Array.flat

if (!Array.prototype.flat) {
  Array.prototype.flat = function (depth = 1) {
    let flatten = (arr, depth) => {
      return depth > 0
        ? arr.reduce(
            (acc, val) =>
              acc.concat(Array.isArray(val) ? flatten(val, depth - 1) : val),
            []
          )
        : arr.slice();
    };
    return flatten(this, depth);
  };
}