What is the Dart null checking idiom or best practice?

Question

I have the following form of assignment & null checks to avoid double lookups in my maps.
Is there a better or more idiomatic way to do this in Dart?

bool isConnected(a, b){   List list;   return (     ((list = outgoing[a]) != null && list.contains(b)) ||     ((list = incoming[a]) != null && list.contains(b))   ); } 

Answer 1

As of Dart 1.12 null-aware operators are available for this type of situation:

bool isConnected(a, b) {   bool outConn = outgoing[a]?.contains(b) ?? false;   bool inConn = incoming[a]?.contains(b) ?? false;   return outConn || inConn; } 

The ?. operator short-circuits to null if the left-hand side is null, and the ?? operator returns the left-hand side if it is not null, and the right-hand side otherwise.

The statement

outgoing[a]?.contains(b) 

will thus either evaluate to null if outgoing[a] is null, or the boolean result of contains(b) if it is not.

That means the resulting statement will be one of the following:

bool outConn = null ?? false; // false bool outConn = false ?? false; // false bool outConn = true ?? false; // true 

The same applies to the inConn boolean, which means both inConn and outConn are guaranteed to be non-null, allowing us to return the result of ||ing the two.