var Immutable = require("immutable");

var a = [1, 2, 3, 5, 6, 7, 8, 9, 10],
    fns = {
        js: function (a) {
            return a.concat(a);
        },
        im: function (a) {
            return Immutable.Seq(a).concat(a).toArray();
        }
    },
    score = {};

for (var name in fns) {
    var fn = fns[name],
        before = Date.now();
    for (var i = 0; i < 3000; i++) {
        fn(a);
    }
    var delta = Date.now() - before;
    score[name] = delta;
}

console.log(score.im/score.js);