> ## 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 Null-Aware Index The `?[]` (null-aware index) operator conditionally evaluates access to a collection's elements. It returns the value at the specified index or key if the collection evaluates to a non-null object, and short-circuits to return `null` if the collection reference evaluates to `null`. ```dart theme={"dark"} expression?[index] ``` ## Evaluation Mechanics When the Dart compiler encounters `expression?[index]`, it executes the following logic: 1. It evaluates the base `expression` exactly once. 2. If the result is `null`, the operation immediately yields `null`. It does not attempt to evaluate the `index` expression or invoke the underlying `[]` method. 3. If the result is a non-null object, it evaluates the `index` expression and invokes the standard `[]` (index) operator on that object. Because the `?[]` operator evaluates the base expression exactly once, it is semantically distinct from a naive ternary operation (`expression != null ? expression[index] : null`), which evaluates the base expression twice. The single-evaluation guarantee of `?[]` prevents redundant computations or unintended side effects when the base expression is a method call. Its behavior is accurately represented by binding the evaluated base expression to a temporary variable: ```dart theme={"dark"} // Semantic equivalent of expression?[index] final temp = expression; temp == null ? null : temp[index]; ``` ## Type Resolution The return type of a `?[]` operation is inherently forced to be nullable, regardless of the collection's generic type arguments. * If a collection is typed as `List?`, the expression `collection?[index]` resolves to type `T?`. * Even if the collection contains non-nullable elements (e.g., `List?`), the result of the null-aware index operation must be assigned to a nullable variable (e.g., `int?`). ## Syntax Examples **List Indexing:** ```dart theme={"dark"} List? nullableList = null; int? result1 = nullableList?[0]; // Evaluates to null nullableList = [10, 20, 30]; int? result2 = nullableList?[0]; // Evaluates to 10 ``` **Map Key Access:** ```dart theme={"dark"} Map? nullableMap = null; double? value1 = nullableMap?['pi']; // Evaluates to null nullableMap = {'pi': 3.14159}; double? value2 = nullableMap?['pi']; // Evaluates to 3.14159 ``` ## Null-Aware Index Assignment Dart allows a null-aware index expression (`?[]`) to be used as an assignable target (l-value) in conjunction with the standard assignment operator (`=`). There is no distinct `?[]=` operator token; rather, the language permits the null-aware index syntax on the left side of an assignment. This allows mutation of a collection at a specific index only if the collection is not null. If the base collection evaluates to `null`, the entire assignment operation short-circuits—the assignment is silently ignored, and the right-hand side expression is never evaluated. ```dart theme={"dark"} List? numbers = null; numbers?[0] = 42; // Operation short-circuits; numbers remains null numbers = [1, 2, 3]; numbers?[0] = 42; // numbers is now [42, 2, 3] ```

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