# Array.flat_map

## Function

First map and then expand the mapping result by depth 1.

## Syntax

```bt
array.flat_map(callback)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| callback | Fn | No | Empty | The callback signature is fn(value, index, current). |

## Return value

| Type | Description |
| ------ | ------ |
| Array | Returns the new array after mapping and expansion. |

## Example

```bt
items = [1, 2]
result = items.flat_map(fn(value) { [value, value * 10] })

// Output: [1,10,2,20]
print result
```

## Notes

- Expand only one level of array.