JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>
<textarea id="textarea" rows="4" cols="50"></textarea>

CSS

textarea { width: 100%; min-height: 300px; }

JavaScript

$(document).ready(function(){
	var res = {"rows": [
  	{"sid":5,"cid":5,"aid":2},
    {"sid":5,"cid":5,"aid":1},
    {"sid":5,"cid":6,"aid":2}
  ]};
  var c = {};
  var a = [];
  _.each(res['rows'], function(row) {
  	if(!(row.sid in c)) {
      c[row.sid] = {
       ref_sid: row.sid,
       ci: []
     };
   }
   c[row.sid].ci.push({
   	ref_cid: row.cid
   });
 });

 var str = JSON.stringify(c, undefined, 4);
 $("#textarea").val(str);
});