Core Format System

/**
 * Creates a new format from a transformation function
 * @param {Function} transformFn - Function that takes (info, opts) and returns modified info or falsy value
 * @returns {Function} Format constructor function that creates format instances
 */
function format(transformFn);

const { format } = require('logform');

// Custom format that uppercases messages
const upperFormat = format((info, opts) => {
  if (opts.enable) {
    info.message = info.message.toUpperCase();
  }
  return info;
});

// Create format instance
const upperInstance = upperFormat({ enable: true });

// Use the format
const result = upperInstance.transform({
  level: 'info',
  message: 'hello world'
});
// Result: { level: 'info', message: 'HELLO WORLD' }

// Filtering example
const filterFormat = format((info, opts) => {
  if (info.private) {
    return false; // Filter out private messages
  }
  return info;
});

/**
 * Registers color configurations for log levels
 * @param {Object} config - Configuration object with colors property or direct color mapping
 * @returns {Object} The configuration object
 */
function levels(config);

const { levels } = require('logform');

// Register custom colors
const config = levels({
  colors: {
    info: 'blue',
    warn: 'yellow',
    error: 'red',
    debug: 'green'
  }
});

// Or directly with color mapping
levels({
  info: 'blue',
  warn: 'yellow'
});

/**
 * Base format class with transform method
 */
class Format {
  /**
   * @param {Object} opts - Configuration options for the format
   */
  constructor(opts);
  
  /** Configuration options passed to constructor */
  options: Object;
  
  /**
   * Transform function that processes info objects
   * @param {Object} info - Log info object to transform
   * @param {Object} opts - Transform options
   * @returns {Object|boolean} Modified info object or false to filter
   */
  transform: Function;
}

/**
 * Error thrown when format function has invalid signature
 * Format functions must take exactly 2 arguments: (info, opts)
 */
class InvalidFormatError extends Error {
  constructor(formatFn);
}

/**
 * Basic info object structure
 */
interface InfoObject {
  /** Log level (required) */
  level: string;
  /** Log message (required) */
  message: any;
  /** Additional metadata properties */
  [key: string]: any;
}

/**
 * Symbol properties for internal state
 */
const SYMBOLS = {
  /** Read-only level symbol - Symbol.for('level') */
  LEVEL: Symbol,
  /** Formatted message symbol - Symbol.for('message') */
  MESSAGE: Symbol,
  /** Splat arguments symbol - Symbol.for('splat') */
  SPLAT: Symbol
};