tofrac.js

function tofrac(x0) {
  let x = x0;
  let a = Math.floor(x);
  let [h, k, h1, k1] = [a, 1, 1, 0];

  while (x - a > Number.EPSILON * k * k) {
    x = 1 / (x - a);
    a = Math.floor(x);
    let [h2, k2] = [h1, k1];
    [h1, k1] = [h, k];
    h = h2 + a * h1;
    k = k2 + a * k1;
  }

  return `${h}/${k}`;
}