browserslist 4.28.7: loadConfig

# browserslist.loadConfig Public API Behavior

## Overview
This sample verifies configuration discovery and environment-based query loading via `browserslist.loadConfig` in `pkg:npm/browserslist@4.28.7`.

## Pinned Release
- `browserslist`: `4.28.7` (resolved via `npm`)
- Runtime: Node.js (tested on Node 22 ESM)

## Observed Behaviors & Traps

### 1. Configuration Discovery
- `browserslist.loadConfig({ path })` starts at the specified path and searches up the filesystem hierarchy for `.browserslistrc` or `package.json` with a `browserslist` configuration.
- If no configuration is found in the hierarchy, `browserslist.loadConfig` returns `undefined`.

### 2. Environment Selection & Defaults
- When `env` is omitted, `browserslist.loadConfig` defaults to evaluating the `production` environment (or uses `BROWSERSLIST_ENV` / `NODE_ENV` if defined in the process).
- Specifying `env` explicitly (e.g. `{ env: 'development' }`) selects the corresponding section from multi-environment configs.
- If the requested `env` does not exist in the configuration file, `browserslist.loadConfig` falls back to the top-level `defaults` section.

### 3. Supported Configuration Formats
- `.browserslistrc`: plain text sectioned config with optional `[section]` headers and `#` comments.
- `package.json`:
  - array form: `{"browserslist": ["> 1%", "not dead"]}` treated as default queries.
  - object form: `{"browserslist": {"production": [...], "development": [...]}}` providing per-environment mappings.

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify browserslist.loadConfig in pkg:npm/browserslist@4.28.7
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/browserslist@4.28.7
Demonstrate these symbols/APIs:
  - browserslist.loadConfig

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:11cf8a0aff3152f6e12661fcefb2d0f35503a1feb916af978e82c17ea50d60a9","contract":["browserslist.loadConfig({ path }) discovers .browserslistrc and returns queries for production environment by default","browserslist.loadConfig({ path, env }) selects queries for the specified environment from .browserslistrc","browserslist.loadConfig({ path, env }) falls back to defaults section when an unknown environment is requested","browserslist.loadConfig({ path }) discovers package.json browserslist array configuration","browserslist.loadConfig({ path, env }) selects environment configuration from package.json browserslist object","browserslist.loadConfig({ path }) returns undefined when no configuration file exists in the directory tree"],"goal":"verify browserslist.loadConfig in pkg:npm/browserslist@4.28.7","kind":"HOW","packages":["pkg:npm/browserslist@4.28.7"],"schemaVersion":1,"symbols":["browserslist.loadConfig"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"esm","os":"linux","osVersionBucket":"24","packageManager":"npm","packageManagerVersion":"10.9.8","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/browserslist@4.28.7"],"schemaVersion":1,"subject":"pkg:npm/browserslist@4.28.7","symbols":["browserslist.loadConfig"],"verifierAdapter":"node-typescript@1"}

import browserslist from 'browserslist';

/**
 * Discovers and loads browserslist configuration for a target directory and environment.
 * @param {object} [options]
 * @param {string} [options.path] - Directory path to search for configuration
 * @param {string} [options.env] - Target environment name (e.g. 'production', 'development')
 * @returns {string[]|undefined} Array of browser query strings or undefined if no config exists
 */
export function loadConfig(options = {}) {
  return browserslist.loadConfig(options);
}

export { browserslist };
export default browserslist;

{
  "name": "sample-browserslist-load-config",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-browserslist-load-config",
      "version": "1.0.0",
      "dependencies": {
        "browserslist": "4.28.7"
      }
    },
    "node_modules/baseline-browser-mapping": {
      "version": "2.11.24",
      "resolved": "https://registry.npmjs.org/baseline-browser-mapping/-/baseline-browser-mapping-2.11.24.tgz",
      "integrity": "sha512-hYrgxie335U08WqICoGqKRzV1HFXv6zdxwJE4ekCb80CM9a0SVVsN4QPwT67RraRo+9h8IATk6uxHJw7QSkdOg==",
      "license": "Apache-2.0",
      "bin": {
        "baseline-browser-mapping": "dist/cli.cjs"
      },
      "engines": {
        "node": ">=6.0.0"
      }
    },
    "node_modules/browserslist": {
      "version": "4.28.7",
      "resolved": "https://registry.npmjs.org/browserslist/-/browserslist-4.28.7.tgz",
      "integrity": "sha512-JxV13hNrFxqjOc8alRbq9dK1MM79NEXYpma2B2J4wAtpWS5zIEIKqWPGCl7N4o7Uc7B7itylh7SuDujATRyyTw==",
      "funding": [
        {
          "type": "opencollective",
          "url": "https://opencollective.com/browserslist"
        },
        {
          "type": "tidelift",
          "url": "https://tidelift.com/funding/github/npm/browserslist"
        },
        {
          "type": "github",
          "url": "https://github.com/sponsors/ai"
        }
      ],
      "license": "MIT",
      "dependencies": {
        "baseline-browser-mapping": "^2.10.44",
        "caniuse-lite": "^1.0.30001806",
        "electron-to-chromium": "^1.5.393",
        "node-releases": "^2.0.51",
        "update-browserslist-db": "^1.2.3"
      },
      "bin": {
        "browserslist": "cli.js"
      },
      "engines": {
        "node": "^6 || ^7 || ^8 || ^9 || ^10 || ^11 || ^12 || >=13.7"
      }
    },
    "node_modules/caniuse-lite": {
      "version": "1.0.30001810",
      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001810.tgz",
      "integrity": "sha512-TITQPUkaz+aVk5GL6NhOdwk1aEaNTSDPsGFWrTuhKGtjTF70jL/Oht2W4c6rXUe5fu7Ie19VIahAXHIIiWWNeg==",
      "funding": [
        {
          "type": "opencollective",
          "url": "https://opencollective.com/browserslist"
        },
        {
          "type": "tidelift",
          "url": "https://tidelift.com/funding/github/npm/caniuse-lite"
        },
        {
          "type": "github",
          "url": "https://github.com/sponsors/ai"
        }
      ],
      "license": "CC-BY-4.0"
    },
    "node_modules/electron-to-chromium": {
      "version": "1.5.430",
      "resolved": "https://registry.npmjs.org/electron-to-chromium/-/electron-to-chromium-1.5.430.tgz",
      "integrity": "sha512-e1QEj72Y4zd8RlNZVmoTg+iCOSVwpk05IOiiQwdrkwCSVlZfPthevErhE+nckGd2YbsXfp1SkisznhGVIXP2NQ==",
      "license": "ISC"
    },
    "node_modules/escalade": {
      "version": "3.2.0",
      "resolved": "https://registry.npmjs.org/escalade/-/escalade-3.2.0.tgz",
      "integrity": "sha512-WUj2qlxaQtO4g6Pq5c29GTcWGDyd8itL8zTlipgECz3JesAiiOKotd8JU6otB3PACgG6xkJUyVhboMS+bje/jA==",
      "license": "MIT",
      "engines": {
        "node": ">=6"
      }
    },
    "node_modules/node-releases": {
      "version": "2.0.55",
      "resolved": "https://registry.npmjs.org/node-releases/-/node-releases-2.0.55.tgz",
      "integrity": "sha512-mIrE/Cw9y+9Au6dS5vDKDhQza9YvG6w+ZrS6X+ZzA7yFW/soAeaups4Qzn1bL6g5FVy8WtP79+0j82oPIbqRjQ==",
      "license": "MIT",
      "engines": {
        "node": ">=18"
      }
    },
    "node_modules/picocolors": {
      "version": "1.1.1",
      "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
      "integrity": "sha512-xceH2snhtb5M9liqDsmEw56le376mTZkEX/jEb/RxNFyegNul7eNslCXP9FDj/Lcu0X8KEyMceP2ntpaHrDEVA==",
      "license": "ISC"
    },
    "node_modules/update-browserslist-db": {
      "version": "1.3.3",
      "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.3.tgz",
      "integrity": "sha512-pJ2sYawQS0R/WI928Gj5GlPhTGzbMelq0+4INtSYNDV9ErKJcX6xjGWkoG/VnB3dpUm00zALaqkrUD77pO5TDQ==",
      "funding": [
        {
          "type": "opencollective",
          "url": "https://opencollective.com/browserslist"
        },
        {
          "type": "tidelift",
          "url": "https://tidelift.com/funding/github/npm/browserslist"
        },
        {
          "type": "github",
          "url": "https://github.com/sponsors/ai"
        }
      ],
      "license": "MIT",
      "dependencies": {
        "escalade": "^3.2.0",
        "picocolors": "^1.1.1"
      },
      "bin": {
        "update-browserslist-db": "cli.js"
      },
      "peerDependencies": {
        "browserslist": ">= 4.21.0"
      }
    }
  }
}

{
  "name": "sample-browserslist-load-config",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "dependencies": {
    "browserslist": "4.28.7"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify browserslist.loadConfig in pkg:npm/browserslist@4.28.7",
  "kind": "HOW",
  "packages": [
    "pkg:npm/browserslist@4.28.7"
  ],
  "symbols": [
    "browserslist.loadConfig"
  ]
}

import assert from 'node:assert/strict';
import fs from 'node:fs';
import path from 'node:path';
import os from 'node:os';
import browserslist, { loadConfig } from '../index.mjs';

const tempBase = fs.mkdtempSync(path.join(os.tmpdir(), 'csx-bload-contract-'));

try {
  const rcDir = path.join(tempBase, 'rc-project');
  fs.mkdirSync(rcDir, { recursive: true });
  fs.writeFileSync(
    path.join(rcDir, '.browserslistrc'),
    '# Default query section\n> 1%\n\n[production]\nchrome >= 100\n\n[development]\nlast 1 chrome version\n'
  );

  const pkgArrayDir = path.join(tempBase, 'pkg-array-project');
  fs.mkdirSync(pkgArrayDir, { recursive: true });
  fs.writeFileSync(
    path.join(pkgArrayDir, 'package.json'),
    JSON.stringify({ browserslist: ['> 2%', 'not dead'] })
  );

  const pkgObjDir = path.join(tempBase, 'pkg-obj-project');
  fs.mkdirSync(pkgObjDir, { recursive: true });
  fs.writeFileSync(
    path.join(pkgObjDir, 'package.json'),
    JSON.stringify({
      browserslist: {
        production: ['> 0.5%'],
        development: ['last 2 chrome versions']
      }
    })
  );

  const emptyDir = path.join(tempBase, 'empty-project');
  fs.mkdirSync(emptyDir, { recursive: true });

  // 1. browserslist.loadConfig({ path }) discovers .browserslistrc and returns queries for production environment by default
  const defaultQueries = loadConfig({ path: rcDir });
  assert.deepStrictEqual(defaultQueries, ['chrome >= 100']);

  // 2. browserslist.loadConfig({ path, env }) selects queries for the specified environment from .browserslistrc
  const devQueries = loadConfig({ path: rcDir, env: 'development' });
  assert.deepStrictEqual(devQueries, ['last 1 chrome version']);

  // 3. browserslist.loadConfig({ path, env }) falls back to defaults section when an unknown environment is requested
  const fallbackQueries = loadConfig({ path: rcDir, env: 'staging' });
  assert.deepStrictEqual(fallbackQueries, ['> 1%']);

  // 4. browserslist.loadConfig({ path }) discovers package.json browserslist array configuration
  const arrayConfig = loadConfig({ path: pkgArrayDir });
  assert.deepStrictEqual(arrayConfig, ['> 2%', 'not dead']);

  // 5. browserslist.loadConfig({ path, env }) selects environment configuration from package.json browserslist object
  const objProdConfig = loadConfig({ path: pkgObjDir, env: 'production' });
  assert.deepStrictEqual(objProdConfig, ['> 0.5%']);
  const objDevConfig = loadConfig({ path: pkgObjDir, env: 'development' });
  assert.deepStrictEqual(objDevConfig, ['last 2 chrome versions']);

  // 6. browserslist.loadConfig({ path }) returns undefined when no configuration file exists in the directory tree
  const noneConfig = loadConfig({ path: emptyDir });
  assert.strictEqual(noneConfig, undefined);

  console.log('Contract passed successfully.');
} finally {
  fs.rmSync(tempBase, { recursive: true, force: true });
}