yew/website/versioned_docs/version-0.21/concepts/function-components/introduction.mdx

---
title: 'Function Components'
slug: /concepts/function-components
---

Let's revisit this previous statement:

> Yew centrally operates on the idea of keeping everything that a reusable piece of
> UI may need in one place - rust files.

We will refine this statement, by introducing the concept that will define the logic and
presentation behavior of an application: "components".

## What are Components?

Components are the building blocks of Yew.

They:

- Take arguments in form of [Props](./properties.mdx)
- Can have their own state
- Compute pieces of HTML visible to the user (DOM)

## Two flavors of Yew Components

You are currently reading about function components - the recommended way to write components
when starting with Yew and when writing simple presentation logic.

There is a more advanced, but less accessible, way to write components - [Struct components](advanced-topics/struct-components/introduction.mdx).
They allow very detailed control, though you will not need that level of detail most of the time.

## Creating function components

To create a function component add the `#[function_component]` attribute to a function.
By convention, the function is named in PascalCase, like all components, to contrast its
use to normal html elements inside the `html!` macro.

```rust
use yew::{function_component, html, Html};

#[function_component]
fn HelloWorld() -> Html {
    html! { "Hello world" }
}

// Then somewhere else you can use the component inside `html!`
#[function_component]
fn App() -> Html {
    html! { <HelloWorld /> }
}
```

## What happens to components

When rendering, Yew will build a virtual tree of these components.
It will call the view function of each (function) component to compute a virtual version (VDOM) of the DOM
that you as the library user see as the `Html` type.
For the previous example, this would look like this:

```xhtml
<App>
  <HelloWorld>
    <p>"Hello world"</p>
  </HelloWord>
</App>
```

When an update is necessary, Yew will again call the view function and reconcile the new virtual DOM with its
previous version and only propagate the new/changed/necessary parts to the actual DOM.
This is what we call **rendering**.

:::note

Behind the scenes, `Html` is just an alias for `VNode` - a virtual node.

:::