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operators - What are the ?? double question marks in Dart?

The following line of code has two question marks:

final myStringList = prefs.getStringList('my_string_list_key') ?? [];

What is the meaning?

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The ?? double question mark operator means "if null". Take the following expression, for example.

String a = b ?? 'hello';

This means a equals b, but if b is null then a equals 'hello'.

Another related operator is ??=. For example:

b ??= 'hello';

This means if b is null then set it equal to hello. Otherwise, don't change it.

Reference

Terms

The Dart 1.12 release news collectively referred to the following as null-aware operators:

  • ?? -- if null operator
  • ??= -- null-aware assignment
  • x?.p -- null-aware access
  • x?.m() -- null-aware method invocation

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