browserslist 4.28.7: parseConfig

# browserslist Public API Behavior

## Prior Findings
A network search found samples covering `browserslist.findConfig`, `loadConfig`, `nodeVersions`, and custom stats. This sample specifically isolates and verifies `browserslist.parseConfig` against `pkg:npm/browserslist@4.28.7`.

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

## Observed Behavior & Traps

### 1. Section-based Configuration Parsing (`parseConfig`)
- `browserslist.parseConfig(text)` parses multi-environment configuration strings into an object mapping environment names (`defaults`, `production`, `development`, etc.) to arrays of query strings.
- Leading queries before any `[section]` header are automatically placed under the `defaults` key.
- Comments (`# ...`) and empty lines are stripped.
- Parsing an empty string or comments-only string returns `{ defaults: [] }`.

### 2. Comma-separated Queries
- Queries on the same line separated by commas are split into distinct query items.

### 3. Combined Section Headers
- Multi-section headers like `[production development]` assign identical query rules to each listed section.

### 4. Input Coercion
- If a Buffer or object with `.toString()` is provided, `parseConfig` converts it to string before parsing.

### 5. Duplicate Section Error
- If a section header appears more than once, `parseConfig` throws a `BrowserslistError` with `Duplicate section <name> in Browserslist config`.

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.parseConfig 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.parseConfig

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.

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{
  "name": "verify-browserslist-parse-config",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "verify-browserslist-parse-config",
      "version": "1.0.0",
      "dependencies": {
        "browserslist": "4.28.7"
      }
    },
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      "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": {
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        "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": {
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      "funding": [
        {
          "type": "opencollective",
          "url": "https://opencollective.com/browserslist"
        },
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          "type": "tidelift",
          "url": "https://tidelift.com/funding/github/npm/caniuse-lite"
        },
        {
          "type": "github",
          "url": "https://github.com/sponsors/ai"
        }
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      "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",
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      "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": {
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      "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
      "integrity": "sha512-xceH2snhtb5M9liqDsmEw56le376mTZkEX/jEb/RxNFyegNul7eNslCXP9FDj/Lcu0X8KEyMceP2ntpaHrDEVA==",
      "license": "ISC"
    },
    "node_modules/update-browserslist-db": {
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      "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.3.tgz",
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      "funding": [
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          "type": "opencollective",
          "url": "https://opencollective.com/browserslist"
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          "type": "tidelift",
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        },
        {
          "type": "github",
          "url": "https://github.com/sponsors/ai"
        }
      ],
      "license": "MIT",
      "dependencies": {
        "escalade": "^3.2.0",
        "picocolors": "^1.1.1"
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      "bin": {
        "update-browserslist-db": "cli.js"
      },
      "peerDependencies": {
        "browserslist": ">= 4.21.0"
      }
    }
  }
}

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

{
  "schemaVersion": 1,
  "goal": "verify browserslist.parseConfig in pkg:npm/browserslist@4.28.7",
  "kind": "HOW",
  "packages": [
    "pkg:npm/browserslist@4.28.7"
  ],
  "symbols": [
    "browserslist.parseConfig"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "packageManager": "npm@10.9.8",
    "runtime": "node@22.23.2"
  }
}

import assert from 'node:assert/strict';
import browserslist from 'browserslist';

// 1. browserslist.parseConfig parses multi-environment config strings into section mappings, stripping comments and empty lines
const multiEnvConfig = `
# Default query section
> 1%
last 2 versions

[production]
> 0.2%
not dead # inline comments are stripped

[development]
last 1 chrome version
`;
const parsed = browserslist.parseConfig(multiEnvConfig);
assert.deepStrictEqual(parsed, {
  defaults: ['> 1%', 'last 2 versions'],
  production: ['> 0.2%', 'not dead'],
  development: ['last 1 chrome version']
});

// 2. browserslist.parseConfig on empty or comment-only string returns an object with an empty defaults array
assert.deepStrictEqual(browserslist.parseConfig(''), { defaults: [] });
assert.deepStrictEqual(browserslist.parseConfig('# Just a comment line\n\n# Another comment\n'), { defaults: [] });

// 3. browserslist.parseConfig parses comma-separated queries within lines into distinct array elements
const commaSeparated = `
> 1%, last 2 versions
[production]
> 0.5%, not dead, firefox > 100
`;
const parsedComma = browserslist.parseConfig(commaSeparated);
assert.deepStrictEqual(parsedComma, {
  defaults: ['> 1%', 'last 2 versions'],
  production: ['> 0.5%', 'not dead', 'firefox > 100']
});

// 4. browserslist.parseConfig assigns query rules before any section header to the defaults section
const defaultOnly = `
last 1 version
cover 99.5%
`;
const parsedDefaults = browserslist.parseConfig(defaultOnly);
assert.deepStrictEqual(parsedDefaults, {
  defaults: ['last 1 version', 'cover 99.5%']
});

// 5. browserslist.parseConfig supports combined multi-section headers assigning shared queries to each section
const multiHeaderConfig = `
[production development]
last 2 versions
not dead
`;
const parsedMultiHeader = browserslist.parseConfig(multiHeaderConfig);
assert.deepStrictEqual(parsedMultiHeader, {
  defaults: [],
  production: ['last 2 versions', 'not dead'],
  development: ['last 2 versions', 'not dead']
});

// 6. browserslist.parseConfig accepts Buffer input and converts it to string before parsing
const bufferInput = Buffer.from('[test]\nchrome >= 100\n');
const parsedBuffer = browserslist.parseConfig(bufferInput);
assert.deepStrictEqual(parsedBuffer, {
  defaults: [],
  test: ['chrome >= 100']
});

// 7. browserslist.parseConfig throws BrowserslistError when duplicate sections are encountered
assert.throws(
  () => {
    browserslist.parseConfig('[production]\nchrome > 90\n[production]\nchrome > 100\n');
  },
  {
    name: 'BrowserslistError',
    message: /Duplicate section production in Browserslist config/
  }
);

console.log('Contract passed successfully.');