A simple implementation of flatten in javascript

flatten.js

function flatten(arr){
  var output = [];
  for (var i = 0; i < arr.length; i++) {
    if(arr[i] instanceof Array){
      output = output.concat(flatten(arr[i]))
    } else {
      output.push(arr[i]);
    }
  };
  return output;
}

// console.log( flatten([1,[2,3,[4,5]],6]) ) -> [1,2,3,4,5,6]