> ## 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 The `[]` (index) operator in Dart is an overloadable instance method that enables subscript access on objects, allowing them to be queried or mutated using array-like or map-like bracket notation. In Dart, subscripting is not a built-in language primitive restricted to core collections; rather, it is syntactic sugar for standard method invocations. The operator is divided into two distinct method signatures: one for retrieval (`operator []`) and one for assignment (`operator []=`). ## Syntax and Implementation To support the index operator, a class must define one or both of the following methods. The following example uses generic type parameters to demonstrate a valid, compilable implementation: ```dart theme={"dark"} class CustomIndexer { final Map _internalData = {}; // Handles read access: var x = obj[index]; V? operator [](K index) { return _internalData[index]; } // Handles write access: obj[index] = value; void operator []=(K index, V value) { _internalData[index] = value; } } ``` ## Technical Characteristics * **Arity Constraints:** * `operator []` must accept exactly one parameter (the index). * `operator []=` must accept exactly two parameters (the index, followed by the value to assign). * **Type Flexibility:** The index parameter is not constrained to integers. It can be any valid Dart type, including `String`, `Enum`, or custom objects. This is the underlying mechanism that allows Dart's `Map` to accept arbitrary objects as keys. * **Return Types:** * `operator []` can return any type, including nullable types (`T?`), which is standard practice when an index might be out of bounds or a key might not exist. * `operator []=` must have a `void` return type, as the assignment expression evaluates to the assigned value inherently, not via the method's return. * **Invocation Translation:** The Dart compiler translates bracket notation directly into method calls at compile time. However, unlike standard methods, operator methods cannot be invoked using dot notation with the `operator` keyword (e.g., `instance.operator[](key)` is strictly invalid Dart syntax and will cause a compile-time error). They must be invoked using the bracket syntax: * `var x = instance[key];` conceptually invokes the `operator []` method with `key` as the argument. * `instance[key] = value;` conceptually invokes the `operator []=` method with `key` and `value` as the arguments. * To invoke a superclass implementation, Dart provides the explicit `super[key]` and `super[key] = value` syntax. * **Null Safety:** If the object being indexed is potentially null, Dart provides the null-aware index operator (`?[]`), which short-circuits the evaluation to `null` without throwing a `NoSuchMethodError` (e.g., `var x = instance?[key];`).

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