Booleans

DO

DO use ?? to convert null to a boolean value.

This rule applies when an expression can evaluate true, false, or null, and you need to pass the result to something that doesn’t accept null. A common case is the result of a null-aware method call being used as a condition:

if (optionalThing?.isEnabled) {
  print("Have enabled thing.");
}

This code throws an exception if optionalThing is null. To fix this, you need to “convert” the null value to either true or false. Although you could do this using ==, we recommend using ??:

// If you want null to be false:
optionalThing?.isEnabled ?? false;

// If you want null to be true:
optionalThing?.isEnabled ?? true;

// If you want null to be false:
optionalThing?.isEnabled == true;

// If you want null to be true:
optionalThing?.isEnabled != false;

Both operations produce the same result and do the right thing, but ?? is preferred for three main reasons:

• The ?? operator clearly signals that the code has something to do with a null value.

• The == true looks like a common new programmer mistake where the equality operator is redundant and can be removed. That’s true when the boolean expression on the left will not produce null, but not when it can.

• The ?? false and ?? true clearly show what value will be used when the expression is null. With == true, you have to think through the boolean logic to realize that means that a null gets converted to false.