JS flatten

// [[1,2,[3]],4] -> [1,2,3,4]

function flatten(a) {
  let result = [];
  
  function searchItemAndAdd(array) {
    array.forEach((item) => {
      if (typeof(item) === 'object') {
        searchItemAndAdd(item);
      } else {
        result.push(item);
      }
    });
  }
  
  searchItemAndAdd(a);
  
  return result;
}

let sample = [1,2,3,[4,5],6, [7,8,[9]]];
console.log('Should contain 9 items: ', flatten(sample).length === 9);
console.log('Should not contain 6 items: ', flatten(sample).length !== 6);
console.log('Should not contain Array inside: ', (flatten(sample).filter((i) => typeof(i) === 'object').length === 0));

let sample2 = [1,[2,3]];
console.log('Should contain 9 items: ', flatten(sample2).length === 3);
console.log('Should not contain 6 items: ', flatten(sample2).length !== 2);
console.log('Should not contain Array inside: ', (flatten(sample2).filter((i) => typeof(i) === 'object').length === 0));