> ## 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 Index Assignment The `[]=` operator, formally known as the index assignment operator, is a syntactic mechanism in Dart used to mutate the state of an object at a specified index or key. When the Dart compiler encounters this operator, it translates the assignment expression into an invocation of the underlying `operator []=` method. ```dart theme={"dark"} void main() { List items = ['a', 'b', 'c']; int index = 1; String value = 'z'; // Standard assignment syntax items[index] = value; } ``` ## Technical Characteristics * **Arity and Parameters:** The `operator []=` method requires exactly two parameters. The first parameter represents the index or key (an expression evaluated to determine the target location), and the second parameter represents the value being assigned. * **Type Flexibility:** The static types of both the index/key expression and the value are arbitrary and defined by the implementing class. For example, in a `List`, the index expression must evaluate to an `int` and the value to `T`. In a `Map`, the key expression evaluates to `K` and the value to `V`. * **Return Type:** The return type of the `operator []=` method must be `void` (attempting to return a value results in a compile-time error). While modern Dart linting practices strongly recommend explicitly declaring the `void` keyword, the compiler does not strictly require it to be written. Note that the assignment expression itself evaluates to the assigned value (allowing chained assignments like `a[0] = b[0] = 5`), even though the underlying operator method returns nothing. ## Custom Implementation Dart allows developers to overload the `[]=` operator within custom classes to define proprietary index assignment behavior. This is achieved by declaring a method named `operator []=`. ```dart theme={"dark"} class Grid { final List _data = List.filled(100, 0); // Overloading the index assignment operator // Parameter 1: The index/key expression (int) // Parameter 2: The value to assign (int) void operator []=(int index, int value) { if (index < 0 || index >= _data.length) { throw RangeError.index(index, _data); } _data[index] = value; } } void main() { Grid myGrid = Grid(); myGrid[10] = 255; // Invokes the custom operator []= } ``` ## Null-Aware Assignment The `[]=` operator interacts with Dart's null safety features. If the target object is potentially null, Dart utilizes a distinct null-aware index operator (`?[]`) to safely perform the assignment. This prevents runtime exceptions by short-circuiting the evaluation if the receiver is null. ```dart theme={"dark"} void main() { List? collection; int index = 0; int value = 42; // If 'collection' is null, the assignment is bypassed. // If 'collection' is non-null, operator []= is invoked. collection?[index] = value; } ```

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