benchmark of vs in

foo.js

'use strict';

const 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 a = [];
for (var i = 0; i < 1000000; i++) {
  a.push(i);
}
let foo = 0;

var t = timer('using of');
for (const i of a) foo = i;
t.stop();

var t = timer('using in');
for (const i in a) foo = i;
t.stop();

Timer: using of finished in 61 ms
Timer: using in finished in 220 ms