How to mock localStorage in JavaScript unit tests?

Question

Are there any libraries out there to mock localStorage?

I\'ve been using Sinon.JS for most of my other javascript mocking and have found it is really gr

Answer 1

Overwriting the localStorage property of the global window object as suggested in some of the answers won't work in most JS engines, because they declare the localStorage data property as not writable and not configurable.

However I found out that at least with PhantomJS's (version 1.9.8) WebKit version you could use the legacy API __defineGetter__ to control what happens if localStorage is accessed. Still it would be interesting if this works in other browsers as well.

var tmpStorage = window.localStorage;

// replace local storage
window.__defineGetter__('localStorage', function () {
    throw new Error("localStorage not available");
    // you could also return some other object here as a mock
});

// do your tests here    

// restore old getter to actual local storage
window.__defineGetter__('localStorage',
                        function () { return tmpStorage });

The benefit of this approach is that you would not have to modify the code you're about to test.