Duff's device in JavaScript — benchmark from lazutkin.com. Run: npm install && npm run bench

README.md

Copying one array into another: hand-written loops and Duff's device

Benchmark from lazutkin.com, the companion to Duff's device in JavaScript.

The follow-up, Duff's device, part 2: copying within an array, asks the same question of a range copied inside one array, where Array#copyWithin competes; its benchmark is in a separate gist.

The files

• bench-duff.mjs — the four contenders, all copying 127 elements from one array into another: the simple loop; unrolled-loop, unrolled by ten with a second loop for the remainder; duff-device, unrolled by ten with a fall-through switch for the remainder; and duff-device-nested, one loop stepping by ten where a switch on the remaining count decides how much of the block runs.

Running it

npm install
npm run bench

The harness is nano-bench; its Concepts page explains how the confidence intervals and the significance tests are computed. Defaults are 100 samples of 50 ms per function.

The work is CPU-bound, so run one process at a time and one machine at a time. Results move with the engine, the engine version and the CPU, so a number from a single box says nothing on its own.

Update 2026-09-13. A fifth contender, duff-device-nested-default, is duff-device-nested with default: in place of its case 10: label. It reproduces the correction in the post's P.S.: the label, not the dispatch, is what made the nested device run at about twice the flat-tail version (on the i3-10110U under Node 26.8.2: 208 ns with the label, 121 without, 107 for duff-device; on an i9-11900K: 138, 70, 69). The original four contenders are unchanged.

bench-duff.mjs

const SIZE = 127;

export const source = new Array(SIZE);
export const target = new Array(source.length);

for (let i = 0; i < source.length; ++i) {
  source[i] = i;
}

export default {
  'simple-loop': n => {
    for (let i = 0; i < n; ++i) {
      for (let j = 0; j < source.length; ++j) {
        target[j] = source[j];
      }
    }
  },
  'unrolled-loop': n => {
    for (let i = 0; i < n; ++i) {
      let j = 0;
      for (; j + 10 <= source.length; j += 10) {
        target[j] = source[j];
        target[j + 1] = source[j + 1];
        target[j + 2] = source[j + 2];
        target[j + 3] = source[j + 3];
        target[j + 4] = source[j + 4];
        target[j + 5] = source[j + 5];
        target[j + 6] = source[j + 6];
        target[j + 7] = source[j + 7];
        target[j + 8] = source[j + 8];
        target[j + 9] = source[j + 9];
      }
      for (; j < source.length; ++j) {
        target[j] = source[j];
      }
    }
  },
  'duff-device': n => {
    for (let i = 0; i < n; ++i) {
      let j = 0;
      for (; j + 10 <= source.length; j += 10) {
        target[j] = source[j];
        target[j + 1] = source[j + 1];
        target[j + 2] = source[j + 2];
        target[j + 3] = source[j + 3];
        target[j + 4] = source[j + 4];
        target[j + 5] = source[j + 5];
        target[j + 6] = source[j + 6];
        target[j + 7] = source[j + 7];
        target[j + 8] = source[j + 8];
        target[j + 9] = source[j + 9];
      }
      switch (source.length - j) {
        case 9: target[j + 8] = source[j + 8];
        case 8: target[j + 7] = source[j + 7];
        case 7: target[j + 6] = source[j + 6];
        case 6: target[j + 5] = source[j + 5];
        case 5: target[j + 4] = source[j + 4];
        case 4: target[j + 3] = source[j + 3];
        case 3: target[j + 2] = source[j + 2];
        case 2: target[j + 1] = source[j + 1];
        case 1: target[j] = source[j];
      }
    }
  },
  'duff-device-nested': n => {
    for (let i = 0; i < n; ++i) {
      for (let j = 0; j < source.length; j += 10) {
        switch (Math.min(source.length - j, 10)) {
          case 10: target[j + 9] = source[j + 9];
          case 9: target[j + 8] = source[j + 8];
          case 8: target[j + 7] = source[j + 7];
          case 7: target[j + 6] = source[j + 6];
          case 6: target[j + 5] = source[j + 5];
          case 5: target[j + 4] = source[j + 4];
          case 4: target[j + 3] = source[j + 3];
          case 3: target[j + 2] = source[j + 2];
          case 2: target[j + 1] = source[j + 1];
          case 1: target[j] = source[j];
        }
      }
    }
  },
  'duff-device-nested-default': n => {
    for (let i = 0; i < n; ++i) {
      for (let j = 0; j < source.length; j += 10) {
        switch (Math.min(source.length - j, 10)) {
          default: target[j + 9] = source[j + 9];
          case 9: target[j + 8] = source[j + 8];
          case 8: target[j + 7] = source[j + 7];
          case 7: target[j + 6] = source[j + 6];
          case 6: target[j + 5] = source[j + 5];
          case 5: target[j + 4] = source[j + 4];
          case 4: target[j + 3] = source[j + 3];
          case 3: target[j + 2] = source[j + 2];
          case 2: target[j + 1] = source[j + 1];
          case 1: target[j] = source[j];
        }
      }
    }
  }
};

package.json

{
  "name": "duff-device-bench",
  "private": true,
  "type": "module",
  "scripts": {
    "bench": "nano-bench bench-duff.mjs"
  },
  "devDependencies": {
    "nano-benchmark": "^1.2.0"
  }
}