citrusbyte

const isFlat = array => {
  if (!array) return true;
  for (let i = 0; i < array.length; i++) {
    if (array[i] instanceof Array) return false;
  }
  return true;
}

const makeFlat = arr => {
  while (!isFlat(arr)) arr = arr.reduce((a,b) => a.concat(b), []);
  return arr
}

console.log(makeFlat([]));
console.log(makeFlat([{ payment: 'cash'}, { test: true}, { activity: 'skiing'}]));
console.log(makeFlat([['mixed', 2], 'abc']));
console.log(makeFlat([[[1], 2], 3]));
console.log(makeFlat([[[{ test: true, other: false}], 2], 3]));
console.log(makeFlat([[50], [60], [70]]));
console.log(makeFlat([[[[1], { test: false}], [3]]]));