they usually come from an external source (like an <input type="file"> element in the browser) or a library. That said, you can create a Blob from one or more string or binary “blob parts”.

const blob = new Blob(["<html>Hello</html>"], {
  type: "text/html",
});

blob.type; // => text/html
blob.size; // => 19

These parts can be string, ArrayBuffer, TypedArray, DataView, or other Blob instances. The parts are concatenated in the order they’re given.

const blob = new Blob([
  "<html>",
  new Blob(["<body>"]),
  new Uint8Array([104, 101, 108, 108, 111]), // "hello" in binary
  "</body></html>",
]);

Read the contents of a Blob asynchronously in various formats.

await blob.text(); // => <html><body>hello</body></html>
await blob.bytes(); // => Uint8Array (copies contents)
await blob.arrayBuffer(); // => ArrayBuffer (copies contents)
await blob.stream(); // => ReadableStream

`BunFile`

BunFile is a subclass of Blob that represents a lazily-loaded file on disk. Like File, it adds a name and lastModified property. Unlike File, it does not require the file to be loaded into memory.

const file = Bun.file("index.txt");
// => BunFile

`File`

Browser only. Experimental support in Node.js 20.

File is a subclass of Blob that adds a name and lastModified property. It’s commonly used in the browser to represent files uploaded with an <input type="file"> element. Node.js and Bun implement File.

// on browser!
// <input type="file" id="file" />

const files = document.getElementById("file").files;
// => File[]

const file = new File(["<html>Hello</html>"], "index.html", {
  type: "text/html",
});

See the MDN documentation.

Streams

Streams let you work with binary data without loading it all into memory at once. They’re commonly used for reading and writing files, sending and receiving network requests, and processing large amounts of data. Bun implements the Web APIs ReadableStream and WritableStream.

Bun also implements the node:stream module, including Readable, Writable, and Duplex. For complete documentation, refer to the Node.js docs.

To create a readable stream:

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("hello");
    controller.enqueue("world");
    controller.close();
  },
});

Read the stream chunk-by-chunk with for await.

for await (const chunk of stream) {
  console.log(chunk);
}

// => "hello"
// => "world"

For more on streams in Bun, see Streams.

Conversion

Use this section as a reference for converting one binary format to another.

From `ArrayBuffer`

Since ArrayBuffer stores the data that underlies other binary structures like TypedArray, the following snippets are not converting from ArrayBuffer to another format. Instead, they create a new instance using the underlying data.

To `TypedArray`

new Uint8Array(buf);

To `DataView`

new DataView(buf);

To `Buffer`

// create Buffer over entire ArrayBuffer
Buffer.from(buf);

// create Buffer over a slice of the ArrayBuffer
Buffer.from(buf, 0, 10);

To `string`

As UTF-8:

new TextDecoder().decode(buf);

To `number[]`

Array.from(new Uint8Array(buf));

To `Blob`

new Blob([buf], { type: "text/plain" });

To `ReadableStream`

The following snippet creates a ReadableStream and enqueues the entire ArrayBuffer as a single chunk.

new ReadableStream({
  start(controller) {
    controller.enqueue(buf);
    controller.close();
  },
});

With chunking

To stream the ArrayBuffer in chunks, use a Uint8Array view and enqueue each chunk.

const view = new Uint8Array(buf);
const chunkSize = 1024;

new ReadableStream({
  start(controller) {
    for (let i = 0; i < view.length; i += chunkSize) {
      controller.enqueue(view.slice(i, i + chunkSize));
    }
    controller.close();
  },
});

From `TypedArray`

To `ArrayBuffer`

The buffer property is the underlying ArrayBuffer. A TypedArray can be a view of a slice of that buffer, so the sizes may differ.

arr.buffer;

To `DataView`

To create a DataView over the same byte range as the TypedArray:

new DataView(arr.buffer, arr.byteOffset, arr.byteLength);

To `Buffer`

Buffer.from(arr);

To `string`

As UTF-8:

new TextDecoder().decode(arr);

To `number[]`

Array.from(arr);

To `Blob`

// only if arr is a view of its entire backing TypedArray
new Blob([arr.buffer], { type: "text/plain" });

To `ReadableStream`

new ReadableStream({
  start(controller) {
    controller.enqueue(arr);
    controller.close();
  },
});

With chunking

To stream the ArrayBuffer in chunks, split the TypedArray into chunks and enqueue each one individually.

new ReadableStream({
  start(controller) {
    for (let i = 0; i < arr.length; i += chunkSize) {
      controller.enqueue(arr.slice(i, i + chunkSize));
    }
    controller.close();
  },
});

From `DataView`

To `ArrayBuffer`

view.buffer;

To `TypedArray`

Only works if the byteLength of the DataView is a multiple of the BYTES_PER_ELEMENT of the TypedArray subclass.

new Uint8Array(view.buffer, view.byteOffset, view.byteLength);
new Uint16Array(view.buffer, view.byteOffset, view.byteLength / 2);
new Uint32Array(view.buffer, view.byteOffset, view.byteLength / 4);
// etc...

To `Buffer`

Buffer.from(view.buffer, view.byteOffset, view.byteLength);

To `string`

As UTF-8:

new TextDecoder().decode(view);

To `number[]`

Array.from(view);

To `Blob`

new Blob([view.buffer], { type: "text/plain" });

To `ReadableStream`

new ReadableStream({
  start(controller) {
    controller.enqueue(view.buffer);
    controller.close();
  },
});

With chunking

To stream the ArrayBuffer in chunks, split the DataView into chunks and enqueue each one individually.

new ReadableStream({
  start(controller) {
    for (let i = 0; i < view.byteLength; i += chunkSize) {
      controller.enqueue(view.buffer.slice(i, i + chunkSize));
    }
    controller.close();
  },
});

From `Buffer`

To `ArrayBuffer`

buf.buffer;

To `TypedArray`

new Uint8Array(buf);

To `DataView`

new DataView(buf.buffer, buf.byteOffset, buf.byteLength);

To `string`

As UTF-8:

buf.toString();

As base64:

buf.toString("base64");

As hex:

buf.toString("hex");

To `number[]`

Array.from(buf);

To `Blob`

new Blob([buf], { type: "text/plain" });

To `ReadableStream`

new ReadableStream({
  start(controller) {
    controller.enqueue(buf);
    controller.close();
  },
});

With chunking

To stream the ArrayBuffer in chunks, split the Buffer into chunks and enqueue each one individually.

new ReadableStream({
  start(controller) {
    for (let i = 0; i < buf.length; i += chunkSize) {
      controller.enqueue(buf.slice(i, i + chunkSize));
    }
    controller.close();
  },
});

From `Blob`

To `ArrayBuffer`

await blob.arrayBuffer();

To `TypedArray`

await blob.bytes();

To `DataView`

new DataView(await blob.arrayBuffer());

To `Buffer`

Buffer.from(await blob.arrayBuffer());

To `string`

As UTF-8:

await blob.text();

To `number[]`

Array.from(await blob.bytes());

To `ReadableStream`

blob.stream();

From `ReadableStream`

Response is a common intermediate for converting a ReadableStream to other formats.

stream; // ReadableStream

const buffer = new Response(stream).arrayBuffer();

But this approach is verbose and adds unnecessary overhead. Bun implements optimized convenience functions for converting a ReadableStream to various binary formats.

To `ArrayBuffer`

// with Response
new Response(stream).arrayBuffer();

// with Bun function
Bun.readableStreamToArrayBuffer(stream);

To `Uint8Array`

// with Response
new Response(stream).bytes();

// with Bun function
Bun.readableStreamToBytes(stream);

To `TypedArray`

// with Response
const buf = await new Response(stream).arrayBuffer();
new Int8Array(buf);

// with Bun function
new Int8Array(Bun.readableStreamToArrayBuffer(stream));

To `DataView`

// with Response
const buf = await new Response(stream).arrayBuffer();
new DataView(buf);

// with Bun function
new DataView(Bun.readableStreamToArrayBuffer(stream));

To `Buffer`

// with Response
const buf = await new Response(stream).arrayBuffer();
Buffer.from(buf);

// with Bun function
Buffer.from(Bun.readableStreamToArrayBuffer(stream));

To `string`

As UTF-8:

// with Response
await new Response(stream).text();

// with Bun function
await Bun.readableStreamToText(stream);

To `number[]`

// with Response
const arr = await new Response(stream).bytes();
Array.from(arr);

// with Bun function
Array.from(new Uint8Array(Bun.readableStreamToArrayBuffer(stream)));

Bun provides a utility for resolving a ReadableStream to an array of its chunks. Each chunk may be a string, typed array, or ArrayBuffer.

// with Bun function
Bun.readableStreamToArray(stream);

To `Blob`

new Response(stream).blob();

To `ReadableStream`

To split a ReadableStream into two streams that can be consumed independently:

const [a, b] = stream.tee();

​ArrayBuffer and views

​DataView

​TypedArray

​Uint8Array

​Buffer

​Blob

​BunFile

​File

​Streams

​Conversion

​From ArrayBuffer

​To TypedArray

​To DataView

​To Buffer

​To string

​To number[]

​To Blob

​To ReadableStream

​From TypedArray

​To ArrayBuffer

​To DataView

​To Buffer

​To string

​To number[]

​To Blob

​To ReadableStream

​From DataView

​To ArrayBuffer

​To TypedArray

​To Buffer

​To string

​To number[]

​To Blob

​To ReadableStream

​From Buffer

​To ArrayBuffer

​To TypedArray

​To DataView

​To string

​To number[]

​To Blob

​To ReadableStream

​From Blob

​To ArrayBuffer

​To TypedArray

​To DataView

​To Buffer

​To string

​To number[]

​To ReadableStream

​From ReadableStream

​To ArrayBuffer

​To Uint8Array

​To TypedArray

​To DataView

​To Buffer

​To string

​To number[]

​To Blob

​To ReadableStream

`ArrayBuffer` and views

`DataView`

`TypedArray`

`Uint8Array`

`Buffer`

`Blob`

`BunFile`

`File`

Streams

Conversion

From `ArrayBuffer`

To `TypedArray`

To `DataView`

To `Buffer`

To `string`

To `number[]`

To `Blob`

To `ReadableStream`

From `TypedArray`

To `ArrayBuffer`

To `DataView`

To `Buffer`

To `string`

To `number[]`

To `Blob`

To `ReadableStream`

From `DataView`

To `ArrayBuffer`

To `TypedArray`

To `Buffer`

To `string`

To `number[]`

To `Blob`

To `ReadableStream`

From `Buffer`

To `ArrayBuffer`

To `TypedArray`

To `DataView`

To `string`

To `number[]`

To `Blob`

To `ReadableStream`

From `Blob`

To `ArrayBuffer`

To `TypedArray`

To `DataView`

To `Buffer`

To `string`

To `number[]`

To `ReadableStream`

From `ReadableStream`

To `ArrayBuffer`

To `Uint8Array`

To `TypedArray`

To `DataView`

To `Buffer`

To `string`

To `number[]`

To `Blob`

To `ReadableStream`