---
type: "fhir-fhirpath-function"
title: "FHIR FHIRPath Function: extension"
source: "source/fhirpath.html"
---

# FHIR FHIRPath Function: extension

This page is part of **FHIR FHIRPath** (FHIR-specific extensions). Base spec lives at [FHIRPath](/fhirpath).

`extension(url : string) : collection`

Will filter the input collection for items named "extension" with the given `url`. This is a syntactical shortcut for `.extension.where(url = string)`, but is simpler to write. If the input collection is empty or the url is empty, the return value will be an empty collection.

Patient.extension('http://hl7.org/fhir/StructureDefinition/patient-mothersMaidenName').value

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