JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = Array(
    {one : 'data1',two: 'x'},
    {one : 'data1',two: 'y'},
    {one : 'data2',two: 'x'},
    {one : 'data1',two: 'y'},
    {one : 'data4',two: 'x'}
)

//Cannot set property '0' of undefined 
//var newArray = Array();
//var i= 0;
//for(key in data){
//	newArray[data[key].one][i] = data[key];
//	i++;
//}

/////// MY SOLUTION ////////
//var newArray
//var i= 0;
//for(key in data){
//    if(!newArray[data[key].one]){
//    	newArray[data[key].one] = new Array();
//    }
//    newArray[data[key].one][i] = data[key];
//	i++;
//}

//Useless but working case (only one index)
var newArray = Array();
var i= 0;
for(key in data){
	newArray[data[key].one] = data[key];
	i++;
}

console.log(newArray);