Simple caching for angular.js

gistfile1.js

"use strict";
angular.module('gatewayApp')
  .factory('Cache', [function(){
    var cache = {};

    var _get = function(key) {
      var cacheHit = cache[key];
      if (_exists(cacheHit) && _notExpired(cacheHit)) {
        return cacheHit.value;
      } else {
        return null;
      }
    }
    var _exists = function(cacheHit) {
      return cacheHit !== undefined;
    }

    var _notExpired = function(cacheHit) {
      return cacheHit.expiry > new Date();
    }

    // hardcoded 1 hour expiry. Would be nice to make configurable
    var _set = function(key, value) {
      cache[key] = {
        expiry: moment().add('hour', 1).toDate(),
        value:  value
      }
      return value;
    }

    var get = function(key, getterFn) {
      var cacheHit = _get(key);
      if (cacheHit !== null) {
        // console.log("cache hit: " + key, cacheHit);
        return cacheHit;
      } else {
        // console.log("cache miss: " + key);
        var value = getterFn()
        _set(key, value);
        return value;
      }
    }

    var invalidate = function(key) {
      if (key === undefined) {
        cache = {}; // invalidate everything
      } else {
        delete cache[key]
      }
    }

    return {
      get: get,
      invalidate: invalidate
    }
  }]);