Convert a binary NodeJS Buffer to JavaScript ArrayBuffer

Question

How can I convert a NodeJS binary buffer into a JavaScript ArrayBuffer?

Astray

I'm curious as to why you would need to do this? – Airy

a good example would be writing a library that worked with File's in browsers and also for NodeJS files? – Dismast

or using a browser library in NodeJS – Farly

Another reason is that a float takes too many bytes of RAM when stored in an Array. So to store many floats you need Float32Array where it takes 4 bytes. And if you want quick serialization of those floats to a file you need a Buffer, as serializing to JSON takes ages. – Fractionate

I want to know exactly the same thing to send generic data using WebRTC and it's unbelievable that so many answers here have so many likes, but don't answer the actual question... – Bos

const file = fs.readFileSync(filePath);, so how do I use this?... 30 minutes later, wow I miss C. – Readability

And ya, I feel ya @WilliamEntriken This is what you get with dynamically typed arrays, a horrible programming interface for basic computer science concepts. – Salman

Answer 1

Instances of Buffer are also instances of Uint8Array in node.js 4.x and higher. Thus, the most efficient solution is to access the buffer's own .buffer property directly, as per https://mcmap.net/q/117961/-convert-a-binary-nodejs-buffer-to-javascript-arraybuffer. 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.

---

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(buffer) {
  const arrayBuffer = new ArrayBuffer(buffer.length);
  const view = new Uint8Array(arrayBuffer);
  for (let i = 0; i < buffer.length; ++i) {
    view[i] = buffer[i];
  }
  return arrayBuffer;
}

From ArrayBuffer to Buffer:

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

Pietro

I'd also recommend you to optimize this by copying integers when possible using DataView. Until size&0xfffffffe, copy 32-bit integers, then, if there's 1 byte remaining, copy 8-bit integer, if 2 bytes, copy 16-bit integer, and if 3 bytes, copy 16-bit and 8-bit integer. – Context

See kraag22's answer for a simpler implementation of half of this. – Farly

Have tested Buffer -> ArrayBuffer with a module intended for browser use and it is working brilliantly. Thanks! – Cupro

@SiPlus are you sure that's an optimization? The DataView methods (e.g. getInt32) are exceptionally slow compared to accessing array views because of a ton of type-checking that goes on. Here, reading from a DataView is 98% slower, which you won't make up for through 4x fewer iterations you'd get from your approach: jsperf.com/haakons-test – Maguire

@Maguire Yes, aligning the offset and using Uint32Array should be much better. – Context

@SiPlus grin "should be" and "is" better are not the same; always test "optimizations" before assuming they're optimizations. First off, you actually can't create a DataView from a node Buffer. However, you can use buffer.readUInt32LE. This is still 80% slower than Martin's method, see here for the benchmark. As for aligning offsets, I don't know what you mean. Typed array views must always be memory-aligned or the engine will throw an error. The user has no control here. (Node seems to allow non-aligned access to its Buffers.) – Maguire

@Maguire I was talking about the typed arrays in browser. Copy the first 0-3 bytes, copy the ints, copy the last 0-3 bytes. – Context

@SiPlus Huh? The question is about node Buffers, which do not exist in browsers. My statements remain true that byte-wise operations are faster than using a read method, anywhere. – Maguire

The 1st example, your returning ab and not view? – Outdare

toArrayBuffer should return the ArrayBuffer instance (ab), not the view. The caller needs to be able to create whatever view they need from ArrayBuffer. You cannot to this from Uint8Array – Libre

Why is ab returned? There is nothing done with ab? I always get {} as a result. – Gongorism

‘The slice() method returns a new ArrayBuffer whose contents are a copy of this ArrayBuffer's bytes from begin, inclusive, up to end, exclusive.’ - MDN ArrayBuffer.prototype.slice() – Proulx

DOWNVOTED, slice()-ing an ArrayBuffer DOES make a copy. – Proulx

TypedArrays have a buffer property which is an ArrayBuffer.. so there is no need to iterate through each index. – Invariable

Why do you call a UintArray a view? – Eustis

Uint8Array is one of several different views of a portion of memory. – Pietro

I had to think about the ++i for a moment. Turns out to have the same effect as i++. So I wonder why you're not incrementing the "usual" way? – Imposing

Note that since NodeJS v15.7.0 and v14.18.0 we can use blob.arrayBuffer() which returns a copy of the blob data. – Aurignacian

Answer 2

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

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

Bootee

That's the opposite of what OP wanted. – Shimberg

But that's what I wanted googling my problem and glad I've found the solution. – Bernita

Answer 3

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);

---

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).

Maguire

It doesn't compile again node > 0.12 – Kat

Use ngossen's fork: github.com/dcodeIO/node-memcpy/pull/6. See also my new answer if you're using node 4+. – Maguire

Where were the .byteLength and .byteOffset documented? – Proulx

@K._ Those properties are inherited from TypedArray: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… and developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Maguire

var ab = b.buffer.slice(b.byteOffset, b.byteOffset + b.byteLength); saved my day – Jorgan

Using slice() is not O(1), it's O(n) because it creates a copy: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Reimburse

@MikeFrysinger oops, thanks. Fixed and refreshed answer. – Maguire

Answer 4

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.

Maryrose

Late addition: @trevnorris says "starting in [V8] 4.3 Buffers are backed by Uint8Array", so possibly this is faster now... – Cecelia

See my answer for the safe way to do this. – Maguire

Tested it with v5.6.0 and it was the fastest – Haubergeon

This only works because instances of Buffer are also instances of Uint8Array in Node.js 4.x and higher. For lower Node.js versions you have to implement a toArrayBuffer function. – Compatriot

Answer 5

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;

---

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));
}

---

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.

---

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

---

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

Parcel

This is all well and good... but did you actually answer OP's question? If you did, it's buried... – Cecilius

Great answer! In obtain_arraybuffer: buf.buffer.subarray does not seem to exist. Did you mean buf.buffer.slice here? – Ardisardisj

@everydayproductive Thank you. As you can see in the edit history, I actually used ArrayBuffer.prototype.slice and later modified it to Uint8Array.prototype.subarray. Oh, and I did it wrong. Probably got a bit confused back then. It's all good right now thanks to you. – Proulx

Answer 6

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)

Building

Joyent took it out – Twinge

Answer 7

A Buffer is a view for an ArrayBuffer. You can get to the internal wrapped ArrayBuffer using the buffer property.

This is shared memory, and no copying is required.

const arrayBuffer = theBuffer.buffer

If you want a copy of the data, create another Buffer from the original Buffer (NOT from the wrapped ArrayBuffer), and then reference its wrapped ArrayBuffer.

const newArrayBuffer = Buffer.from(theBuffer).buffer

For reference, going the other direction, from an ArrayBuffer to a Buffer

const arrayBuffer = getArrayBuffer()
const sharedBuffer = Buffer.from(arrayBuffer)

const copiedBuffer = Buffer.from(sharedBuffer)
const copiedArrayBuffer = copiedBuffer.buffer

Hutment

Answer 8

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!"

Compatriot

Answer 9

Now there is a very useful npm package for this: buffer https://github.com/feross/buffer

It tries to provide an API that is 100% identical to node's Buffer API and allow:

• convert typed array to buffer: https://github.com/feross/buffer#convert-typed-array-to-buffer
• convert buffer to typed array: https://github.com/feross/buffer#convert-buffer-to-typed-array

and few more.

Anthropogeography

Answer 10

Surprisingly, in my case (Electron: sending buffer to renderer as ArrayBuffer) this simply worked

function bufferToArrayBuffer(buffer: Buffer): ArrayBuffer {
  return buffer.buffer as ArrayBuffer;
}

and is like 100x faster than

function bufferToArrayBuffer(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;
}

Outgroup

Answer 11

I tried the above for a Float64Array and it just did not work.

I ended up realising that really the data needed to be read 'INTO' the view in correct chunks. This means reading 8 bytes at a time from the source Buffer.

Anyway this is what I ended up with...

var buff = new Buffer("40100000000000004014000000000000", "hex");
var ab = new ArrayBuffer(buff.length);
var view = new Float64Array(ab);

var viewIndex = 0;
for (var bufferIndex=0;bufferIndex<buff.length;bufferIndex=bufferIndex+8)            {

    view[viewIndex] = buff.readDoubleLE(bufferIndex);
    viewIndex++;
}

Ernie

That's why Martin Thomson's answer uses Uint8Array -- it is agnostic to the size of the elements. The Buffer.read* methods are all slow, also. – Maguire

Multiple typed array views can reference the same ArrayBuffer using the same memory. Each value in a Buffer is one byte, so you need to put it into an array with element size of 1 byte. You can use Martin's method, then make a new Float64Array using the same arraybuffer in the constructor. – Maguire

Answer 12

This Proxy will expose the buffer as any of the TypedArrays, without any copy. :

https://www.npmjs.com/package/node-buffer-as-typedarray

It only works on LE, but can be easily ported to BE. Also, never got to actually test how efficient this is.

Lobotomy

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes – Echeverria

My wording might not sound very official, but it does provide enough information to recreate the solution. The solution relies on JavaScript Proxy Object to wrap a native NodeJS Buffer with getters and setters used by TypedArrays. This makes the Buffer instance compatible with any library that requires Typed Array interface. This is the answer original poster was hoping for, but feel free to dismiss it as it doesn't fit your academic/corporate lingo. See if I care. – Lobotomy

Answer 13

NodeJS, at one point (I think it was v0.6.x) had ArrayBuffer support. I created a small library for base64 encoding and decoding here, but since updating to v0.7, the tests (on NodeJS) fail. I'm thinking of creating something that normalizes this, but till then, I suppose Node's native Buffer should be used.

Kropp

Answer 14

I have already update my node to Version 5.0.0 And I work work with this:

function toArrayBuffer(buffer){
    var array = [];
    var json = buffer.toJSON();
    var list = json.data

    for(var key in list){
        array.push(fixcode(list[key].toString(16)))
    }

    function fixcode(key){
        if(key.length==1){
            return '0'+key.toUpperCase()
        }else{
            return key.toUpperCase()
        }
    }

    return array
}

I use it to check my vhd disk image.

Concision

This looks like a specialized (and slow) serialization-based method, not a generic method for converting to/from Buffer/ArrayBuffer? – Maguire

@Maguire it is generic method for V5.0.0+[only] = =. – Concision

toArrayBuffer(new Buffer([1,2,3])) -> ['01', '02', '03'] -- this is returning an array of strings, not integers/bytes. – Maguire

@Maguire return array ->return list. i fix int->string for stdout – Concision

In that case it's the same as https://mcmap.net/q/117961/-convert-a-binary-nodejs-buffer-to-javascript-arraybuffer, and still without necessary the byte offset checks in https://mcmap.net/q/117961/-convert-a-binary-nodejs-buffer-to-javascript-arraybuffer. – Maguire