code: function required(rule, value, source, errors, options, type) {
    if (rule.required &&
      (!source.hasOwnProperty(rule.field) || util.isEmptyValue(value, type || rule.type))) {
      errors.push(util.format(options.messages.required, rule.fullField));
    }
  }
docs: Rule for validating required fields.
  
  @param rule The validation rule.
  @param value The value of the field on the source object.
  @param source The source object being validated.
  @param errors An array of errors that this rule may add
  validation errors to.
  @param options The validation options.
  @param options.messages The validation messages.