Adding Content from custom fields to Yoast SEO Analysis Using YoastJS

yoastSeo.js

console.log('script ready');

(function($) {
	
    var MyYoastPlugin = function()
    {
        YoastSEO.app.registerPlugin('myYoastPlugin', {status: 'loading'});

        this.getData();
    };

    MyYoastPlugin.prototype.getData = function()
    {

        var _self = this,
            $text = $('#rw-product_cat_details');

        _self.custom_content = $text.val();

        YoastSEO.app.pluginReady('myYoastPlugin');

        YoastSEO.app.registerModification('content', $.proxy(_self.getCustomContent, _self), 'myYoastPlugin', 5);

    };

    MyYoastPlugin.prototype.getCustomContent = function (content)
    {
      var mceid = $('#rw-product_cat_details textarea').prop('id');
	  console.log('mceid');
	  var mycontent = isEditor( mceid ) ? tinymce.get( mceid ).getContent() : content;
	  
	  return this.custom_content + mycontent + content;	   
    };
	
	
	/**
	 * Check if the field is a TinyMCE editor.
	 *
	 * @param fieldId The field ID
	 * @returns boolean
	 */
	function isEditor( fieldId ) {
		return typeof tinymce !== 'undefined' && tinymce.get( fieldId ) !== null;
	}

    $(window).on('YoastSEO:ready', function ()
    {
      new MyYoastPlugin();
    });

})(jQuery);