---
title: 'Callbacks'
---

Callbacks are used to asynchronously communicate upwards the components tree and with other things like agents or the DOM during event handling.
Internally their type is just an `Fn` wrapped in `Rc` to allow them to be cheaply cloned.

They have an `emit` function if you want to call them manually.

```rust
use yew::{html, Component, Context, Html, Callback};

let cb: Callback<String, String> = Callback::from(move |name: String| {
    format!("Bye {}", name)
});

let result = cb.emit(String::from("Bob")); // call the callback
// web_sys::console::log_1(&result.into()); // if uncommented will print "Bye Bob"
```

## Passing callbacks as props

A common pattern in yew is to create a callback and pass it down as a prop.

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

#[derive(Properties, PartialEq)]
pub struct Props {
    pub on_name_entry: Callback<String>,
}

#[function_component]
fn HelloWorld(props: &Props) -> Html {

    props.on_name_entry.emit(String::from("Bob"));

    html! { "Hello" }
}

// Then supply the prop
#[function_component]
fn App() -> Html {
    let on_name_entry: Callback<String> = Callback::from(move |name: String| {
        let greeting = format!("Hey, {}!", name);
        // web_sys::console::log_1(&greeting.into()); // if uncommented will print
    });

    html! { <HelloWorld {on_name_entry} /> }
}

```

## DOM Events and Callbacks

Callbacks are also used to hook into DOM events.

For example here we define a callback that will be called when the user clicks the button:

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

#[function_component]
fn App() -> Html {
    let onclick = Callback::from(move |_| {
        let greeting = String::from("Hi there");
        // web_sys::console::log_1(&greeting.into()); // if uncommented will print
    });

    html! {
        <button {onclick}>{ "Click" }</button>
    }
}
```
