build(type-plus): drop the redundant esm/package.json marker

[unional]

What

• build:esm no longer runs emit-module-type-marker.mjs, so esm/package.json is no longer emitted.
• The cjs/package.json marker now writes {"type":"commonjs","sideEffects":false}.
• scripts/emit-module-type-marker.mjs drops its two-arg form (it only ever has one job now) and its header comment is rewritten to explain why cjs/ needs a marker and esm/ must not have one.

Why

The package root declares "type": "module", so everything under esm/ is already ESM by inheritance. The esm/package.json marker shipped in 8.0.0-beta.10 (e58fac6) "for symmetry" and restated what was already true.

It was not free. packages/type-plus/package.json carries "sideEffects": false. Bundlers read that field from the nearest package.json describing a module, and esm/package.json became that file for every ESM entry point — while carrying no sideEffects field of its own. So bundlers fell back to assuming side effects on exactly the build consumers tree-shake.

Tree-shaking evidence

Built the package, symlinked it into a throwaway project, and bundled this consumer:

import { reduceWhile } from 'type-plus'
console.log(reduceWhile([1, 2, 3], (a, b) => a + b, 0, () => true))

webpack 5.110.3, mode: 'production', optimization.minimize: false, ESM output:

	bundle	modules kept
with esm/package.json	32,779 bytes	21
without it	707 bytes	2

With the marker present webpack pulled in esm/index.js and dragged along nominal/{index,brand,flavor,constants,nominal_match}.js, testing/{stub,test_type}.js, assertion/assert_type.js and the rest of the barrel. Without it, only array/reduce_while.js survives. The regression is real.

One correction to the premise: rollup 4 with @rollup/plugin-node-resolve produced byte-identical output (522 bytes) in both configurations. Its own static side-effect analysis eliminated the unused modules regardless of the sideEffects hint, so this is a webpack-class regression, not a universal one. The changeset says so rather than overclaiming.

require() regression check

The cjs marker is load-bearing — without it Node reads cjs/index.js as ESM and its require() calls resolve against the caller. Verified against the actual clean build output, not in theory:

$ node req.cjs
require ok: function function 54 exports
$ node -e "import('type-plus').then(...)"
import ok: function 54 exports

Both entry points resolve, and re-running the webpack test after the change reproduces the 707-byte bundle.

Verification

• pnpm -w turbo build lint test — 4/4 green
• pnpm --filter type-plus test:type — 5.4, 5.5, 5.6, 6.0 and latest green

patch changeset included (the package is in beta pre mode on the open 8.0.0 train).

🤖 Generated with Claude Code

https://claude.ai/code/session_019f2P5bUGxqe1niopUctKqP

[changeset-bot]

🦋 Changeset detected

Latest commit: e4a3bc0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
type-plus	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5413511) to head (e4a3bc0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #666   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           43        43           
  Lines          232       232           
  Branches        51        51           
=========================================
  Hits           232       232           

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