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Learn about one of the ways to store dynamic information in React: Props.

Props

Every component's `props` object has a property named `children`.

`props.children` will return everything in between a component's opening and closing JSX tags. 

So far, all of the components that you've seen have been *self-closing tags*, such as `<MyFunctionComponent />`. They don't have to be! You could write `<MyFunctionComponent></MyFunctionComponent>`, and it would still work.

`props.children` would return everything in between `<MyFunctionComponent>` and `</MyFunctionComponent>`. 

By using `props.children`, we can separate the outer component, `MyFunctionComponent` in this case, from the content, which makes it flexible and reusable.

Look at **BigButton.js**. 
* In *Example 1,* `<BigButton>`'s `props.children` would equal the text, "I am a child of BigButton."
* In *Example 2,* `<BigButton>`'s `props.children` would equal a `<LilButton />` component.
* In *Example 3,* `<BigButton>`'s `props.children` would equal `undefined`.

If a component has more than one child between its JSX tags, then `props.children` will return those children in an array. However, if a component has only one child, then `props.children` will return the single child, *not* wrapped in an array.

props.children

Every component has something called `props`.

A component's `props` is an object. It holds information about that component.

You've seen this before, but you might not have realized it! Let's take a look at the HTML button tag. There are several pieces of information we can pass to the button tag, such as the `type` of the button. 

```js
<button type="submit" value="Submit"> Submit </button> 
```

In this example, we've passed two pieces of information to the button tag, a `type` and a `value`. Depending on what `type` attribute we give to the `<button>` element, it will treat the form differently. In the same way, we can pass information to our own components to specify how they behave!

Props serve the same purpose for components as arguments do for functions.

To access a component's `props` object, you can reference the `props` object and the dot notation for its properties. Here's an example: 

```js
props.name
```

This would retrieve the `name` property from the `props` object.

Access a Component's props

Let's talk about naming things.

When you pass an *event handler* as a prop, as you just did, there are two names that you have to choose. Both naming choices occur in the *parent* component, the component that defines the event handler and passes it.

The first name that you have to choose is the name of the event handler itself.  

Look at **Talker.js**, lines 5 through 11. This is our event handler. We chose to name it `talk`.

The second name that you have to choose is the name of the prop that you will use to *pass* the event handler. This is the same thing as the attribute name.

For our prop name, we also chose `talk`, as shown on line 12:

```js
return <Button talk={talk} />;
```

These two names can be whatever we want. However, there is a naming convention that is commonly used.

Here's how the naming convention works: first, think about what *type of event* you are listening for. In our example, the event type was "click". If you are listening for a "click" event, then you name your *event handler* `handleClick`. If you are listening for a "hover" event, then you name your event handler `handleHover`:

```js
function myClass() {
  function handleHover() {
    alert('I am an event handler.');
    alert('I will be called in response to "hover" events.');
  }
}
```

Your prop name should be the word `on`, plus your event type. If you are listening for a "click" event, then you name your prop `onClick`. If you are listening for a "hover" event, then you name your prop `onHover`:

```js
function myClass(){
  function handleHover() {
    alert('I am an event handler.');
    alert('I will listen for a "hover" event.');
  }
   return <Child onHover={handleHover} />;
}
```

handleEvent, onEvent, and props.onEvent

That completes our lesson on props! Here are some of the skills that you've learned:
- Passing a prop by giving an *attribute* to a component instance.
- Accessing a passed-in prop via `props.propName`.
- Displaying a prop.
- Using a prop to make decisions about what to display.
- Defining an event handler in a function component.
- Passing an event handler as a prop.
- Receiving a prop event handler and attaching it to an event listener.
- Naming event handlers and event handler attributes according to a convention.
- Accessing `props.children`.
- Assigning default values to props.
  
That's a lot! Don't worry if it's all a bit of a blur. You'll soon get plenty of practice!

Review

When thinking in the frame of a React application, components are small pieces of a whole. Together, they make up the interface that users will see. 

With each component playing a role in the interface, there are times when components must be able to communicate with other components.

In this lesson, you will learn another way that components can interact: a component *passing information* to another component.

Information that gets passed from one component to another is known as **props**.

Props can be used to customize the output of each component depending on the information that is passed in. 

By allowing components to communicate with each other, we can add a level of flexibility that was not possible before.

By the end of this lesson, you should be able to: 
* Pass, access, and display props.
* Use props to create conditional statements.
* Define event handlers in a component and pass them to other components.
* Work with a component's children.
* Assign default values to props.

Let's get started!

props

Props allow us to customize the component by passing it information. 

We've learned how to *pass* information to a component's `props` object. You will often want a component to *display* the information that you pass.

To make sure that a function component can use the `props` object, define your function component with `props` as the parameter:

``` js
function Button(props) {
  return <button>{props.displayText}</button>;
}
```

In the example, `props` is accepted as a parameter, and the object values are accessed with the dot notation accessors pattern (`object.propertyName`).

Alternatively, since `props` is an object, you can also use destructuring syntax like so:
 
``` js
function Button({displayText}) {
  return <button>{displayText}</button>;
}
```

Render a Component's props

Good! You've defined a new method inside the `Talker` component. Now you're ready to *pass* that function to another component. 

You can pass a method in the exact same way that you pass any other data—using curly braces.

Pass an Event Handler as a prop

You can do more with props than just display them. You can also use props to make decisions.

```js
function LoginMsg(props) {
  if (props.password === 'a-tough-password') {
    return <h2>Sign In Successful.</h2>
  } else {
    return <h2>Sign In Failed..</h2>
  }
}
```

In this example, we use the props passed in to make a decision rather than rendering the value to the screen. 

If the `password` received is equal to `'a-tough-password'`, the resulting message in `<h2></h2>` will be different!

The passed-in `password` is not displayed in either case! The prop is used to *decide* what will be displayed. This is a common technique. 

Render Different UI Based on props

To take advantage of `props`, we need to *pass information*  to a React component. In the previous exercise, we rendered an empty `props` object because we did not pass any `props` to our `PropsDisplayer` component.

How do we pass `props`? By giving the component an *attribute*:

```js
<Greeting name="Jamel" />
```

Let's say that you want to pass a component the message, `"We're great!"`. Here's how you can do it:

```js
<SloganDisplay message="We're great!" />
```

As you can see, to pass information to a component, you need a *name* for the information that you want to pass.

In the above example, we used the name `message`. You can use any name you want. 

If you want to pass information that isn't a string, then wrap that information in curly braces. Here's how you would pass an array:

```js
<Greeting myInfo={["Astronaut", "Narek", "43"]} />
```

In this next example, we pass several pieces of information to `<Greeting />`. The values that aren't strings are wrapped in curly braces:

```js
<Greeting name="The Queen Mary" city="Long Beach, California" age={56} haunted={true} />
```

Pass `props` to a Component

You have learned how to pass a `prop` to a component:

```js
<Greeting firstName="Esmerelda" />
```

You have also learned how to access and display a passed-in `prop`:

```js
return <h1>{props.firstName}</h1>;
```

The most common use of `props` is to pass information to a component from *a different component*.

Props in React travel in a one-way direction, from the top to bottom, parent to child.

Let's explore the parent-child relationship of passing `props` a bit further. 

```js
function App() {
    return <Product name="Apple Watch" price = {399} rating = "4.5/5.0" />;
}
```

In this example, `App` is the parent and `Product` is the child. `App` passes three props to `Product` (`name`, `price`, and `rating`), which can then be read inside the child component. 

Props passed down are immutable, meaning they cannot be changed. If a component wants new values for its props, it needs to rely on the parent component to pass it new ones.

Let's practice this!

Pass props From Component To Component

Great! You just passed a function from `<Talker />` to `<Button />`.

Take a look at **Button.js** in the code editor. Notice that `Button` returns a `<button>` element.

If a user clicks on this `<button>` element, you want your passed-in `talk()` function to get called. This means that you need to attach `talk()` to the `<button>` element as an *event handler*.

How do you do that? In the same way that you attach any event handler to a JSX element: you give that JSX element a special *attribute*. The attribute's *name* should be an event name like `onClick` or `onHover`. The attribute's *value* should be the event handler that you want to attach.

Receive an Event Handler as a prop

You can, and often will, pass functions as `props`. It is especially common to pass *event handler* functions.

In the next exercise, we will pass an event handler function as a prop. However, we have to *define* an event handler before we can pass one anywhere. In this exercise, we will define an event handler function.
 
How do we define an event handler in React?

We define an event handler as a method on the function component!

Take a look at the **Example.js** file in the code editor. On lines 4 through 8, an *event handler* method is defined. On line 10, that event handler method is attached to an *event* (a click event, in this case).

Put an Event Handler in a Function Component

Take a look at the `Button` component. Notice that on line 6, `Button` expects to receive a prop named `text`. The received `text` will be displayed inside of a `<button>` element.

What if nobody passes any `text` to `Button`?

If nobody passes any `text` to `Button`, then `Button`'s display will be blank. It would be better if `Button` could display a default message instead.

You can make this happen by specifying a default value for the prop. There are three ways to do this!

The first method is adding a `defaultProps` static property to the component: 

```js
function Example(props) {
  return <h1>{props.text}</h1>
}

Example.defaultProps = {
  text: 'This is default text',
};
```

You can also specify the default value directly in the function definition:

```js
function Example({text='This is default text'}) {
   return <h1>{text}</h1>
}
```

Lastly, you can also set the default value in the function body:

```js
function Example(props) {
  const {text = 'This is default text'} = props;
  return <h1>{text}</h1>
}
```

If an `<Example />` doesn't get passed any text, then it will display "This is default text".

If an `<Example />` *does* get passed some text, then it will display that passed-in text.