flatten.js

function flattenArr(arr) {
  let flattenedArr = [];

  function flatten(subArr) {
    for (let i of subArr) {
      if (Array.isArray(i)) {
        flatten(i);
        continue;
      }
      flattenedArr.push(i);
    }
  }
  flatten(arr);
  return flattenedArr;
}
console.log(flattenArr([1, 2, [3, 4], [5, 6, [7]]]));