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?

Answer 1

Posting the following, as I ended up here by searching the title.

As per https://github.com/dart-lang/sdk/issues/66, it seems that the answer to "What is the Dart null checking idiom or best practice?" is:

There is no null checking idiom or best practice in the general case. If null-aware operators don't fit your case use direct comparison as in if (object == null) or if (object != null).

Answer 2

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.

Answer 3

bool ok<T>(T obj) => obj != null;
bool nul<T>(T obj) => obj == null;

Answer 4

There are now 4 null aware operators

??     provides a default if the subject is null

return subject ?? "defaultIfNull";

??=     sets the subject to a default only if the subject is null

This is similar to ?? but sets the subject variable to a default if it is null.

subject ??= "defaultIfNull";

?.     avoid an exception if subject is null when accessing subject's property

object?.x will return null if object is null, object.x would cause an exception if object were null

...?     from a spread collection, avoid a null item in the final list if the subject list is null

the result of the following

[
  ...[1, 2],
  null,
]

is [1, 2, null]

to avoid the null value use ...?

var resultingList = [
  ...[1, 2],
  ...?subjectList,
];