---
title: Guard
slug: Glossary/Guard
tags:
  - API
  - CodingScripting
  - Glossary
  - guard
---
Guard is a feature of {{domxref("Headers")}} objects (as defined in the {{domxref("Fetch_API","Fetch spec")}}, which affects whether methods such as {{domxref("Headers.set","set()")}} and {{domxref("Headers.append","append()")}} can change the header's contents. For example, `immutable` guard means that headers can't be changed. For more information, read [Fetch basic concepts: guard](/en-US/docs/Web/API/Fetch_API/Basic_concepts#guard).
