flatten using reduce

flattenReduce.js

function each(collection, callback) {
  if (Array.isArray(collection)) {
    for (var i = 0; i < collection.length; i++) {
      callback(collection[i]);
    }
  } else {
    for (var j in collection) {
      callback(collection[j]);
    }
  }
}

function reduce(collection, callback, initial) {
  var initialFlag = arguments.length < 3? false : true;
  each(collection, function (e) {
    if (!initialFlag) {
      initial = e;
      initialFlag = true;
    } else {
      initial = callback(initial, e);
    }
    return initial;
  });
  return initial;
}

function flattenReduce(array) {
  // on each call of flattenReduce return array transformed to flattened array
  return reduce(array, function(total, curr){
    // if value is not an array, you can just push to total
    if (!Array.isArray(curr)) {
      total.push(curr);
    } else {
      // transform current value to flattened array with recursive call
      curr = flattenReduce(curr);
      // flattenReduce returns flat array
      // for each value in flattened array, push to total
      each(curr, function(e) {total.push(e);})
    }
    return total;
  }, []);
}