Question: `document.cookie` is like a string, but it is not a string. To quote the example from the [Mozilla doc](https://developer.mozilla.org/En/Document.cookie):

```
document.cookie = "name=oeschger";
document.cookie = "favorite_food=tripe";
alert(document.cookie);
// displays: name=oeschger;favorite_food=tripe

```

If you tried to make a mock cookie using only a string, you would *not* get the same results:

```
var mockCookie = "";
mockCookie = "name=oeschger";
mockCookie = "favorite_food=tripe";
alert(mockCookie);
// displays: favorite_food=tripe

```

So, if you wanted to unit test a module that operates on the cookie, and if you wanted to use a mock cookie for those tests, could you? **How?**

Answer: 
You could create an object with a `cookie` setter and getter. Here is a very simple implementation:

```
var mock = {
    value_: '', 

    get cookie() {
        return this.value_;
    },

    set cookie(value) {
        this.value_ += value + ';';
    }
};

```

Might not work in all browsers though (especially IE). **Update:** It only works in browsers supporting ECMAScript 5!

[**More about getter and setters**](https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Working_with_Objects#Defining_Getters_and_Setters).

```
mock.cookie = "name=oeschger";
mock.cookie = "favorite_food=tripe";
alert(mock.cookie);
// displays: name=oeschger;favorite_food=tripe;

```

[**DEMO**](http://jsfiddle.net/fkling/xEVBs/)
Here is how I ended up doing it in Jest:

My cookie wasn't added anymore to `document.cookie` after adding the `Secure` attribute (because `document.location` is the insecure `http://localhost`).

So after lots of trials and errors *(trying to intercept `document.cookie`'s setter and dropping its `;Secure` from it, but then calling again `document.cookie =`  with the new value triggered an infinite loop as it entered again the setter...)*, I ended up with this quite simple solution:

```js
beforeEach(function() {
  let cookieJar = document.cookie;
  jest.spyOn(document, 'cookie', 'set').mockImplementation(cookie => {
    cookieJar += cookie;
  });
  jest.spyOn(document, 'cookie', 'get').mockImplementation(() => cookieJar);
})

```