Nullable Variables in Dart

Declaring Nullable Variables

String? username;      // null by default
int? age = null;       // explicitly null
double? score = 98.5;  // has a value, but could be null later

// Nullable parameters in functions
void printName(String? name) {
  print(name ?? 'Anonymous');
}

printName(null);     // Anonymous
printName('Alice');  // Alice

Null Checks

Before using a nullable variable, check if it is null:

String? email = getUserEmail(); // might return null

// Method 1: if check (promotes to non-nullable inside if)
if (email != null) {
  print(email.toUpperCase()); // safe — email is String here
}

// Method 2: ?? operator
print(email ?? 'no-email@example.com');

// Method 3: ?.  null-aware access
print(email?.toLowerCase());

Type Promotion

Inside an if (variable != null) check, Dart automatically promotes the nullable type to its non-nullable counterpart:

String? text = maybeGetText();

if (text != null) {
  // Here, text is treated as String (not String?)
  // All String methods are available without ?
  int len = text.length;  // no error
  print(text.toUpperCase());
}

Example

String? getUserEmail() => 'user@example.com';
int? getAge() => null;

void main() {
  String? email = getUserEmail();
  int? age = getAge();

  // Pattern: check and use
  if (email != null) {
    print('Email domain: $${email.split("@").last}');
  }

  // Pattern: provide default
  int userAge = age ?? 0;
  print('Age: $userAge');  // 0

  // Pattern: early return
  void processAge(int? a) {
    if (a == null) { print('Age unknown'); return; }
    print('In $${18 - a} years you will be 18');
  }
  processAge(age);   // Age unknown
  processAge(14);    // In 4 years you will be 18
}

Email domain: example.com
Age: 0
Age unknown
In 4 years you will be 18

🧠 Quiz

Summary

Nullable variables hold either a value or null. Always check with if (x != null), ??, or ?. before using them. Dart's type promotion automatically narrows the type inside null checks so you can use all non-nullable methods safely.