# JS Command Pattern

> Encapsulate operations as objects to support undo, queue, and logging

## When to Use

- You need undo/redo functionality
- Operations should be queued, deferred, or logged
- You want to decouple the sender of an operation from its executor

## Instructions

1. Define a command interface with `execute()` and optionally `undo()` methods.
2. Each command class captures the receiver and parameters needed to perform (and reverse) the operation.
3. Store executed commands in a history stack for undo support.
4. The invoker (button, menu, keyboard shortcut) calls `command.execute()` without knowing the implementation.

```javascript
class AddTextCommand {
  constructor(editor, text) {
    this.editor = editor;
    this.text = text;
    this.prevContent = null;
  }

  execute() {
    this.prevContent = this.editor.content;
    this.editor.content += this.text;
  }

  undo() {
    this.editor.content = this.prevContent;
  }
}

const editor = { content: 'Hello' };
const history = [];

const cmd = new AddTextCommand(editor, ' World');
cmd.execute();
history.push(cmd);
console.log(editor.content); // 'Hello World'

history.pop().undo();
console.log(editor.content); // 'Hello'
```

## Details

The Command pattern turns function calls into first-class objects. This makes operations serializable, loggable, and reversible. Redux actions are a functional variant of this pattern.

**Trade-offs:**

- More boilerplate than a direct function call
- Memory cost for storing command history
- Undo logic can be complex for operations with side effects

**When NOT to use:**

- When undo/redo is not needed and operations are one-shot
- For simple event handlers — a plain callback is sufficient

## Source

https://patterns.dev/javascript/command-pattern

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
