Flatten an array of numbers

flatten_array.js

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

  arr.map((elem, index) => {
    if (Array.isArray(elem)) {
      found = found.concat(flattenArray(elem));
    } else {
      found.push(elem);
    }
  });
  return found;
}

let testArray = [[1, 2, [3]], 4, [5, 6]];
let output = flattenArray(testArray);
console.log(output);