Drupal js filesize validation

custom.js

(function ($) {
  
  var mytheme = mytheme || {};

  /**
   * Client-side file input validation of filesize.
   */
  mytheme.validateFileSize = function (event) {
    // Define variables.
    var filesize = event.data.size;
    var error = Drupal.t('The file size exceeding the maximum file size');
    var $this = $(this);
    var filesize_exceeds = false;

    // Check uploaded files if they are exists and filesize is defined.
    if (!navigator.userAgent.match(/MSIE\s(?!10.0)/)) {
      if (filesize != undefined && this.value.length > 0 && this.files.length > 0) {
        var $this_files = this;
        $.each(this.files, function (ind, val) {
          var current_filesize = val.size;

          // Check filesize of uploaded file.
          if (current_filesize > filesize) {
            // Remove old errors and display the new one.
            $('.file-upload-js-error').remove();
            $('.file-upload-js-error-size').remove();
            $this.closest('div.form-managed-file').prepend('<div class="messages error file-upload-js-error-size" aria-live="polite">' + error + '</div>');
            $this_files.value = '';
            filesize_exceeds = true;
            return false;
          }
        });
      }

      // Remove error if filesize is ok.
      if (!filesize_exceeds) {
        $('.file-upload-js-error-size').remove();
      }
    }
  };

  /**
   * Attach behaviors to managed file element upload fields.
   */
  Drupal.behaviors.cnamtsFileValidateAutoAttach = {
    attach: function (context, settings) {
      // Bind filesize validation.
      if (settings.file && settings.file.size) {
        $.each(settings.file.size, function (selector) {
          var size = settings.file.size[selector];
          $(selector, context).once().bind('change', {size: size}, mytheme.validateFileSize);
        });
      }
    },
    detach: function (context, settings) {
      // Unbind filesize validation.
      if (settings.file && settings.file.size) {
        $.each(settings.file.size, function (selector) {
          $(selector, context).unbind('change', mytheme.validateFileSize);
        });
      }
    }
  };
  
})(jQuery);

mymodule.module

/**
 * Implements hook_element_info_alter().
 */
function mymodule_element_info_alter(&$type) {
  if (!empty($type['managed_file'])) {
    $type['managed_file']['#process'][] = 'mymodule_managed_file_process';
  }
}

/**
 * Process function to extends the managed_file element type.
 */
function mymodule_managed_file_process($element, &$form_state, $form) {
  // Add the field size to the page as JavaScript settings.
  if (isset($element['#upload_validators']['file_validate_size'][0])) {
    $file_size = $element['#upload_validators']['file_validate_size'][0];
    $element['upload']['#attached']['js'][0]['data']['file']['size'] = array('#' . $element['#id'] => $file_size);
  }

  return $element;
}