browserslist 4.28.9: readConfig

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.readConfig in pkg:npm/browserslist@4.28.9
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/browserslist@4.28.9
Demonstrate these symbols/APIs:
  - browserslist.readConfig

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:sha256:9471491ccc6ccd67d0a35d57cf2bdfd4eb302e8d0c7fdcbde41c149b5cf02b3a","contract":["browserslist.readConfig(file) parses defaults section from a browserslist configuration file","browserslist.readConfig(file) parses named environment sections specified with bracket syntax","browserslist.readConfig(file) supports multiple environments sharing the same section header","browserslist.readConfig(file) strips comment lines starting with hash and ignores empty lines","browserslist.readConfig(file) supports comma-separated queries on a single line","browserslist.readConfig(file) throws BrowserslistError when duplicate section header is encountered","browserslist.readConfig(file) throws BrowserslistError when the specified configuration file does not exist","browserslist.readConfig(file) throws BrowserslistError when target path is a directory instead of a regular file"],"goal":"verify browserslist.readConfig in pkg:npm/browserslist@4.28.9","kind":"HOW","packages":["pkg:npm/browserslist@4.28.9"],"schemaVersion":1,"symbols":["browserslist.readConfig"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"cjs","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@4.28.9"],"schemaVersion":1,"subject":"pkg:npm/browserslist@4.28.9","symbols":["browserslist.readConfig"],"verifierAdapter":"node-typescript@1"}

const browserslist = require('browserslist');

/**
 * Reads and parses a browserslist configuration file.
 *
 * @param {string} file - Path to configuration file.
 * @returns {Record<string, string[]>} Parsed section map with environments and query lists.
 */
function readConfig(file) {
  return browserslist.readConfig(file);
}

module.exports = {
  browserslist,
  readConfig,
};

{
  "name": "sample-browserslist-read-config",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-browserslist-read-config",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "browserslist": "4.28.9"
      }
    },
    "node_modules/baseline-browser-mapping": {
      "version": "2.11.21",
      "resolved": "https://registry.npmjs.org/baseline-browser-mapping/-/baseline-browser-mapping-2.11.21.tgz",
      "integrity": "sha512-uh8vpY/1/YyFkunIDFH/12p7/7VdPKA1hejMVEbdkEaWnUz0Hesvx5EbiU6XxjyHZIOju+ZMbQJkRh+es3/spQ==",
      "license": "Apache-2.0",
      "bin": {
        "baseline-browser-mapping": "dist/cli.cjs"
      },
      "engines": {
        "node": ">=6.0.0"
      }
    },
    "node_modules/browserslist": {
      "version": "4.28.9",
      "resolved": "https://registry.npmjs.org/browserslist/-/browserslist-4.28.9.tgz",
      "integrity": "sha512-EWazOblFYUvlGZcfGhPUPmYh3nikUxBVb+y9MJun5f3hBi812X+8MSQTujLBtgK3cf51fJWbWfOjyeO954d+Eg==",
      "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": "^2.11.20",
        "caniuse-lite": "^1.0.30001810",
        "electron-to-chromium": "^1.5.420",
        "node-releases": "^2.0.54",
        "update-browserslist-db": "^1.3.2"
      },
      "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": "1.5.422",
      "resolved": "https://registry.npmjs.org/electron-to-chromium/-/electron-to-chromium-1.5.422.tgz",
      "integrity": "sha512-UvA/32XqrLDdZSn7Jllo1AYNcWji/G0d5M0GTViE7KoGBiMunw3a34Sb2KO4ZZyrSEhqsxFoVhWWJshdyfKqJA==",
      "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": "2.0.54",
      "resolved": "https://registry.npmjs.org/node-releases/-/node-releases-2.0.54.tgz",
      "integrity": "sha512-YHs7BmmcsdAI5Ozuf8JZo6PT0mv2GIWC9vMfvUC3dp65M8hn7Ux8CPL+2oBI7juNuj9d0ndhTcznq2ODBps9cQ==",
      "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": "1.3.2",
      "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.2.tgz",
      "integrity": "sha512-UQ+MSxlhRm1bzjhU+DcuXfjFO1FzNtqhK5+9Yvlp90ItDLk5vT932A0rFu619nf7RVS+Y/VeaUW1jaRDqZ8VJw==",
      "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-read-config",
  "version": "1.0.0",
  "private": true,
  "description": "Clean-room sample verifying browserslist.readConfig",
  "main": "index.js",
  "license": "MIT-0",
  "dependencies": {
    "browserslist": "4.28.9"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify browserslist.readConfig in pkg:npm/browserslist@4.28.9",
  "kind": "HOW",
  "packages": [
    "pkg:npm/browserslist@4.28.9"
  ],
  "symbols": [
    "browserslist.readConfig"
  ]
}

import assert from 'node:assert/strict';
import fs from 'node:fs';
import path from 'node:path';
import os from 'node:os';
import browserslist from 'browserslist';
import { readConfig } from '../index.js';

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

try {
  // 1. browserslist.readConfig(file) parses defaults section from a browserslist configuration file
  const f1 = path.join(tmpDir, 'c1');
  fs.writeFileSync(f1, 'last 2 versions\n> 1%\n');
  assert.deepStrictEqual(browserslist.readConfig(f1), { defaults: ['last 2 versions', '> 1%'] });
  assert.deepStrictEqual(readConfig(f1), { defaults: ['last 2 versions', '> 1%'] });

  // 2. browserslist.readConfig(file) parses named environment sections specified with bracket syntax
  const f2 = path.join(tmpDir, 'c2');
  fs.writeFileSync(f2, 'defaults\n\n[production]\n> 0.2%\nnot dead\n\n[development]\nlast 1 chrome version\n');
  const expected2 = {
    defaults: ['defaults'],
    production: ['> 0.2%', 'not dead'],
    development: ['last 1 chrome version']
  };
  assert.deepStrictEqual(browserslist.readConfig(f2), expected2);
  assert.deepStrictEqual(readConfig(f2), expected2);

  // 3. browserslist.readConfig(file) supports multiple environments sharing the same section header
  const f3 = path.join(tmpDir, 'c3');
  fs.writeFileSync(f3, '[production modern]\n> 1%\n');
  const expected3 = {
    defaults: [],
    production: ['> 1%'],
    modern: ['> 1%']
  };
  assert.deepStrictEqual(browserslist.readConfig(f3), expected3);
  assert.deepStrictEqual(readConfig(f3), expected3);

  // 4. browserslist.readConfig(file) strips comment lines starting with hash and ignores empty lines
  const f4 = path.join(tmpDir, 'c4');
  fs.writeFileSync(f4, '# Main config\nlast 2 versions # inline comment\n\n# another comment\n> 5%\n');
  const expected4 = {
    defaults: ['last 2 versions', '> 5%']
  };
  assert.deepStrictEqual(browserslist.readConfig(f4), expected4);
  assert.deepStrictEqual(readConfig(f4), expected4);

  // 5. browserslist.readConfig(file) supports comma-separated queries on a single line
  const f5 = path.join(tmpDir, 'c5');
  fs.writeFileSync(f5, 'last 1 chrome version, last 1 firefox version\n');
  const expected5 = {
    defaults: ['last 1 chrome version', 'last 1 firefox version']
  };
  assert.deepStrictEqual(browserslist.readConfig(f5), expected5);
  assert.deepStrictEqual(readConfig(f5), expected5);

  // 6. browserslist.readConfig(file) throws BrowserslistError when duplicate section header is encountered
  const f6 = path.join(tmpDir, 'c6');
  fs.writeFileSync(f6, '[production]\n> 1%\n\n[production]\n> 2%\n');
  assert.throws(
    () => browserslist.readConfig(f6),
    (err) => err.name === 'BrowserslistError' && err.message.includes('Duplicate section production in Browserslist config')
  );
  assert.throws(
    () => readConfig(f6),
    (err) => err.name === 'BrowserslistError' && err.message.includes('Duplicate section production in Browserslist config')
  );

  // 7. browserslist.readConfig(file) throws BrowserslistError when the specified configuration file does not exist
  const f7 = path.join(tmpDir, 'non-existent-config');
  assert.throws(
    () => browserslist.readConfig(f7),
    (err) => err.name === 'BrowserslistError' && err.message === `Can't read ${f7} config`
  );
  assert.throws(
    () => readConfig(f7),
    (err) => err.name === 'BrowserslistError' && err.message === `Can't read ${f7} config`
  );

  // 8. browserslist.readConfig(file) throws BrowserslistError when target path is a directory instead of a regular file
  const f8 = path.join(tmpDir, 'dir-target');
  fs.mkdirSync(f8);
  assert.throws(
    () => browserslist.readConfig(f8),
    (err) => err.name === 'BrowserslistError' && err.message === `Can't read ${f8} config`
  );
  assert.throws(
    () => readConfig(f8),
    (err) => err.name === 'BrowserslistError' && err.message === `Can't read ${f8} config`
  );

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