Methods for flattening nested array (array of arrays) in JavaScript

flattenArray.js

//Classic recursive array walk algorithm
var flatten = function(nestedArray, output) {
  output = output || [];
  var i = 0;
  
  for (;i < nestedArray.length; i++) {
    if (nestedArray[i] instanceof Array) {
      flatten(nestedArray[i], output);
    } else {
      output.push(nestedArray[i]);
    }
  }
  
  return output;
}

//Using newer array methods (reduce, some, isArray)
var flattenReduce = function(nestedArray) {
    return nestedArray.reduce(function(previous, current) {
      return previous.concat(Array.isArray(current) && current.some(Array.isArray) ? flattenReduce(current) : current);
   }, []);
}