flatten-array-2.js

//[1, [2, [3, 4], 5], 6, [7]]
// ->
//[1, 2, 3, 4, 5, 6, 7]

function reducer(acc, newElement) {
  if (Array.isArray(newElement)) {
    return acc.concat(newElement.reduce(reducer, []));
  } else {
    return acc.concat([newElement]);
  }
}

console.log([1, [2, [3, 4], 5], 6, [7]].reduce(reducer, []));