> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Dart If-Null Assignment
The `??=` operator is a null-aware compound assignment operator in Dart. It evaluates the left-hand operand and assigns the value of the right-hand expression to it strictly if the left-hand operand evaluates to `null`. As an expression, the operation yields the final value of the left-hand operand, allowing it to be used inline within larger expressions.
```dart theme={"dark"}
leftOperand ??= rightExpression;
```
## Evaluation Mechanics
1. **Expression Yield:** Because `??=` is an expression rather than a statement, it evaluates to a value. This distinguishes it from an `if (x == null) { x = y; }` block, which is a statement that yields no value.
```dart theme={"dark"}
```
int? x;
print(x ??= 5); // Assigns 5 to x, evaluates to 5, and prints 5
````
2. **Single Receiver Evaluation:** When the left-hand operand involves a property access, the compound `??=` operator guarantees the receiver is evaluated exactly once. The expression `obj.value ??= y` is semantically distinct from the expanded form `obj.value = obj.value ?? y`, as the expanded form evaluates the receiver (`obj`) twice.
3. **Short-Circuiting:** The operator utilizes short-circuit evaluation. If the left-hand operand evaluates to a non-null value, the right-hand expression is completely ignored and never executed.
4. **Type Constraints:** The right-hand expression must yield a value whose type is assignable to the left-hand operand. While typically applied to nullable types, Dart permits the use of `??=` on non-nullable types. Doing so is not a compile-time error, though it triggers a static analyzer warning for dead code since the left side can never be `null` and the right side is unreachable.
## Behavioral Examples
**Null Operand Assignment:**
```dart
int? a;
a ??= 10; // 'a' is null, 10 is assigned. Expression evaluates to 10.
````
**Non-Null Operand Bypass:**
```dart theme={"dark"}
int? b = 5;
b ??= 20; // 'b' is not null, assignment is bypassed. Expression evaluates to 5.
```
**Short-Circuit Execution:**
```dart theme={"dark"}
int? c = 100;
int computeValue() {
print("Evaluated");
return 50;
}
c ??= computeValue(); // computeValue() is never called; 'c' remains 100.
```
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