# Handlebars Helper Function: flatten

Transforms a list of lists into a flat list. This function is useful for normalizing nested data structures in templates.

## Availability

This helper is available in the Winter ’26 release of Marketing Cloud Next.

## Syntax

```handlebars
{{flatten nestedList}}
```

## Parameters

| Parameter    | Type  | Description                             |
| ------------ | ----- | --------------------------------------- |
| `nestedList` | Array | Required. The list of lists to flatten. |

## Return Value

The function returns a new list containing all elements from the nested lists. If the input isn’t a list of lists, or if a parameter is missing or an element isn’t a list, the function throws an exception.

## Usage Examples

This example shows how to use the `flatten` function to flatten this example array.

```javascript
myArray = [[1, 2], [3, 4], [5]];
```

To flatten the array, use the `flatten` function, as shown in this example.

```handlebars
{{flatten myArray}}
```

The function returns `[1, 2, 3, 4, 5]`.