format.js

'use strict';

/*
 * Displays a helpful message and the source of
 * the format when it is invalid.
 */
class InvalidFormatError extends Error {
  constructor(formatFn) {
    super(`Format functions must be synchronous taking a two arguments: (info, opts)
Found: ${formatFn.toString().split('\n')[0]}\n`);

    Error.captureStackTrace(this, InvalidFormatError);
  }
}

/*
 * function format (formatFn)
 * Returns a create function for the `formatFn`.
 */
module.exports = formatFn => {
  if (formatFn.length > 2) {
    throw new InvalidFormatError(formatFn);
  }

  /*
   * function Format (options)
   * Base prototype which calls a `_format`
   * function and pushes the result.
   */
  function Format(options = {}) {
    this.options = options;
  }

  Format.prototype.transform = formatFn;

  //
  // Create a function which returns new instances of
  // FormatWrap for simple syntax like:
  //
  // require('winston').formats.json();
  //
  function createFormatWrap(opts) {
    return new Format(opts);
  }

  //
  // Expose the FormatWrap through the create function
  // for testability.
  //
  createFormatWrap.Format = Format;
  return createFormatWrap;
};