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byte-snap

A byte counter for your build. Snap before, snap after, see the delta.

Install

npm i -D byte-snap
# or
bun add -d byte-snap

Quick start

// vite.config.js
import { snapBuild } from 'byte-snap';

export default {
  plugins: [snapBuild.vite({ dir: 'dist' })],
};

$ vite build
✓ built in 1.24s

your-package
────────────
264.36 KB → 86.21 KB
saved: 178.15 KB (67.39% smaller)
files: 4 → 2

Option	Default	Description
dir	'dist'	Directory to measure

measureSize is a deprecated alias of snapBuild — same plugin, kept for back-compat.

Measure one plugin

snapPlugins shows what a single plugin (or group) changed in the final build, by rebuilding without it and diffing. Drop it into plugins: [...] where that plugin would go:

// vite.config.js
import { snapPlugins } from 'byte-snap';
import compress from 'some-compression-plugin';

export default {
  plugins: [snapPlugins([compress])],
};

$ vite build

size: plugin some-compression-plugin
────────────
1.91 KB → 338.00 B
saved: 1.58 KB (82.74% smaller)
files: 1 → 1

snapPlugins re-runs your build command once (in a child process, without the measured plugin) to produce the baseline — so the diff is byte-identical except for that plugin's effect. It defaults to npm run build, which runs your package.json build script (PM-agnostic, with local bins on PATH). Pass buildCmd for a non-standard command.

Option	Default	Description
buildCmd	'npm run build'	Build command re-run (in a child process) for the baseline.

savedPercent is always relative to the whole snapshot's beforeBytes — not to some subsystem within it. If you snapshot a whole bundle to measure one plugin's effect, the percent shown is "that many bytes off the whole bundle," which can look small even when the plugin's own footprint shrank a lot. Use savedBytes for the absolute number if that's what matters.

Custom usage

Two functions. Snapshot, do the work, diff:

import { diff, snap } from 'byte-snap';

const before = snap.path('./dist'); // file or directory (recursive)
await runYourMinifier(); // eg. codemod, image squash, anything
const after = snap.path('./dist');

diff(before, after).print();

Want the numbers? .json():

const stats = diff(before, after).json();
// {
//   beforeBytes: 1268776, afterBytes: 968496,
//   savedBytes: 300280, savedPercent: 23.67,
//   beforeFiles: 34, afterFiles: 34, fileDelta: 0
// }

Exotic use cases

Fail build if a custom plugin didn't save enough

const before = snap.path('./dist');
await runYourMinifier();

const { savedBytes } = diff(before, snap.path('./dist')).json();

// expect at least 100 bytes saved
if (savedBytes < 100) {
  console.error(`Baseline not met. Saved only ${savedBytes} bytes 🚨`);
  process.exit(1);
}

Compare gzip vs brotli on a single file

import { diff, snap } from 'byte-snap';
import { readFileSync } from 'node:fs';
import { brotliCompressSync, gzipSync } from 'node:zlib';

const raw = readFileSync('dist/index.js');
diff(snap.buffer(raw), snap.buffer(gzipSync(raw))).print();
diff(snap.buffer(raw), snap.buffer(brotliCompressSync(raw))).print();

Measure a raw string transform

diff(snap.text(source), snap.text(minify(source))).print();

API

Call	Returns / does
snap.path(target)	Snapshot a file or directory (recursive). Missing path → empty.
snap.text(str)	Snapshot a string's UTF-8 byte length.
snap.buffer(buf)	Snapshot a Buffer or ArrayBuffer.
diff(a, b)	Compare two snapshots → { print(title?), json() }.
snapBuild.vite()	Whole-build plugin (and .rollup, .webpack, .esbuild, …).
snapPlugins([p], o)	Plugin array measuring what plugin p changed (o.buildCmd).

Each snapshot also exposes per-file detail: { files, bytes: { total, average, largest, smallest }, entries }.

License

MIT © jayF0x