---
type: "fhirpath-function"
title: "FHIRPath Function: join"
function: "join"
category: "Additional String Functions"
section: "5.7.7"
source: "fhirpath/functions.json"
---
# FHIRPath Function: join
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The join function takes a collection of strings and _joins_ them into a single string, optionally using the given separator.
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If the input is empty, the result is empty.
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If no separator is specified, the strings are directly concatenated.
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The following example illustrates the behavior of the `.join` operator:
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``` fhirpath
('A' | 'B' | 'C').join() // 'ABC'
('A' | 'B' | 'C').join(',') // 'A,B,C'
```
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## Summary
- **Category**: Additional String Functions
- **Section**: `5.7.7`
- **Return Type**: `String`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `false`
## Description
Combines the elements in the input collection into a single string, separated by `separator`. If no separator is specified, the elements are concatenated directly.
## Arguments
- `separator` (optional): `String` - Optional separator to use between elements
## Type Mapping
- `String-String`
## Example
```fhirpath
{'a', 'b', 'c'}.join(',') // 'a,b,c'
{'a', 'b', 'c'}.join() // 'abc'
```