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The fastest function to swap bytes (a.k.a. reverse the byte ordering, change endianness) of TypedArrays in-place for Node.js, Bun and browsers. Uses SIMD when available. Works with all of the TypedArray types, including BigUint64Array and BigInt64Array. Also works on Buffers if you construct a TypedArray view on the underlying ArrayBuffer (see below).


$ npm install bswap
$ bun install bswap  # but see


import bswap from "bswap";
const x = new Uint16Array([1, 2, 3, 4, 5, 6, 7, 8]);
// now: Uint16Array [ 256, 512, 768, 1024, 1280, 1536, 1792, 2048 ]

// With buffers:
const b = Buffer.alloc(128);
// This constructs a "view" on the same memory; it does not allocate new memory:
const ui32 = new Uint32Array(b.buffer, b.byteOffset, b.byteLength / Uint32Array.BYTES_PER_ELEMENT);

In Node.js/Bun when native code and a recent x86 or ARM processor is available, this library uses the fastest available SIMD instructions (PSHUFB (SSSE3) or VPSHUFB (AVX2), REVn (NEON)), which process multiple array elements simultaneously.

In the browser or when native code is unavailable, this library falls back to the fastest JavaScript implementation. The JavaScript implementation is also always explicitly available:

import {js} from "bswap"; // Use javascript implementation explicitly


Showing millions of elements processed per second when invoked with a 10,000-element array. (Run the benchmark suite to see results for varying array lengths and other libraries.) Ran on an Intel i9-11900H 2.50 GHz processor (AVX2 supported) or Cavium ThunderX 2.0 GHz processor (ARM NEON); Node.js v16.x; Windows 11 (MSVC) or Ubuntu 20.04 (GCC, Clang). (Note that a 10,000-element Int16Array fits in L1 cache, whereas a 10,000-element Int32Array or Float64Array does not.)

compiler C++ JS Native:JS Node.js Native:Node
16 bit types (Uint16Array, Int16Array)
MSVC 2022 46,221 722 64.0x 18,213 2.5x
GCC 9.4 40,945 56.8x 13,720 2.9x
Clang 15 47,398 65.6x 3.5x
GCC-ARM 2,677 183 14.6x 297 9.0x
32 bits types (Uint32Array, Int32Array, Float32Array)
MSVC 2022 27,459 342 36.7x 9,431 2.9x
GCC 9.4 23,613 61.9x 2,842 8.3x
Clang 15 29,013 84.8x 10.2x
GCC-ARM 670 94 7.1x 249 2.7x
64 bit types (Float64Array)
MSVC 2022 9,005 179 38.2x 4,348 2.1x
GCC 9.4 8,774 49.1x 2,642 3.3x
Clang 15 8,937 49.9x 3.4x
GCC-ARM 382 49 7.8x 213 1.8x

There's an AVX512 implementation that is disabled by default. On the Cascade Lake CPU that I tested on, it is ~28% faster than the AVX2 version when the data fit in the L1 cache. However, it is ~10% slower than the AVX2 version when the data come from L2 and ~15% slower from L3. Under the assumption that this module is more often used with arrays larger than 32KB, I've thus left it disabled. Sometime maybe I'll make it select between AVX2 and AVX512 depending on the array length, but this module has no ability to know if the data is resident in the L1 cache.

Comparison to other libraries

Library Operand In-Place 64-bit Type Support Browser Speed (vs. bswap)*
bswap (this) TypedArray yes yes yes 1.00
node buffer.swap16/32/64 Buffer yes since 6.3.0 no 0.05 to 0.38
network-byte-order Number/[Octet] no no yes 0.010
endian-toggle Buffer no yes no 0.0056

* Higher is better. For 16-bit types, 10k-element arrays. Range given for Node.js version reflects Windows vs. Linux benchmark.

  • Node.js' built-in buffer.swap16|32|64 methods (16/32 since v5.10.0; 64 since 6.3.0). Operates in-place. No browser support. Slower except for tiny arrays (where it uses the JS implementation).

    In 6.3.0 I added some optimizations to Node.js' implementation. The optimizations are effective on Windows, but GCC does not do the same automatic vectorization that MSVC does, nor does Node's default build config enable the newer SIMD instructions that this library uses.

    > Buffer.from(typedArray.buffer).swap16()
  • endian-toggle. Simple usage, operates on a Node.js Buffer, handles any byte size, returns a new buffer (does not operate in-place).

    > const x = new Uint16Array([2048])
    > toggle(Buffer.from(x.buffer), x.BYTES_PER_ELEMENT * 8)
    <Buffer d2 04 09 07>
  • network-byte-order. Operates on a single value at a time (i.e. needs to be looped to operate on an array) and has separate hton and ntoh methods, which do effectively the same thing but have different syntaxes. It can operate on strings, but it cannot swap 64-bit types.

    // Using hton
    > const b = [];
    > nbo.htons(b, 0, 2048);
    > b
    [8, 0]
    // or using ntoh
    > const x = new Uint16Array([2048])
    > nbo.ntohs(new Uint8Array(x.buffer, x.byteOffset, 2), 0)
    > const z = new Uint16Array([8])
    > new Uint8Array(z.buffer, z.byteOffset, 2)
    Uint8Array [ 8, 0 ]

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