This is kind of tricky... You need to call app.evaluate to get this done. But this method is not available in the App API. Instead you need to use enigma.js. Since enigma is autogenerated, all methods defined in the engine API are available. Do note that enigma is flagged as experimental, though.
In an extension you can find enigma at this.backendApi.model.enigmaModel. That is a reference to the GenericObject wrapper, and in this case you need the app wrapper at this.backendApi.model.enigmaModel.app. This means you could do:
And you will get a printout like this:
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Hope this helps