# Applying Custom Formatting

In the previous guide we learned how to create custom block types that render chunks of text inside different containers. But Slate allows for more than just "blocks".

In this guide, we'll show you how to add custom formatting options, like **bold**, _italic_, `code` or ~~strikethrough~~.

So we start with our app from earlier:

```jsx
const renderElement = props => {
  switch (props.element.type) {
    case 'code':
      return <CodeElement {...props} />
    default:
      return <DefaultElement {...props} />
  }
}

const initialValue = [
  {
    type: 'paragraph',
    children: [{ text: 'A line of text in a paragraph.' }],
  },
]

const App = () => {
  const [editor] = useState(() => withReact(createEditor()))

  return (
    <Slate editor={editor} initialValue={initialValue}>
      <Editable
        renderElement={renderElement}
        onKeyDown={event => {
          if (event.key === '`' && event.ctrlKey) {
            event.preventDefault()
            const [match] = Editor.nodes(editor, {
              match: n => n.type === 'code',
            })
            Transforms.setNodes(
              editor,
              { type: match ? 'paragraph' : 'code' },
              { match: n => Element.isElement(n) && Editor.isBlock(editor, n) }
            )
          }
        }}
      />
    </Slate>
  )
}
```

And now, we'll edit the `onKeyDown` handler to make it so that when you press `control-B`, it will add a `bold` format to the currently selected text:

```jsx
const initialValue = [
  {
    type: 'paragraph',
    children: [{ text: 'A line of text in a paragraph.' }],
  },
]

const App = () => {
  const [editor] = useState(() => withReact(createEditor()))

  const renderElement = useCallback(props => {
    switch (props.element.type) {
      case 'code':
        return <CodeElement {...props} />
      default:
        return <DefaultElement {...props} />
    }
  }, [])

  return (
    <Slate editor={editor} initialValue={initialValue}>
      <Editable
        renderElement={renderElement}
        onKeyDown={event => {
          if (!event.ctrlKey) {
            return
          }

          switch (event.key) {
            // When "`" is pressed, keep our existing code block logic.
            case '`': {
              event.preventDefault()
              const [match] = Editor.nodes(editor, {
                match: n => n.type === 'code',
              })
              Transforms.setNodes(
                editor,
                { type: match ? 'paragraph' : 'code' },
                {
                  match: n => Element.isElement(n) && Editor.isBlock(editor, n),
                }
              )
              break
            }

            // When "B" is pressed, bold the text in the selection.
            case 'b': {
              event.preventDefault()
              Editor.addMark(editor, 'bold', true)
              break
            }
          }
        }}
      />
    </Slate>
  )
}
```

Unlike the code format from the previous step, which is a block-level format, bold is a character-level format. Slate manages text contained within blocks (or any other element) using "leaves". Slate's character-level formats/styles are called "marks". Adjacent text with the same marks (styles) applied will be grouped within the same "leaf". When we use `addMark` to add our bold mark to the selected text, Slate will automatically break up the "leaves" using the selection boundaries and produce a new "leaf" with the bold mark added.

Okay, so we've got the hotkey handler setup... but! If you happen to now try selecting text and hitting `Ctrl-B`, you won't notice any change. That's because we haven't told Slate how to render a "bold" mark.

For every format you add, you need to tell Slate how to render it, just like for elements. So let's define a `Leaf` component:

```jsx
// Define a React component to render leaves with bold text.
const Leaf = props => {
  return (
    <span
      {...props.attributes}
      style={{ fontWeight: props.leaf.bold ? 'bold' : 'normal' }}
    >
      {props.children}
    </span>
  )
}
```

Pretty familiar, right? Note that it is described with a `span` - This is because all leaves must be an [inline element](https://developer.mozilla.org/en-US/docs/Web/HTML/Inline_elements). You can learn more about leaves in the [Rendering section](../concepts/09-rendering.md#leaves).

And now, let's tell Slate about that leaf. To do that, we'll pass in the `renderLeaf` prop to our editor.

```jsx
const initialValue = [
  {
    type: 'paragraph',
    children: [{ text: 'A line of text in a paragraph.' }],
  },
]

const App = () => {
  const [editor] = useState(() => withReact(createEditor()))

  const renderElement = useCallback(props => {
    switch (props.element.type) {
      case 'code':
        return <CodeElement {...props} />
      default:
        return <DefaultElement {...props} />
    }
  }, [])

  // Define a leaf rendering function that is memoized with `useCallback`.
  const renderLeaf = useCallback(props => {
    return <Leaf {...props} />
  }, [])

  return (
    <Slate editor={editor} initialValue={initialValue}>
      <Editable
        renderElement={renderElement}
        // Pass in the `renderLeaf` function.
        renderLeaf={renderLeaf}
        onKeyDown={event => {
          if (!event.ctrlKey) {
            return
          }

          switch (event.key) {
            case '`': {
              event.preventDefault()
              const [match] = Editor.nodes(editor, {
                match: n => n.type === 'code',
              })
              Transforms.setNodes(
                editor,
                { type: match ? null : 'code' },
                {
                  match: n => Element.isElement(n) && Editor.isBlock(editor, n),
                }
              )
              break
            }

            case 'b': {
              event.preventDefault()
              Editor.addMark(editor, 'bold', true)
              break
            }
          }
        }}
      />
    </Slate>
  )
}

const Leaf = props => {
  return (
    <span
      {...props.attributes}
      style={{ fontWeight: props.leaf.bold ? 'bold' : 'normal' }}
    >
      {props.children}
    </span>
  )
}
```

Now, if you try selecting a piece of text and hitting `Ctrl-B` you should see it turn bold! Magic!