JSFiddle - React, Tailwind, and code Playground

JavaScript

var timer = function(name) {
    var start = new Date();
    return {
        stop: function() {
            var end  = new Date();
            var time = end.getTime() - start.getTime();
            console.log('Timer:', name, 'finished in', time, 'ms');
        }
    }
};

var t = timer('Benchmark'); // <-- START BENCHMARK

var reformattedArray;
var rObj;
var kvArray = [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}];
for(var i = 0; i < 1000000; i++) {
    reformattedArray = kvArray.map(function(obj){ 
        rObj = {};
        rObj[obj.key] = obj.value;
        return rObj;
    });
}

t.stop();  // <-- STOP BENCHMARK