# APPEND

### Description

`APPEND` returns a new list with a single element added to the end. Unlike [`CONCAT`](/rule-language/operators/string-operators/concat.md), which merges two lists together, `APPEND` takes an existing list and a single value and returns a new list containing all original elements followed by that value. The original list is not mutated.

### Syntax

```
APPEND(list, element)
```

**Returns:** A new list with `element` appended.

### Example

```
APPEND(['a', 'b', 'c'], 'd')
```

This returns `['a', 'b', 'c', 'd']`.

### Difference from CONCAT

`CONCAT` merges two lists:

```
CONCAT(['a', 'b'], ['c', 'd'])
```

Returns `['a', 'b', 'c', 'd']`.

`APPEND` adds a single element, not a list:

```
APPEND(['a', 'b'], 'c')
```

Returns `['a', 'b', 'c']`.

If you pass a list as the element to `APPEND`, it will be added as a nested list, not merged:

```
APPEND(['a', 'b'], ['c', 'd'])
```

Returns `['a', 'b', ['c', 'd']]`.


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