NodeJS mandelbrot bench

class Complex {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }

  /** Return the multiplication of this and another complex num */
  mul(other) {
    return new Complex(other.x * this.x - other.y * this.y,
                       other.x * this.y + other.y * this.x);
  }

  add(other) {
    return new Complex(other.x + this.x, other.y + this.y);
  }

  len_2() {
    return this.x * this.x + this.y * this.y;
  }
}

function mandel(c) {
  let iter = function(z) {
    return (z.mul(z)).add(c);
  };
  let z = new Complex(0, 0);
  for (let ii = 0; ii < 100000; ++ii) {
    z = iter(z);
    if (z.len_2() > 4) {
      return ii;
    }
  }
  return -1;
}

let start = (new Date()).getTime();
let values = [];
for (let ii = 0; ii < 100; ++ii) {
  values.push([]);
  for (let jj = 0; jj < 100; ++jj) {
    let c = new Complex(ii / 20, jj / 20);
    values[values.length - 1].push(mandel(c));
  }
}
let end = (new Date()).getTime();

console.log("Result: " + values);
console.log("Time taken (millis): " + (end - start));