Does this work as expected? It seems to me you want to delete `cache[url]`?

```javascript
var cache = {foo: 'FOO', bar: 'BAR'};
// undefined
var foo = cache.foo;
// undefined
foo;
// "FOO"
cache.foo;
// "FOO"
delete foo;
// true
cache.foo;
// "FOO"
foo;
// ReferenceError: foo is not defined
```