Simple Javascript speed testing

test.js

let a = [
    [1, 2, 3, 4],
    [1, 2, 3, 4],
    [1, 2, 3, 4],
    [1, 2, 3, 4],
];

let t1 = new Date().getTime();

for (let i = 0; i < 1000000; i++) {
    for (let j in a) {
        for (let k in a[j]) {
            let x1 = a[j];
            let x2 = a[j];
            let x3 = a[j];
            let x4 = a[j];
            // console.log(x);
        }
    }
}

let t2 = new Date().getTime();

console.log(t2 - t1);

for (let i = 0; i < 1000000; i++) {
    for (let j in a) {
        let y = a[j];
        for (let k in y) {
            let x1 = y[k];
            let x2 = y[k];
            let x3 = y[k];
            let x4 = y[k];
            // console.log(x);
        }
    }
}

let t3 = new Date().getTime();

console.log(t3 - t2);