Underline text when cmd + u is pressed

packages/format-library/src/index.js

@@ -7,6 +7,7 @@ import { image } from './image';
 import { italic } from './italic';
 import { link } from './link';
 import { strikethrough } from './strikethrough';
+import { underline } from './underline';
 
 /**
  * WordPress dependencies
@@ -22,4 +23,5 @@ import {
 	italic,
 	link,
 	strikethrough,
+	underline,
 ].forEach( ( { name, ...settings } ) => registerFormatType( name, settings ) );

packages/format-library/src/underline/index.js

@@ -0,0 +1,38 @@
+/**
+ * WordPress dependencies
+ */
+import { __ } from '@wordpress/i18n';
+import { Fragment } from '@wordpress/element';
+import { toggleFormat } from '@wordpress/rich-text';
+import { RichTextToolbarButton, RichTextShortcut } from '@wordpress/editor';
+
+const name = 'core/underline';
+
+export const underline = {
+	name,
+	title: __( 'Underline' ),
+	tagName: 'u',
+	className: null,
+	edit( { isActive, value, onChange } ) {
+		const onToggle = () => onChange( toggleFormat( value, { type: name } ) );
+
+		return (
+			<Fragment>
+				<RichTextShortcut
+					type="primary"
+					character="u"
+					onUse={ onToggle }
+				/>
+				<RichTextToolbarButton
+					name="underline"
+					icon="editor-underline"
+					title={ __( 'Underline' ) }
+					onClick={ onToggle }
+					isActive={ isActive }
+					shortcutType="primary"
+					shortcutCharacter="u"
+				/>
+			</Fragment>
+		);
+	},
+};