JSFiddle - React, Tailwind, and code Playground

JavaScript

var aaData=[{"country":"USA", "month":"1", "earnings":"1000"}, {"country":"USA", "month":"2", "earnings":"1001"}, {"country":"USA", "month":"3", "earnings":"1002"}, {"country":"Germany", "month":"1", "earnings":"1000"}, {"country":"Germany", "month":"2", "earnings":"1001"}, {"country":"Germany", "month":"3", "earnings":"1002"}]

var result=aaData.reduce(function(o, n){
    if (!o.hasOwnProperty(n.country)) o[n.country]={};
    o[n.country][n.month]=n.earnings;
    return o;
}, {})

console.log(result);