command-pattern

What this skill does

# Command Pattern

With the **Command Pattern**, we can _decouple_ objects that execute a certain task from the object that calls the method.

Let's say we have an online food delivery platform. Users can place, track, and cancel orders.

## When to Use

- Use this when you need to decouple the object invoking an operation from the object performing it
- This is helpful when commands need a certain lifespan or should be queued and executed at specific times

## When NOT to Use

- For simple one-off operations that don't need undo/redo, queuing, or logging
- When direct function calls are clear enough and the extra abstraction adds complexity without benefit
- When the system has few operations and the command infrastructure would be over-engineering

## Instructions

- Create a Command class with an `execute` method that encapsulates the action
- Replace direct method calls with command objects passed to a single `execute` method on the manager
- Use this pattern sparingly as it can add unnecessary boilerplate in simpler JavaScript applications

## Details

```js
class OrderManager {
  constructor() {
    this.orders = []
  }

  placeOrder(order, id) {
    this.orders.push(id)
    return `You have successfully ordered ${order} (${id})`;
  }

  trackOrder(id) {
    return `Your order ${id} will arrive in 20 minutes.`
  }

  cancelOrder(id) {
    this.orders = this.orders.filter(order => order !== id)
    return `You have canceled your order ${id}`
  }
}
```

On the `OrderManager` class, we have access to the `placeOrder`, `trackOrder` and `cancelOrder` methods. It would be totally valid JavaScript to just use these methods directly!

```js
const manager = new OrderManager();

manager.placeOrder("Pad Thai", "1234");
manager.trackOrder("1234");
manager.cancelOrder("1234");
```

However, there are downsides to invoking the methods directly on the `manager` instance. It could happen that we decide to rename certain methods later on, or the functionality of the methods change.

Say that instead of calling it `placeOrder`, we now rename it to `addOrder`! This would mean that we would have to make sure that we don't call the `placeOrder` method anywhere in our codebase, which could be very tricky in larger applications. Instead, we want to decouple the methods from the `manager` object, and create separate command functions for each command!

Let's refactor the `OrderManager` class: instead of having the `placeOrder`, `cancelOrder` and `trackOrder` methods, it will have one single method: `execute`. This method will execute any command it's given.

Each command should have access to the `orders` of the manager, which we'll pass as its first argument.

```js
class OrderManager {
  constructor() {
    this.orders = [];
  }

  execute(command, ...args) {
    return command.execute(this.orders, ...args);
  }
}
```

We need to create three `Command`s for the order manager:

- `PlaceOrderCommand`
- `CancelOrderCommand`
- `TrackOrderCommand`

```js
class Command {
  constructor(execute) {
    this.execute = execute;
  }
}

function PlaceOrderCommand(order, id) {
  return new Command((orders) => {
    orders.push(id);
    return `You have successfully ordered ${order} (${id})`;
  });
}

function CancelOrderCommand(id) {
  return new Command((orders) => {
    const index = orders.indexOf(id);
    if (index > -1) orders.splice(index, 1);
    return `You have canceled your order ${id}`;
  });
}

function TrackOrderCommand(id) {
  return new Command(() => `Your order ${id} will arrive in 20 minutes.`);
}
```

Perfect! Instead of having the methods directly coupled to the `OrderManager` instance, they're now separate, decoupled functions that we can invoke through the `execute` method that's available on the `OrderManager`.

### Pros

The command pattern allows us to decouple methods from the object that executes the operation. It gives you more control if you're dealing with commands that have a certain lifespan, or commands that should be queued and executed at specific times.

### Cons

The use cases for the command pattern are quite limited, and often adds unnecessary boilerplate to an application.

## Source

- [patterns.dev/vanilla/command-pattern](https://patterns.dev/vanilla/command-pattern)

### References

- [Command Design Pattern](https://sourcemaking.com/design_patterns/command) - SourceMaking
- [Command Pattern](https://refactoring.guru/design-patterns/command) - Refactoring Guru
- [Command Pattern](https://www.carloscaballero.io/design-patterns-command/) - Carlos Caballero