# mixin-pattern

Teaches the AI agent how to implement the mixin pattern in JavaScript and TypeScript without inheritance chains.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 616 installs · ★ 257
- Author: patterns.dev
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/PatternsDev/skills/tree/HEAD/javascript/mixin-pattern/SKILL.md
- Updated: 2026-04-15
- HTML: https://affpapa.org/en/nevibecode/skills/mixin-pattern

## What it does

This skill provides instructions on sharing behavior across objects or classes using `Object.assign()` on target prototypes. It guides the agent when composing functionality from multiple sources if classes do not share an ancestor. Examples demonstrate method injection and chaining with `super`.

Mutating prototypes brings real hazards. The skill flags prototype pollution and property collisions as main failure points. In React projects, it instructs the agent to choose Hooks over mixins. If a simple utility module solves the problem, modifying prototypes is strongly discouraged.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add PatternsDev/skills --skill mixin-pattern -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/mixin-pattern/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/mixin-pattern/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add PatternsDev/skills --skill mixin-pattern -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/mixin-pattern/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add PatternsDev/skills --skill mixin-pattern -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/mixin-pattern/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install PatternsDev/skills mixin-pattern
```

2. **Manually: put the files in place**
- File: `.github/skills/mixin-pattern/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add PatternsDev/skills --skill mixin-pattern -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/PatternsDev/skills --path javascript/mixin-pattern
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/mixin-pattern/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add PatternsDev/skills --skill mixin-pattern -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/PatternsDev/skills/tree/HEAD/javascript/mixin-pattern/SKILL.md

## When to use it
- share methods across classes without deep class hierarchies
- compose object behavior from multiple mixin sources
- prevent prototype pollution and property collisions in JavaScript
- replace legacy React mixins with modern Hooks

## Example prompts
- Show how to add shared methods to a class prototype using `Object.assign`
- Combine two functional mixins into a single target prototype in `services.ts`
- Refactor this legacy React mixin into a custom hook

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 56c70ddb751cc193.

BEGIN THIRD-PARTY FILE 56c70ddb751cc193
````text
---
name: mixin-pattern
description: Teaches the mixin pattern for sharing functionality without inheritance. Use when you need to add reusable behavior to multiple objects or classes that don't share a common ancestor.
paths:
  - "**/*.js"
  - "**/*.ts"
license: MIT
metadata:
  author: patterns.dev
  version: "1.1"
related_skills:
  - "module-pattern"
  - "singleton-pattern"
---

# Mixin Pattern

A **mixin** is an object that we can use in order to add reusable functionality to another object or class, without using inheritance. We can't use mixins on their own: their sole purpose is to _add functionality_ to objects or classes without inheritance.

Let's say that for our application, we need to create multiple dogs. However, the basic dog that we create doesn't have any properties but a `name` property.

## When to Use

- Use this when you need to add reusable functionality to multiple classes without creating an inheritance chain
- This is helpful when you want to compose behavior from multiple sources

## When NOT to Use

- When composition via hooks (React) or composables (Vue) achieves the same result with better traceability
- When prototype pollution is a risk — mixins modify shared prototypes and can cause naming collisions
- When the added functionality is simple enough that a utility function or module import suffices

## Instructions

- Use `Object.assign()` to add mixin properties to a class prototype
- Be cautious with prototype pollution — modifying prototypes can lead to unexpected behavior
- In React, prefer Hooks over mixins (mixins are discouraged by the React team)
- Consider composition over inheritance when designing reusable behavior

## Details

```js
class Dog {
  constructor(name) {
    this.name = name;
  }
}
```

A dog should be able to do more than just have a name. It should be able to bark, wag its tail, and play! Instead of adding this directly to the `Dog`, we can create a mixin that provides the `bark`, `wagTail` and `play` property for us.

```js
const dogFunctionality = {
  bark: () => console.log("Woof!"),
  wagTail: () => console.log("Wagging my tail!"),
  play: () => console.log("Playing!"),
};
```

We can add the `dogFunctionality` mixin to the `Dog` prototype with the `Object.assign` method. This method lets us add properties to the _target object_: `Dog.prototype` in this case. Each new instance of `Dog` will have access to the properties of `dogFunctionality`, as they're added to the `Dog`'s prototype!

```js
class Dog {
  constructor(name) {
    this.name = name;
  }
}

const dogFunctionality = {
  bark: () => console.log("Woof!"),
  wagTail: () => console.log("Wagging my tail!"),
  play: () => console.log("Playing!"),
};

Object.assign(Dog.prototype, dogFunctionality);
```

Let's create our first pet, `pet1`, called Daisy. As we just added the `dogFunctionality` mixin to the `Dog`'s prototype, Daisy should be able to walk, wag her tail, and play!

```js
const pet1 = new Dog("Daisy");

pet1.name; // Daisy
pet1.bark(); // Woof!
pet1.play(); // Playing!
```

Perfect! Mixins make it easy for us to add custom functionality to classes or objects without using inheritance.
````
END THIRD-PARTY FILE 56c70ddb751cc193

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