Cache data in the localStorage

getCache.js

var getCache = (function() {
  var supportsLocalStorage = 'localStorage' in window;

  // both functions return a promise, so no matter which function
  // gets called inside getCache, you get the same API.
  function getJSON(key) {
    return jQuery.getJSON(key).then(function(data) {
      localStorage.setItem(key, JSON.stringify(data));
    }).promise();
  }

  function getStorage(key) {
    var storageDfd = new jQuery.Deferred(),
        storedData = localStorage.getItem(key);

    if (!storedData) return getJSON(key);

    setTimeout(function() {
      storageDfd.resolveWith(null, [JSON.parse(storedData)]);
    });

    return storageDfd.promise();
  }

  return supportsLocalStorage ? getJSON : getStorage;

}());

usage.js

// first time uses an ajax request
getCache('/cache/1234').then(function (data) {
  console.log(data);
});

// second time pulls from local storage
getCache('/cache/1234').then(function (data) {
  console.log(data);
});

// always executes in asynchronous order, uses ajax always if localStorage isn't supported.

I've never heard of that happening--but it would make sense to check for the availability of local storage first.

updated to check for localStorage support