converts multi dimensional array into single dimensional array

flatten.js

var multi = [3, 4, [5, 6, [7, 8], 9], [10, 11]];

function flatten(array) {
  var simple = []
  function slice(array) {
    for (item of array) {
      if (Array.isArray(item)) {
        slice(item)
      } else {
        simple.push(item)
      }
    }
  }
  slice(array)
  return simple
}

console.log(flatten(multi))