debug 4.3.7: formatters

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 debug.formatters in pkg:npm/debug@4.3.7
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/debug@4.3.7
Demonstrate these symbols/APIs:
  - debug.formatters

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:c84c74f590253dc275b9187b1a47e995d1a365b8d90a770f8c76291187272a70","contract":["debug.formatters exposes built-in formatters for %o (compact) and %O (multi-line) inspection","debug.formatters allows registering custom formatters invoked during log formatting","debug.formatters executes custom formatters with the logger instance as this context","debug.formatters inlines the formatted token and shifts subsequent unformatted arguments","debug.formatters supports uppercase and lowercase single-letter format specifiers"],"goal":"verify debug.formatters in pkg:npm/debug@4.3.7","kind":"HOW","packages":["pkg:npm/debug@4.3.7"],"schemaVersion":1,"symbols":["debug.formatters"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"npm","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/debug@4.3.7"],"schemaVersion":1,"subject":"pkg:npm/debug@4.3.7","symbols":["debug.formatters"],"verifierAdapter":"node-typescript@1"}

import debug from 'debug';

/**
 * Registers a custom single-letter formatter on debug.formatters.
 *
 * @param {string} letter - A single letter format specifier.
 * @param {Function} formatter - Formatting function receiving the value and bound to logger.
 */
export function registerFormatter(letter, formatter) {
  debug.formatters[letter] = formatter;
}

/**
 * Retrieves a registered formatter function from debug.formatters.
 *
 * @param {string} letter - The single letter format specifier.
 * @returns {Function|undefined} The registered formatter function.
 */
export function getFormatter(letter) {
  return debug.formatters[letter];
}

/**
 * Creates a configured debug logger for a namespace.
 *
 * @param {string} namespace - Logger namespace.
 * @returns {Function} Debug logger function.
 */
export function createLogger(namespace) {
  return debug(namespace);
}

export default debug;

{
  "name": "sample",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample",
      "version": "1.0.0",
      "dependencies": {
        "debug": "4.3.7"
      }
    },
    "node_modules/debug": {
      "version": "4.3.7",
      "resolved": "https://registry.npmjs.org/debug/-/debug-4.3.7.tgz",
      "integrity": "sha512-Er2nc/H7RrMXZBFCEim6TCmMk02Z8vLC2Rbi1KEBggpo0fS6l0S1nnapwmIi3yW/+GOJap1Krg4w0Hg80oCqgQ==",
      "license": "MIT",
      "dependencies": {
        "ms": "^2.1.3"
      },
      "engines": {
        "node": ">=6.0"
      },
      "peerDependenciesMeta": {
        "supports-color": {
          "optional": true
        }
      }
    },
    "node_modules/ms": {
      "version": "2.1.3",
      "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.3.tgz",
      "integrity": "sha512-6FlzubTLZG3J2a/NVCAleEhjzq5oxgHyaCU9yYXvcLsvoVaHJq/s5xXI6/XXP6tz7R9xAOtHnSO/tXtF3WRTlA==",
      "license": "MIT"
    }
  }
}

{
  "name": "sample",
  "version": "1.0.0",
  "type": "module",
  "dependencies": {
    "debug": "4.3.7"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify debug.formatters in pkg:npm/debug@4.3.7",
  "kind": "HOW",
  "packages": [
    "pkg:npm/debug@4.3.7"
  ],
  "symbols": [
    "debug.formatters"
  ]
}

import assert from 'node:assert/strict';
import debug, {
  registerFormatter,
  getFormatter,
  createLogger
} from '../index.mjs';

// Case 1: debug.formatters exposes built-in formatters for %o (compact) and %O (multi-line) inspection
{
  assert.equal(typeof debug.formatters.o, 'function', 'debug.formatters.o is a function');
  assert.equal(typeof debug.formatters.O, 'function', 'debug.formatters.O is a function');

  const obj = { nested: { longProp1: 'value one', longProp2: 'value two', longProp3: 'value three', longProp4: 'value four' } };
  const single = debug.formatters.o.call({ inspectOpts: {}, useColors: false }, obj);
  assert.ok(!single.includes('\n'), 'o formatter flattens to a single line');

  const multi = debug.formatters.O.call({ inspectOpts: {}, useColors: false }, obj);
  assert.ok(multi.includes('\n'), 'O formatter retains multiple lines');
}

// Case 2: debug.formatters allows registering custom formatters invoked during log formatting
{
  registerFormatter('h', (val) => '0x' + Number(val).toString(16));
  assert.equal(typeof getFormatter('h'), 'function', 'h formatter registered');

  debug.enable('test:custom');
  const log = createLogger('test:custom');
  log.useColors = false;

  let captured = '';
  log.log = (...args) => {
    captured = args.join(' ');
  };

  log('offset: %h', 255);
  assert.match(captured, /offset: 0xff/, '%h formatter renders hex string');
}

// Case 3: debug.formatters executes custom formatters with the logger instance as this context
{
  registerFormatter('u', function(val) {
    return `[${this.namespace}] ${String(val).toUpperCase()}`;
  });

  debug.enable('test:context');
  const log = createLogger('test:context');
  log.useColors = false;

  let captured = '';
  log.log = (...args) => {
    captured = args.join(' ');
  };

  log('user: %u', 'alice');
  assert.match(captured, /user: \[test:context\] ALICE/, 'formatter accesses this.namespace');
}

// Case 4: debug.formatters inlines the formatted token and shifts subsequent unformatted arguments
{
  registerFormatter('b', (val) => (val ? 'TRUE' : 'FALSE'));

  debug.enable('test:shift');
  const log = createLogger('test:shift');
  log.useColors = false;

  let capturedArgs = [];
  log.log = (...args) => {
    capturedArgs = args;
  };

  log('active: %b result', true, 'trailing-arg');
  assert.match(capturedArgs[0], /active: TRUE result/, 'inlines formatted argument');
  assert.equal(capturedArgs[1], 'trailing-arg', 'shifts subsequent argument');
}

// Case 5: debug.formatters supports uppercase and lowercase single-letter format specifiers
{
  registerFormatter('J', (val) => JSON.stringify(val));
  registerFormatter('j', (val) => JSON.stringify(val).toLowerCase());

  debug.enable('test:cases');
  const log = createLogger('test:cases');
  log.useColors = false;

  let captured = '';
  log.log = (...args) => {
    captured = args.join(' ');
  };

  log('upper: %J lower: %j', { State: 'OK' }, { State: 'OK' });
  assert.match(captured, /upper: \{"State":"OK"\} lower: \{"state":"ok"\}/, 'supports both uppercase and lowercase keys');
}

console.log('All contract assertions passed.');