Get the device width in javascript

Is there a way to get the users device width, as opposed to viewport width, using javascript? CSS media queries offer this, as I can say @media screen and (max-width:640px) { /* ... */ } and @media screen and (max-device-width:960px) { /* ... */ } This is useful if I'm targeting smartphones in landscape orientation. For example, on iOS a declaration of max-width:640px will target both landscape an