flatteningarray.js

/**
 * Converts nested arrays into one-dimensional flattening array (recursively)
 * @param {Array} arr Input nested arrays 
 * @param {Array} result Output one-dimensional flattening array
 * @return Output one-dimensional flattening array
 */
function flattenNestedArrays(arr, result = []) {
  const length = arr.length;
  for (let i = 0; i < length; i++) {
    const value = arr[i];
    if (Array.isArray(value)) { // if the current element is an Array then call flattenNestedArrays recursively to flattening this array
      flattenNestedArrays(value, result);
    } else {
      result.push(value); // if it is normal element then just push into array
    }
  }
  return result;
};


// execution test
const inputArray = [[1, 2, [3]], 4];
const result = flattenNestedArrays(inputArray);
console.log(result);