presentational-container-pattern

What this skill does

# Container/Presentational Pattern

In React, one way to enforce separation of concerns is by using the **Container/Presentational pattern**. With this pattern, we can separate the view from the application logic.

## When to Use

- Use this when you want a clear separation between data-fetching logic and UI rendering
- This is helpful for making presentational components reusable and easy to test

## When NOT to Use

- For small components where the separation into two files adds overhead without meaningful benefit
- When hooks already encapsulate the data logic, making a separate container component redundant
- When the component is a one-off view with no reuse potential for either layer

## Instructions

- Container components handle data fetching and state; presentational components handle rendering
- Prefer custom Hooks over container components in modern React for the same separation of concerns
- Keep presentational components as pure functions that receive data through props
- Use this pattern when it genuinely simplifies your architecture — avoid it for small components

## Details

Let's say we want to create an application that fetches 6 dog images, and renders these images on the screen.

Ideally, we want to enforce separation of concerns by separating this process into two parts:

1. **Presentational Components**: Components that care about _**how**_ data is shown to the user. In this example, that's the _rendering the list of dog images_.
2. **Container Components**: Components that care about _**what**_ data is shown to the user. In this example, that's _fetching the dog images_.

Fetching the dog images deals with **application logic**, whereas displaying the images only deals with the **view**.

### Presentational Component

A presentational component receives its data through `props`. Its primary function is to simply **display the data it receives** the way we want them to, including styles, _without modifying_ that data.

Let's take a look at the example that displays the dog images. When rendering the dog images, we simply want to map over each dog image that was fetched from the API, and render those images. In order to do so, we can create a functional component that receives the data through `props`, and renders the data it received.

The `DogImages` component is a presentational component. Presentational components are _usually_ stateless: they do not contain their own React state, unless they need a state for UI purposes. The data they receive, is not altered by the presentational components themselves.

Presentational components receive their data from **container components**.

### Container Components

The primary function of container components is to **pass data** to presentational components, which they _contain_. Container components themselves usually don't render any other components besides the presentational components that care about their data. Since they don't render anything themselves, they usually do not contain any styling either.

In our example, we want to pass dog images to the `DogsImages` presentational component. Before being able to do so, we need to fetch the images from an external API. We need to create a **container component** that fetches this data, and passes this data to the presentational component `DogImages` in order to display it on the screen.

Combining these two components together makes it possible to separate handling application logic with the view.

### Hooks

In many cases, the Container/Presentational pattern can be replaced with React Hooks. The introduction of Hooks made it easy for developers to add statefulness without needing a container component to provide that state.

Instead of having the data fetching logic in the `DogImagesContainer` component, we can create a custom hook that fetches the images, and returns the array of dogs.

```js
export default function useDogImages() {
  const [dogs, setDogs] = useState([]);

  useEffect(() => {
    fetch("https://dog.ceo/api/breed/labrador/images/random/6")
      .then((res) => res.json())
      .then(({ message }) => setDogs(message));
  }, []);

  return dogs;
}
```

By using this hook, we no longer need the wrapping `DogImagesContainer` container component to fetch the data, and send this to the presentational `DogImages` component. Instead, we can use this hook directly in our presentational `DogImages` component!

By using the `useDogImages` hook, we still separated the application logic from the view. We're simply using the returned data from the `useDogImages` hook, without modifying that data within the `DogImages` component.

Hooks make it easy to separate logic and view in a component, just like the Container/Presentational pattern. It saves us the extra layer that was necessary in order to wrap the presentational component within the container component.

### Pros

There are many benefits to using the Container/Presentational pattern.

The Container/Presentational pattern encourages the separation of concerns. Presentational components can be pure functions which are responsible for the UI, whereas container components are responsible for the state and data of the application. This makes it easy to enforce the separation of concerns.

Presentational components are easily made reusable, as they simply _display_ data without altering this data. We can reuse the presentational components throughout our application for different purposes.

Since presentational components don't alter the application logic, the appearance of presentational components can easily be altered by someone without knowledge of the codebase, for example a designer. If the presentational component was reused in many parts of the application, the change can be consistent throughout the app.

Testing presentational components is easy, as they are usually pure functions. We know what the components will render based on which data we pass, without having to mock a data store.

### Cons

The Container/Presentational pattern makes it easy to separate application logic from rendering logic. However, Hooks make it possible to achieve the same result without having to use the Container/Presentational pattern, and without having to rewrite a stateless functional component into a class component. Note that today, we don't need to create class components to use state anymore.

Although we can still use the Container/Presentational pattern, even with React Hooks, this pattern can easily be an overkill in smaller sized application.

## Source

- [patterns.dev/react/presentational-container-pattern](https://patterns.dev/react/presentational-container-pattern)

### References

- [Presentational and Container Components - Dan Abramov](https://medium.com/@dan_abramov/smart-and-dumb-components-7ca2f9a7c7d0)