---
type: "fhirpath-function"
title: "FHIRPath Function: indexOf"
function: "indexOf"
category: "String Manipulation"
section: "5.6.1"
source: "fhirpath/functions.json"
---
# FHIRPath Function: indexOf
Returns the 0-based index of the first position `substring` is found in the input string, or -1 if it is not found.

If `substring` is an empty string (`''`), the function returns 0.

If the input or `substring` is empty (`{ }`), the result is empty (`{ }`).

If the input collection contains multiple items, the evaluation of the expression will end and signal an error to the calling environment.

``` fhirpath
'abcdefg'.indexOf('bc') // 1
'abcdefg'.indexOf('x') // -1
'abcdefg'.indexOf('abcdefg') // 0
```
## Summary
- **Category**: String Manipulation
- **Section**: `5.6.1`
- **Return Type**: `Integer`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `true`
## Description
Returns the 0-based index of the first position `substring` is found in the input string, or -1 if it is not found.
## Arguments
- `substring`: `String` - Substring to find
## Type Mapping
- `String-Integer`
## Example
```fhirpath
'abcdefg'.indexOf('bc') // 1
'abcdefg'.indexOf('x') // -1
'abcdefg'.indexOf('abcdefg') // 0
```