LocalStorage is not accessible inside WorkerScript

I want to run a long time  DB query inside WorkerScript.

The WS script is look like the following:

.import LocalStorage 2.10 as LS 
WorkerScript.onMessage = function(message) { 
};

But for some reason I get error: 

ReferenceError: LocalStorage is not defined

Running the LS query from a QML script works fine.