Flatten array

flatten-array.js

const arr = [[1,2,[3]],4];

function flattenArray(arr = []) {
  let flattenedArray = [];

  arr.forEach(item => {
      if (Array.isArray(item)) {
          flattenedArray = [...flattenedArray, ...flattenArray(item)];
      } else {
          flattenedArray.push(item);
      }
  });

  return flattenedArray;
}

flattenArray(arr);