simple array flat prototype

prototype.js

Object.defineProperty(Array.prototype, 'flat', {
  value: function(res = []) {
    Object(this).forEach(cursor => {
      if (Object.prototype.toString.call(cursor) === '[object Array]') {
        cursor.flat(res);
      } else {
        res.push(cursor);
      }
    });

    return res;
  }
})

console.log([[1,2,[3]],4].flat(), 'flat');