JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://dsmorse.github.io/gridster.js/demos/assets/css/demo.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/jquery.gridster/0.5.6/jquery.gridster.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://dsmorse.github.io/gridster.js/dist/jquery.gridster.min.js"></script>
<div class="gridster">
    <ul>
    
    </ul>
        
</div>

	<button class="btn btn-outline-info" id="check"> Check If unique selected </button>

CSS

.red{
  background-color:red !important;
}

JavaScript

var gridster;

gridster = $(".gridster ul").gridster({
  widget_base_dimensions: [100, 100],
  widget_margins: [5, 5],
  helper: 'clone',
   serialize_params: function($w, wgd) {return {images: $w.find('textarea').val().trim() , col: wgd.col, row: wgd.row, size_x: wgd.size_x, size_y: wgd.size_y}},
  resize: {
    enabled: true
  }
}).data('gridster');

var json = [{
   
    "col": 1,
    "row": 1,
    "size_y": 2,
    "size_x": 2
}, {
   
    "col": 4,
    "row": 1,
    "size_y": 2,
    "size_x": 2
},

{
    
    "col": 6,
    "row": 1,
    "size_y": 2,
    "size_x": 2
},


{
    "col": 1,
    "row": 3,
    "size_y": 1,
    "size_x": 1
}, {
   
    "col": 4,
    "row": 3,
    "size_y": 1,
    "size_x": 1
},

{
    "col": 6,
    "row": 3,
    "size_y": 1,
    "size_x": 1
}

];


for(var index=0;index<json.length;index++) {
   
		

gridster.add_widget('<li class="new" ><button class="select-element" style="float: right;">-</button></li>',json[index].size_x,json[index].size_y,json[index].col,json[index].row);
}

var parentElement = {};
var uniqueId = 0;

$(document).on("click", ".select-element", function() {
 var closestLI = $(this).closest('li');
			closestLI.toggleClass("red");
			
			
			if(parentElement[closestLI.attr('id')] !== undefined){
				delete parentElement[closestLI.attr('id')];
			}else {
				closestLI.attr('id', uniqueId);
				parentElement[uniqueId++] = closestLI;
			
			}
		   
       console.log("Parent Element Array Length", Object.keys(parentElement).length);
			$.each(parentElement, function(i, v){
			console.log(v);
			
			});
			
});