Closures

example.js

var factoryAdder = function(initialValue) {
  // this function "closes over" initialValue, 
  // effectively sealing it into the context
  return function(numberToAdd) {
    return initialValue + numberToAdd;
  };
};

var add5 = factoryAdder(5);
console.log(add5(7)); // logs 12

var add100 = factoryAdder(100);
console.log(add100(add5(20))); // logs 125