JSFiddle - React, Tailwind, and code Playground

JavaScript

var aSelectedDataSet = [
	{
  	__metadata:{},
		to_ListTypeGroupAssignment:{
			ListTypeGroup:undefined,
      ListTypeGroupDescription:undefined
		}
  },
	{
  	__metadata:{},
		to_ListTypeGroupAssignment:{
			ListTypeGroup:undefined,
      ListTypeGroupDescription:undefined
		}
  }
];

var oData = {
	__metadata:{results:[]},
  to_ExcludedTerms:{results:[]},
  to_ListTypeGroupAssignment:{
  	results:[
    	{
      	AuthorisationGroup:'AuthorisationGroup 1',
        DbKey:'839a45f6-de2b-4bab-90dd-311695868c55',
        ListTypeGroup:'ListTypeGroup1',
        ListTypeGroupDescription:'ListTypeGroupDescription 1',
        ParentKey:'8ae25d47-c3cc-4ee3-a040-ea00505692111',
        __metadata:{}
      },
      {
      	AuthorisationGroup:'AuthorisationGroup 2',
        DbKey:'786a50a2-91e0-4dfd-b1e7-5320c0c16f2c',
        ListTypeGroup:'ListTypeGroup2',
        ListTypeGroupDescription:'ListTypeGroupDescription 2',
        ParentKey:'34bcdc74-ab42-4538-8657-0a2b0473fcb7',
        __metadata:{}
      }      
    ]
  }
};



$(function(){

		aSelectedDataSet.forEach(function(currentObject,index){
         
         for (var childObject in currentObject) {
         			if (! currentObject.hasOwnProperty(childObject))
              continue;
              
              var objectToClone = oData[childObject]['results'][index];
              
              if(objectToClone)
                  $.extend(true,currentObject[childObject],objectToClone);
         }
    });

});

console.log(aSelectedDataSet);