My First Component
- Components: UI Building Blocks
- Defining a Component
- Using a Component
`, and an (abbreviated) table of contents as an ordered list ``. Markup like this, combined with CSS for style, and JavaScript for interactivity, lies behind every sidebar, avatar, modal, dropdown—every piece of UI you see on the Web.
React lets you combine your markup, CSS, and JavaScript into custom "components", **reusable UI elements for your app.** The table of contents code you saw above could be turned into a ``, ``, etc.
Just like with HTML tags, you can compose, order and nest components to design whole pages. For example, the documentation page you're reading is made out of React components:
```js
```
As your project grows, you will notice that many of your designs can be composed by reusing components you already wrote, speeding up your development. Our table of contents above could be added to any screen with `
```js
export default function Profile() {
return (
)
}
```
```css
img { height: 200px; }
```
And here's how to build a component:
### Step 1: Export the component {/*step-1-export-the-component*/}
The `export default` prefix is a [standard JavaScript syntax](https://developer.mozilla.org/docs/web/javascript/reference/statements/export) (not specific to React). It lets you mark the main function in a file so that you can later import it from other files. (More on importing in [Importing and Exporting Components](/learn/importing-and-exporting-components)!)
### Step 2: Define the function {/*step-2-define-the-function*/}
With `function Profile() { }` you define a JavaScript function with the name `Profile`.
React components are regular JavaScript functions, but **their names must start with a capital letter** or they won't work!
### Step 3: Add markup {/*step-3-add-markup*/}
The component returns an `![]()
` tag with `src` and `alt` attributes. `![]()
` is written like HTML, but it is actually JavaScript under the hood! This syntax is called [JSX](/learn/writing-markup-with-jsx), and it lets you embed markup inside JavaScript.
Return statements can be written all on one line, as in this component:
```js
return 
;
```
But if your markup isn't all on the same line as the `return` keyword, you must wrap it in a pair of parentheses like this:
```js
return (
);
```
Without parentheses, any code on the lines after `return` [will be ignored](https://stackoverflow.com/questions/2846283/what-are-the-rules-for-javascripts-automatic-semicolon-insertion-asi)!
## Using a component {/*using-a-component*/}
Now that you've defined your `Profile` component, you can nest it inside other components. For example, you can export a `Gallery` component that uses multiple `Profile` components:
```js
function Profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; height: 90px; }
```
### What the browser sees {/*what-the-browser-sees*/}
Notice the difference in casing:
* `` is lowercase, so React knows we refer to an HTML tag.
* `![]()
`. In the end, this is what the browser sees:
```html
Amazing scientists
```
### Nesting and organizing components {/*nesting-and-organizing-components*/}
Components are regular JavaScript functions, so you can keep multiple components in the same file. This is convenient when components are relatively small or tightly related to each other. If this file gets crowded, you can always move `Profile` to a separate file. You will learn how to do this shortly on the [page about imports.](/learn/importing-and-exporting-components)
Because the `Profile` components are rendered inside `Gallery`—even several times!—we can say that `Gallery` is a **parent component,** rendering each `Profile` as a "child". This is part of the magic of React: you can define a component once, and then use it in as many places and as many times as you like.
Components can render other components, but **you must never nest their definitions:**
```js {2-5}
export default function Gallery() {
// 🔴 Never define a component inside another component!
function Profile() {
// ...
}
// ...
}
```
The snippet above is [very slow and causes bugs.](/learn/preserving-and-resetting-state#different-components-at-the-same-position-reset-state) Instead, define every component at the top level:
```js {5-8}
export default function Gallery() {
// ...
}
// ✅ Declare components at the top level
function Profile() {
// ...
}
```
When a child component needs some data from a parent, [pass it by props](/learn/passing-props-to-a-component) instead of nesting definitions.
#### Components all the way down {/*components-all-the-way-down*/}
Your React application begins at a "root" component. Usually, it is created automatically when you start a new project. For example, if you use [CodeSandbox](https://codesandbox.io/) or [Create React App](https://create-react-app.dev/), the root component is defined in `src/App.js`. If you use the framework [Next.js](https://nextjs.org/), the root component is defined in `pages/index.js`. In these examples, you've been exporting root components.
Most React apps use components all the way down. This means that you won't only use components for reusable pieces like buttons, but also for larger pieces like sidebars, lists, and ultimately, complete pages! Components are a handy way to organize UI code and markup, even if some of them are only used once.
Frameworks like Next.js take this a step further. Instead of using an empty HTML file and letting React "take over" managing the page with JavaScript, they *also* generate the HTML automatically from your React components. This allows your app to show some content before the JavaScript code loads.
Still, many websites only use React to [add "sprinkles of interactivity".](/learn/add-react-to-a-website) They have many root components instead of a single one for the entire page. You can use as much—or as little—React as you need.
You've just gotten your first taste of React! Let's recap some key points.
* React lets you create components, **reusable UI elements for your app.**
* In a React app, every piece of UI is a component.
* React components are regular JavaScript functions except:
1. Their names always begin with a capital letter.
2. They return JSX markup.
#### Export the component {/*export-the-component*/}
This sandbox doesn't work because the root component is not exported:
```js
function Profile() {
return (
);
}
```
```css
img { height: 181px; }
```
Try to fix it yourself before looking at the solution!
Add `export default` before the function definition like so:
```js
export default function Profile() {
return (
);
}
```
```css
img { height: 181px; }
```
You might be wondering why writing `export` alone is not enough to fix this example. You can learn the difference between `export` and `export default` in [Importing and Exporting Components.](/learn/importing-and-exporting-components)
#### Fix the return statement {/*fix-the-return-statement*/}
Something isn't right about this `return` statement. Can you fix it?
You may get an "Unexpected token" error while trying to fix this. In that case, check the that semicolon appears *after* the closing parenthesis. Leaving a semicolon inside `return ( )` will cause an error.
```js
export default function Profile() {
return
;
}
```
```css
img { height: 180px; }
```
You can fix this component by moving the return statement to one line like so:
```js
export default function Profile() {
return ;
}
```
```css
img { height: 180px; }
```
Or by wrapping the returned JSX markup in parentheses that open right after `return`:
```js
export default function Profile() {
return (
);
}
```
```css
img { height: 180px; }
```
#### Spot the mistake {/*spot-the-mistake*/}
Something's wrong with how the `Profile` component is declared and used. Can you spot the mistake? (Try to remember how React distinguishes components from the regular HTML tags!)
```js
function profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; height: 90px; }
```
React component names must start with a capital letter.
Change `function profile()` to `function Profile()`, and then change every `
```js
function Profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; }
```
#### Your own component {/*your-own-component*/}
Write a component from scratch. You can give it any valid name and return any markup. If you're out of ideas, you can write a `Congratulations` component that shows `Good job!
`. Don't forget to export it!
```js
// Write your component below!
```
```js
export default function Congratulations() {
return (
Good job!
);
}
```
`, etc.
Just like with HTML tags, you can compose, order and nest components to design whole pages. For example, the documentation page you're reading is made out of React components:
```js
```
As your project grows, you will notice that many of your designs can be composed by reusing components you already wrote, speeding up your development. Our table of contents above could be added to any screen with `
```js
export default function Profile() {
return (
)
}
```
```css
img { height: 200px; }
```
And here's how to build a component:
### Step 1: Export the component {/*step-1-export-the-component*/}
The `export default` prefix is a [standard JavaScript syntax](https://developer.mozilla.org/docs/web/javascript/reference/statements/export) (not specific to React). It lets you mark the main function in a file so that you can later import it from other files. (More on importing in [Importing and Exporting Components](/learn/importing-and-exporting-components)!)
### Step 2: Define the function {/*step-2-define-the-function*/}
With `function Profile() { }` you define a JavaScript function with the name `Profile`.
React components are regular JavaScript functions, but **their names must start with a capital letter** or they won't work!
### Step 3: Add markup {/*step-3-add-markup*/}
The component returns an `![]()
` tag with `src` and `alt` attributes. `![]()
` is written like HTML, but it is actually JavaScript under the hood! This syntax is called [JSX](/learn/writing-markup-with-jsx), and it lets you embed markup inside JavaScript.
Return statements can be written all on one line, as in this component:
```js
return ;
```
But if your markup isn't all on the same line as the `return` keyword, you must wrap it in a pair of parentheses like this:
```js
return (
);
```
Without parentheses, any code on the lines after `return` [will be ignored](https://stackoverflow.com/questions/2846283/what-are-the-rules-for-javascripts-automatic-semicolon-insertion-asi)!
## Using a component {/*using-a-component*/}
Now that you've defined your `Profile` component, you can nest it inside other components. For example, you can export a `Gallery` component that uses multiple `Profile` components:
```js
function Profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; height: 90px; }
```
### What the browser sees {/*what-the-browser-sees*/}
Notice the difference in casing:
* `` is lowercase, so React knows we refer to an HTML tag.
* `![]()
`. In the end, this is what the browser sees:
```html
Amazing scientists
```
### Nesting and organizing components {/*nesting-and-organizing-components*/}
Components are regular JavaScript functions, so you can keep multiple components in the same file. This is convenient when components are relatively small or tightly related to each other. If this file gets crowded, you can always move `Profile` to a separate file. You will learn how to do this shortly on the [page about imports.](/learn/importing-and-exporting-components)
Because the `Profile` components are rendered inside `Gallery`—even several times!—we can say that `Gallery` is a **parent component,** rendering each `Profile` as a "child". This is part of the magic of React: you can define a component once, and then use it in as many places and as many times as you like.
Components can render other components, but **you must never nest their definitions:**
```js {2-5}
export default function Gallery() {
// 🔴 Never define a component inside another component!
function Profile() {
// ...
}
// ...
}
```
The snippet above is [very slow and causes bugs.](/learn/preserving-and-resetting-state#different-components-at-the-same-position-reset-state) Instead, define every component at the top level:
```js {5-8}
export default function Gallery() {
// ...
}
// ✅ Declare components at the top level
function Profile() {
// ...
}
```
When a child component needs some data from a parent, [pass it by props](/learn/passing-props-to-a-component) instead of nesting definitions.
#### Components all the way down {/*components-all-the-way-down*/}
Your React application begins at a "root" component. Usually, it is created automatically when you start a new project. For example, if you use [CodeSandbox](https://codesandbox.io/) or [Create React App](https://create-react-app.dev/), the root component is defined in `src/App.js`. If you use the framework [Next.js](https://nextjs.org/), the root component is defined in `pages/index.js`. In these examples, you've been exporting root components.
Most React apps use components all the way down. This means that you won't only use components for reusable pieces like buttons, but also for larger pieces like sidebars, lists, and ultimately, complete pages! Components are a handy way to organize UI code and markup, even if some of them are only used once.
Frameworks like Next.js take this a step further. Instead of using an empty HTML file and letting React "take over" managing the page with JavaScript, they *also* generate the HTML automatically from your React components. This allows your app to show some content before the JavaScript code loads.
Still, many websites only use React to [add "sprinkles of interactivity".](/learn/add-react-to-a-website) They have many root components instead of a single one for the entire page. You can use as much—or as little—React as you need.
You've just gotten your first taste of React! Let's recap some key points.
* React lets you create components, **reusable UI elements for your app.**
* In a React app, every piece of UI is a component.
* React components are regular JavaScript functions except:
1. Their names always begin with a capital letter.
2. They return JSX markup.
#### Export the component {/*export-the-component*/}
This sandbox doesn't work because the root component is not exported:
```js
function Profile() {
return (
);
}
```
```css
img { height: 181px; }
```
Try to fix it yourself before looking at the solution!
Add `export default` before the function definition like so:
```js
export default function Profile() {
return (
);
}
```
```css
img { height: 181px; }
```
You might be wondering why writing `export` alone is not enough to fix this example. You can learn the difference between `export` and `export default` in [Importing and Exporting Components.](/learn/importing-and-exporting-components)
#### Fix the return statement {/*fix-the-return-statement*/}
Something isn't right about this `return` statement. Can you fix it?
You may get an "Unexpected token" error while trying to fix this. In that case, check the that semicolon appears *after* the closing parenthesis. Leaving a semicolon inside `return ( )` will cause an error.
```js
export default function Profile() {
return
;
}
```
```css
img { height: 180px; }
```
You can fix this component by moving the return statement to one line like so:
```js
export default function Profile() {
return ;
}
```
```css
img { height: 180px; }
```
Or by wrapping the returned JSX markup in parentheses that open right after `return`:
```js
export default function Profile() {
return (
);
}
```
```css
img { height: 180px; }
```
#### Spot the mistake {/*spot-the-mistake*/}
Something's wrong with how the `Profile` component is declared and used. Can you spot the mistake? (Try to remember how React distinguishes components from the regular HTML tags!)
```js
function profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; height: 90px; }
```
React component names must start with a capital letter.
Change `function profile()` to `function Profile()`, and then change every `
```js
function Profile() {
return (
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
```
```css
img { margin: 0 10px 10px 0; }
```
#### Your own component {/*your-own-component*/}
Write a component from scratch. You can give it any valid name and return any markup. If you're out of ideas, you can write a `Congratulations` component that shows `Good job!
`. Don't forget to export it!
```js
// Write your component below!
```
```js
export default function Congratulations() {
return (
Good job!
);
}
```
)
}
```
```css
img { height: 200px; }
```
);
}
export default function Gallery() {
return (
Amazing scientists
Amazing scientists
);
}
```
```css
img { height: 181px; }
```
);
}
```
```css
img { height: 181px; }
```
;
}
```
```css
img { height: 180px; }
```
;
}
```
```css
img { height: 180px; }
```
);
}
```
```css
img { height: 180px; }
```
);
}
export default function Gallery() {
return (
Amazing scientists
);
}
export default function Gallery() {
return (