Dart: What does a question mark (?) after a variable name mean?

Question

itemCount: _contacts?.length ?? 0,

and I have two question on that,

what is meant by variableName? ( variableName followed by question mark '?') e.g in above _contacts?

The second question was on two question marks and is at What are the ?? double question marks in Dart?

user_10273524 · Answer

"Variable?" is use for null safety. If _contact has value null then we can't check length of variable or it's throw error.

"??" is used for if condition. It's assign value 0.

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