Prividing expectations in PEG.js generated parser (last lines)

Expecations in PEG.js error

      /*
       * The parser is now in one of the following three states:
       *
       * 1. The parser successfully parsed the whole input.
       *
       *    - |result !== null|
       *    - |pos === input.length|
       *    - |rightmostMatchFailuresExpected| may or may not contain something
       *
       * 2. The parser successfully parsed only a part of the input.
       *
       *    - |result !== null|
       *    - |pos < input.length|
       *    - |rightmostMatchFailuresExpected| may or may not contain something
       *
       * 3. The parser did not successfully parse any part of the input.
       *
       *   - |result === null|
       *   - |pos === 0|
       *   - |rightmostMatchFailuresExpected| contains at least one failure
       *
       * All code following this comment (including called functions) must
       * handle these states.
       */
      if (result === null || pos !== input.length) {
        var errorPosition = computeErrorPosition();
        throw new this.SyntaxError(
          buildErrorMessage(),
          errorPosition.line,
          errorPosition.column,
          rightmostMatchFailuresPos,
          rightmostMatchFailuresExpected
        );
      }
      
      return result;
    },
    
    /* Returns the parser source code. */
    toSource: function() { return this._source; }
  };
  
  /* Thrown when a parser encounters a syntax error. */
  
  result.SyntaxError = function(message, line, column, position, expectations) {
    this.name = 'SyntaxError';
    this.message = message;
    this.line = line;
    this.column = column;
    this.position = position;
    this.expectations = expectations;
  };
  
  result.SyntaxError.prototype = Error.prototype;
  
  return result;
})();