How to check if module function exist in Node js

I have created a module and defined functions in it. Sometimes I need to check if a certain function is actually already created.

For example

var newyork = function() { 
   console.log("newyork");
};

var washington = function() { 
   console.log("washington");
};

exports.newyork = newyork;
exports.washington = washington;

Now in the different file, I want to first check if the function exists, something like:

var cities = require('./city');

if(cities.newyork) {
  console.log("city function exist");
}
else {
  //false
}