Unexpected 'null' in non-nullable properties of the Navigator class

[miquelbeltran]

Hello, we are getting reports of this issue from device_info_plus users:

[Log] TypeError: null: type 'Null' is not a subtype of type 'double' (dart_sdk.js, line 38043)

The deviceMemory is declared as double but sometimes it may be null.

The following comment has more details: fluttercommunity/plus_plugins#3391 (comment)

I am not sure if this may be the case for other navigator properties.

[srujzs]

deviceMemory is unavailable on Firefox and Safari: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/deviceMemory.

In general, we use the IDL as the source of truth. Making a property nullable to tackle various browser capabilities and edge-cases means excessive null-checks for users and even more breaking changes when those eventually change. dart:html's body member is one such example of leading to excessive null-checks. We have discussed emitting metadata though on various members to signal which browser supports it, but that may be too verbose.

As for the fix, I'd avoid catching cast errors, and instead use extension members:

extension on web.Navigator {
  @JS('deviceMemory')
  external double? get browserSafeDeviceMemory;
}