Assign value if not exist in Dart

In Dart, you can assign a value to a variable only if it does not already have a value using the null-aware assignment operator ??=. The syntax of the null-aware assignment operator is variableName ??= valueToAssignIfNull.

Here's an example:

void main() {
  int a;
  a ??= 10; // if a is null, assign 10 to it
  print(a); // Output: 10
int b = 20;

  b ??= 30; // if b is not null, it will retain its value (20)

  print(b); // Output: 20

}

In the above example, we declared two variables a and b. We assigned null to a and 20 to b. We then used the null-aware assignment operator ??= to assign 10 to a if it was null, and 30 to b if it was null. Since a was null, it got assigned the value 10, and b was not null, so it retained its value of 20.

This is a handy operator when you want to assign a default value to a variable that may or may not have been initialized.