Sadly, this does not work as you expect. I guess by _ you mean the _() function passed to the callback passed to the get() function of MockLazyL10n (see https://github.com/mozilla-b2g/gaia/blob/master/shared/test/unit/mocks/dialer/mock_lazy_l10n.js). This _() function returns the key and consequently your previous code will store 'totalDurationMinutes' in the variable totalCallDuration. Definitely not what we want.