const intervalls = [[0, 3], [2, 6], [3, 4], [6, 9]];
const getLargestMinAndSmallestMax = (acc, intervall) => {
    if (acc[1] === undefined || intervall[0] > acc[1]) {
      acc[1] = intervall[0];
    }
    if (acc[0] === undefined || intervall[1] < acc[0]) {
      acc[0] = intervall[1];
    }
    return acc;
  };
const smallestIntervallCoveringAllIntervalls = list => list.reduce(getLargestMinAndSmallestMax, []);

console.log(smallestIntervallCoveringAllIntervalls(intervalls));