# ItemSelector (Map)
<a name="input-output-itemselector"></a>

**Managing state and transforming data**  
Learn about [Passing data between states with variables](workflow-variables.md) and [Transforming data with JSONata](transforming-data.md).

By default, the effective input for the `Map` state is the set of individual data items present in the raw state input. With the `ItemSelector` field, you can override the data items’ values before they’re passed on to the `Map` state. 

To override the values, specify a valid JSON input that contains a collection of key-value pairs. The pairs can be static values provided in your state machine definition, values selected from the state input using a [path](amazon-states-language-paths.md), or values accessed from the [Context object](input-output-contextobject.md). 

If you specify key-value pairs using a path or Context object, the key name must end in `.$`.

**Note**  
The `ItemSelector` field replaces the `Parameters` field within the `Map` state. If you use the `Parameters` field in your `Map` state definitions to create custom input, we recommend that you replace them with `ItemSelector`.

You can specify the `ItemSelector` field in both an *Inline Map state* and a *Distributed Map state*.

For example, consider the following JSON input that contains an array of three items within the `imageData` node. For each *`Map` state iteration*, an array item is passed to the iteration as input.

```
[
  {
    "resize": "true",
    "format": "jpg"
  },
  {
    "resize": "false",
    "format": "png"
  },
  {
    "resize": "true",
    "format": "jpg"
  }
]
```

Using the `ItemSelector` field, you can define a custom JSON input to override the original input as shown in the following example. Step Functions then passes this custom input to each *`Map` state iteration*. The custom input contains a static value for `size` and the value of a Context object data for `Map` state. The `$$.Map.Item.Value` Context object contains the value of each individual data item.

```
{
  "ItemSelector": {
    "size": 10,
    "value.$": "$$.Map.Item.Value"
  }
}
```

The following example shows the input received by one iteration of the *Inline Map state*:

```
{
  "size": 10,
  "value": {
    "resize": "true",
    "format": "jpg"
  }
}
```

**Tip**  
For a complete example of a *Distributed Map state* that uses the `ItemSelector` field, see [Copy large-scale CSV using Distributed Map](tutorial-map-distributed.md).