Flatten an array of nested arrays

flatten_arr.js

/*
 * flatten_arr(array) 
 * Purpose: Flatten an array of nested arrays
 *
 * Usage: 
 * var arrayToFlatten = flatten_arr([[1,2,[3]],4]); // [1,2,3,4]
 *
 * // If not passing an array it returns the initial value
 * var notArray = flatten_arr('5'); // '5' returns initial value
*/

function flatten_arr(arr) {

  // If argument is not an array return
  if(!Array.isArray(arr)) {
    console.error("flatten_arr - Argument is not an array");
    // Return initial value.
    return arr;
  }
  
  var reduced = arr.reduce(function(outputArr, currentVal) {
    return outputArr.concat(currentVal);
  }, []);
  
  // Check if any arrays are nested past the inital reduce
  for(prop of reduced) {
    if(Array.isArray(reduced[prop])) {
      return flatten_arr(reduced);
    }
  }
  
  return reduced;
  
}