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# Dart Addition Assignment
The `+=` (addition assignment) operator is a compound assignment operator that evaluates the right-hand expression, adds its value to the current value of the left-hand variable, and assigns the computed result back to the left-hand variable.
## Syntax
```dart theme={"dark"}
variable += expression;
```
This operation is syntactic sugar for the following standard assignment:
```dart theme={"dark"}
variable = variable + expression;
```
## Type-Specific Behavior
The underlying operation executed by `+=` is dictated by the static type of the left operand and its specific implementation of the `+` operator:
* **Numeric Types (`int`, `double`, `num`):** Performs standard arithmetic addition.
* **Strings (`String`):** Performs string concatenation.
* **Custom Classes:** Invokes the overridden `operator +` method defined on the left operand's class.
## Technical Characteristics
* **Single Evaluation:** The left operand is evaluated exactly once. In a complex expression such as `collection[computeIndex()] += value`, the `computeIndex()` function is executed only one time. This differs from the expanded form `collection[computeIndex()] = collection[computeIndex()] + value`, which would evaluate the index twice.
* **Type Compatibility:** The return type of the underlying `+` operation must be assignable to the static type of the left operand. For example, if `a` is an `int` and `b` is a `double`, the expression `a += b` will throw a compile-time error. The operation `a + b` yields a `double`, which cannot be assigned back to the `int` variable `a` without an explicit cast.
* **Mutability Requirement:** The left operand must be a mutable reference. It cannot be a variable declared with the `final` or `const` modifiers.
* **Null Safety:** Neither the left nor the right operand can be `null` unless the left operand's class explicitly overrides `operator +` to accept nullable types (which is highly non-standard in Dart). Attempting `+=` on a nullable numeric or string type without prior null-checking will result in a compile-time error.
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