Convert a binary NodeJS Buffer to JavaScript ArrayBuffer

Question

Instances of Buffer are also instances of Uint8Array in node.js 4.x and higher. Thus, the most efficient solution is to access the buf.buffer property directly, as per https://stackoverflow.com/a/31394257/1375574. The Buffer constructor also takes an ArrayBufferView argument if you need to go the other direction.

Note that this will not create a copy, which means that writes to any ArrayBufferView will write through to the original Buffer instance.

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In older versions, node.js has both ArrayBuffer as part of v8, but the Buffer class provides a more flexible API. In order to read or write to an ArrayBuffer, you only need to create a view and copy across.

From Buffer to ArrayBuffer:

function toArrayBuffer(buf) {
    const ab = new ArrayBuffer(buf.length);
    const view = new Uint8Array(ab);
    for (let i = 0; i < buf.length; ++i) {
        view[i] = buf[i];
    }
    return ab;
}

From ArrayBuffer to Buffer:

function toBuffer(ab) {
    const buf = Buffer.alloc(ab.byteLength);
    const view = new Uint8Array(ab);
    for (let i = 0; i < buf.length; ++i) {
        buf[i] = view[i];
    }
    return buf;
}

No dependencies, fastest, Node.js 4.x and later

Buffers are Uint8Arrays, so you just need to slice (copy) its region of the backing ArrayBuffer.

// Original Buffer
let b = Buffer.alloc(512);
// Slice (copy) its segment of the underlying ArrayBuffer
let ab = b.buffer.slice(b.byteOffset, b.byteOffset + b.byteLength);

The slice and offset stuff is required because small Buffers (less than 4 kB by default, half the pool size) can be views on a shared ArrayBuffer. Without slicing, you can end up with an ArrayBuffer containing data from another Buffer. See explanation in the docs.

If you ultimately need a TypedArray, you can create one without copying the data:

// Create a new view of the ArrayBuffer without copying
let ui32 = new Uint32Array(b.buffer, b.byteOffset, b.byteLength / Uint32Array.BYTES_PER_ELEMENT);

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No dependencies, moderate speed, any version of Node.js

Use Martin Thomson's answer, which runs in O(n) time. (See also my replies to comments on his answer about non-optimizations. Using a DataView is slow. Even if you need to flip bytes, there are faster ways to do so.)

Dependency, fast, Node.js ≤ 0.12 or iojs 3.x

You can use https://www.npmjs.com/package/memcpy to go in either direction (Buffer to ArrayBuffer and back). It's faster than the other answers posted here and is a well-written library. Node 0.12 through iojs 3.x require ngossen's fork (see this).

"From ArrayBuffer to Buffer" could be done this way:

var buffer = Buffer.from( new Uint8Array(ab) );

A quicker way to write it

var arrayBuffer = new Uint8Array(nodeBuffer).buffer;

However, this appears to run roughly 4 times slower than the suggested toArrayBuffer function on a buffer with 1024 elements.

1. A Buffer is just a view for looking into an ArrayBuffer.

A Buffer, in fact, is a FastBuffer, which extends (inherits from) Uint8Array, which is an octet-unit view (“partial accessor”) of the actual memory, an ArrayBuffer.

  📜/lib/buffer.js#L65-L73 ^{Node.js 9.4.0}

class FastBuffer extends Uint8Array {
  constructor(arg1, arg2, arg3) {
    super(arg1, arg2, arg3);
  }
}
FastBuffer.prototype.constructor = Buffer;
internalBuffer.FastBuffer = FastBuffer;

Buffer.prototype = FastBuffer.prototype;

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2. The size of an ArrayBuffer and the size of its view may vary.

Reason #1: Buffer.from(arrayBuffer[, byteOffset[, length]]).

With Buffer.from(arrayBuffer[, byteOffset[, length]]), you can create a Buffer with specifying its underlying ArrayBuffer and the view's position and size.

const test_buffer = Buffer.from(new ArrayBuffer(50), 40, 10);
console.info(test_buffer.buffer.byteLength); // 50; the size of the memory.
console.info(test_buffer.length); // 10; the size of the view.

Reason #2: FastBuffer's memory allocation.

It allocates the memory in two different ways depending on the size.

• If the size is less than the half of the size of a memory pool and is not 0 (“small”): it makes use of a memory pool to prepare the required memory.
• Else: it creates a dedicated ArrayBuffer that exactly fits the required memory.

  📜/lib/buffer.js#L306-L320 ^{Node.js 9.4.0}

function allocate(size) {
  if (size <= 0) {
    return new FastBuffer();
  }
  if (size < (Buffer.poolSize >>> 1)) {
    if (size > (poolSize - poolOffset))
      createPool();
    var b = new FastBuffer(allocPool, poolOffset, size);
    poolOffset += size;
    alignPool();
    return b;
  } else {
    return createUnsafeBuffer(size);
  }
}

  📜/lib/buffer.js#L98-L100 ^{Node.js 9.4.0}

function createUnsafeBuffer(size) {
  return new FastBuffer(createUnsafeArrayBuffer(size));
}

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What do you mean by a “memory pool?”

A memory pool is a fixed-size pre-allocated memory block for keeping small-size memory chunks for Buffers. Using it keeps the small-size memory chunks tightly together, so prevents fragmentation caused by separate management (allocation and deallocation) of small-size memory chunks.

In this case, the memory pools are ArrayBuffers whose size is 8 KiB by default, which is specified in Buffer.poolSize. When it is to provide a small-size memory chunk for a Buffer, it checks if the last memory pool has enough available memory to handle this; if so, it creates a Buffer that “views” the given partial chunk of the memory pool, otherwise, it creates a new memory pool and so on.

---

You can access the underlying ArrayBuffer of a Buffer. The Buffer's buffer property (that is, inherited from Uint8Array) holds it. A “small” Buffer's buffer property is an ArrayBuffer that represents the entire memory pool. So in this case, the ArrayBuffer and the Buffer varies in size.

const zero_sized_buffer = Buffer.allocUnsafe(0);
const small_buffer = Buffer.from([0xC0, 0xFF, 0xEE]);
const big_buffer = Buffer.allocUnsafe(Buffer.poolSize >>> 1);

// A `Buffer`'s `length` property holds the size, in octets, of the view.
// An `ArrayBuffer`'s `byteLength` property holds the size, in octets, of its data.

console.info(zero_sized_buffer.length); /// 0; the view's size.
console.info(zero_sized_buffer.buffer.byteLength); /// 0; the memory..'s size.
console.info(Buffer.poolSize); /// 8192; a memory pool's size.

console.info(small_buffer.length); /// 3; the view's size.
console.info(small_buffer.buffer.byteLength); /// 8192; the memory pool's size.
console.info(Buffer.poolSize); /// 8192; a memory pool's size.

console.info(big_buffer.length); /// 4096; the view's size.
console.info(big_buffer.buffer.byteLength); /// 4096; the memory's size.
console.info(Buffer.poolSize); /// 8192; a memory pool's size.

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3. So we need to extract the memory it “views.”

An ArrayBuffer is fixed in size, so we need to extract it out by making a copy of the part. To do this, we use Buffer's byteOffset property and length property, which are inherited from Uint8Array, and the ArrayBuffer.prototype.slice method, which makes a copy of a part of an ArrayBuffer. The slice()-ing method herein was inspired by @ZachB.

const test_buffer = Buffer.from(new ArrayBuffer(10));
const zero_sized_buffer = Buffer.allocUnsafe(0);
const small_buffer = Buffer.from([0xC0, 0xFF, 0xEE]);
const big_buffer = Buffer.allocUnsafe(Buffer.poolSize >>> 1);

function extract_arraybuffer(buf)
{
    // You may use the `byteLength` property instead of the `length` one.
    return buf.buffer.slice(buf.byteOffset, buf.byteOffset + buf.length);
}

// A copy -
const test_arraybuffer = extract_arraybuffer(test_buffer); // of the memory.
const zero_sized_arraybuffer = extract_arraybuffer(zero_sized_buffer); // of the... void.
const small_arraybuffer = extract_arraybuffer(small_buffer); // of the part of the memory.
const big_arraybuffer = extract_arraybuffer(big_buffer); // of the memory.

console.info(test_arraybuffer.byteLength); // 10
console.info(zero_sized_arraybuffer.byteLength); // 0
console.info(small_arraybuffer.byteLength); // 3
console.info(big_arraybuffer.byteLength); // 4096

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4. Performance improvement

If you're to use the results as read-only, or it is okay to modify the input Buffers' contents, you can avoid unnecessary memory copying.

const test_buffer = Buffer.from(new ArrayBuffer(10));
const zero_sized_buffer = Buffer.allocUnsafe(0);
const small_buffer = Buffer.from([0xC0, 0xFF, 0xEE]);
const big_buffer = Buffer.allocUnsafe(Buffer.poolSize >>> 1);

function obtain_arraybuffer(buf)
{
    if(buf.length === buf.buffer.byteLength)
    {
        return buf.buffer;
    } // else:
    // You may use the `byteLength` property instead of the `length` one.
    return buf.subarray(0, buf.length);
}

// Its underlying `ArrayBuffer`.
const test_arraybuffer = obtain_arraybuffer(test_buffer);
// Just a zero-sized `ArrayBuffer`.
const zero_sized_arraybuffer = obtain_arraybuffer(zero_sized_buffer);
// A copy of the part of the memory.
const small_arraybuffer = obtain_arraybuffer(small_buffer);
// Its underlying `ArrayBuffer`.
const big_arraybuffer = obtain_arraybuffer(big_buffer);

console.info(test_arraybuffer.byteLength); // 10
console.info(zero_sized_arraybuffer.byteLength); // 0
console.info(small_arraybuffer.byteLength); // 3
console.info(big_arraybuffer.byteLength); // 4096

Use the following excellent npm package: to-arraybuffer.

Or, you can implement it yourself. If your buffer is called buf, do this:

buf.buffer.slice(buf.byteOffset, buf.byteOffset + buf.byteLength)

You can think of an ArrayBuffer as a typed Buffer.

An ArrayBuffer therefore always needs a type (the so-called "Array Buffer View"). Typically, the Array Buffer View has a type of Uint8Array or Uint16Array.

There is a good article from Renato Mangini on converting between an ArrayBuffer and a String.

I have summarized the essential parts in a code example (for Node.js). It also shows how to convert between the typed ArrayBuffer and the untyped Buffer.

function stringToArrayBuffer(string) {
  const arrayBuffer = new ArrayBuffer(string.length);
  const arrayBufferView = new Uint8Array(arrayBuffer);
  for (let i = 0; i < string.length; i++) {
    arrayBufferView[i] = string.charCodeAt(i);
  }
  return arrayBuffer;
}

function arrayBufferToString(buffer) {
  return String.fromCharCode.apply(null, new Uint8Array(buffer));
}

const helloWorld = stringToArrayBuffer('Hello, World!'); // "ArrayBuffer" (Uint8Array)
const encodedString = new Buffer(helloWorld).toString('base64'); // "string"
const decodedBuffer = Buffer.from(encodedString, 'base64'); // "Buffer"
const decodedArrayBuffer = new Uint8Array(decodedBuffer).buffer; // "ArrayBuffer" (Uint8Array)

console.log(arrayBufferToString(decodedArrayBuffer)); // prints "Hello, World!"