How to use the Immutable Data Patterns agent skill

What does the immutable-patterns skill make the agent do?

An agent skill is a folder of instructions that an AI agent can load when a task matches its description. This one covers a single JavaScript best practice: prefer immutable data patterns. Mutating shared objects makes it hard to track where a value changed. Immutable updates create a new object for each version of the data, so you can compare references to detect changes and find which part of the code changed the data.

When you ask for a review, the agent looks for direct property assignments on function parameters and push or splice calls on arrays. It suggests spread syntax and non-mutating methods such as map(), filter() and reduce(), and it can suggest Object.freeze() for configuration objects that should never change.

How do you install the immutable-patterns skill?

Download the ZIP for the skill from our page and unzip it. The skill contains two files: SKILL.md and references/rule.md. Place the unzipped folder in one of these locations:

You can install it for yourself in ~/.claude/skills/, or for one project in that project's .claude/skills/ folder.

In the Claude apps, you can instead upload the skill under Settings → Capabilities → Skills.

• Personal: ~/.claude/skills/ (unzip the folder here)
• Project: .claude/skills/ inside your project folder
• Claude apps: Settings → Capabilities → Skills, then upload

How do you trigger the skill with a natural request?

You do not need a special command. The skill's description tells the agent when to use it: reviewing scripts, client components, bundles, or runtime behaviour related to immutable data patterns. Ask for a review in plain language that names the kind of code and the concern.

Mention mutation, state or immutability in your request so the match is clear.

• Review this React reducer for places where state is mutated in place.
• Check this Zustand store update for push, splice or direct property assignments.
• Look at this component's event handler and tell me where it changes props or state objects directly.
• Explain why this Redux update could cause a bug and suggest a non-mutating version.

What does a typical result look like?

The agent first flags the exact lines that mutate data, such as a direct assignment to user.name or a call to items.push(). It then rewrites them with spread or non-mutating methods. The reference guide shows the pattern: return { ...user, ...changes } instead of changing user in place, and a nested update that spreads the address object as well.

For component code, the skill also asks the agent to check the browser execution path, flagging runtime side effects or blocking operations and stating how to verify the change in the browser.

• Before: user.name = changes.name; return user
• After: return { ...user, ...changes }
• Nested update: return { ...user, address: { ...user.address, ...address } }

What does the skill need, and what should you check first?

The skill lists no required tools or dependencies. It does not run scripts and does not use the network, so it only provides instructions the agent reads.

Because the skill pushes toward copies without weighing cost, review its suggestions before applying them to performance-critical code. Copying large data structures can be expensive, and sometimes mutation is intentional.

• Requirements: none listed
• Runs scripts: no
• Uses network: no

What are the limits and tips?

The guidance is specific to JavaScript. For other languages, or for projects that are not JavaScript, the skill is not the right fit.

Use it as a review pass for mutation bugs across a team, so that changes follow a consistent checklist. Ask the agent to explain the reasoning if you are learning why immutable updates matter.

• Good for: reviewing React, Redux, Zustand or similar state code
• Good for: a consistent review pass for mutation bugs
• Be careful with: large data copies in performance-critical code
• Not for: non-JavaScript projects