function flatten(array) {
  var l = array.length;
  while (l--) {
    if (Array.isArray(array[l])) {
      flatten(array[l]);
      array.splice(l, 1, ...array[l]);
    }
  }
}


var array = [['1', '2', '3'], ['4', '5', ['6'], ['7', '8']]];

flatten(array);

console.log(array);