Pure Functions.

pure_functions.js

//this is not a pure function because it has a 'hidden input' of the current date.
function addDay(days) {
  var currentDate = new Date();
  return currentDate.setDate(currentDate.getDate() + days);
}

//this is not a pure function because it has a 'hidden output' of the times called.
var timesCalled = 0;

function addDay(days, currentDate) {
  timesCalled = timesCalled + 1;
  return currentDate.setDate(currentDate.getDate() + days);
}

//this is a pure function because it has no hidden inputs or outputs.
function addDays(days, currentDate, timesCalled) {
  return [currentDate.setDate(currentDate.getDate() + days), timesCalled + 1]
}