function doit() {

    function make_x_first(x,y) {
        return {x, y};
    }

    function make_y_first(x, y) {
        return {y, x};
    }

    function make_random(x, y) {
        if (Math.random() > 0.5) {
            return make_y_first(x, y);
        } else {
            return make_x_first(x, y);
        }
    }

    const million = 1000 * 1000;
    const n = 10 * million;

    function bulk(filler) {
        let a = [];
        for (let i = 0; i < n; i++) {
            a.push(filler(i, i * i));
        }
        return a;
    }

    const x_first = bulk(make_x_first);
    const y_first = bulk(make_y_first);
    const mixed = bulk(make_random);

    function time(func) {
        const start = Date.now();
        func();
        const delta = Date.now() - start;
        return delta;
    }

    function report(func) {
        let delta = time(func);
        console.log(`${delta} ms`);
    }

    function demo(dataSet) {
        for (let i = 0; i < 3; i++) {
            console.log(`run ${i+1}`);
            report(() => {
                let sumX = 0, sumY = 0;
                for (let i = 0; i < dataSet.length; i++) {
                    let item = dataSet[i];
                    sumX += item.x;
                    sumY += item.y;
                }
                return {x: sumX, y: sumY};
            });
            console.log(``);
        }
    }

    console.log(`** x_first`);
    demo(x_first);

    console.log(`** y_first`);
    demo(y_first);

    console.log(`** mixed`);
    demo(mixed);

}

doit();