diff --git a/landing/button.react.js b/landing/button.react.js
--- a/landing/button.react.js
+++ b/landing/button.react.js
@@ -2,10 +2,12 @@
 
 import * as React from 'react';
 
+import type { OnClick } from 'lib/types/core';
+
 import css from './button.css';
 
 type Props = {
-  +onClick: (event: SyntheticEvent<HTMLButtonElement>) => mixed,
+  +onClick: OnClick,
   +children?: React.Node,
 };
 
diff --git a/lib/types/core.js b/lib/types/core.js
--- a/lib/types/core.js
+++ b/lib/types/core.js
@@ -1,3 +1,5 @@
 // @flow
 
 export type Shape<O> = $ReadOnly<$Rest<O, { ... }>>;
+
+export type OnClick = (event: SyntheticEvent<HTMLButtonElement>) => mixed;
diff --git a/web/components/button.react.js b/web/components/button.react.js
--- a/web/components/button.react.js
+++ b/web/components/button.react.js
@@ -3,10 +3,12 @@
 import classnames from 'classnames';
 import * as React from 'react';
 
+import type { OnClick } from 'lib/types/core';
+
 import css from './button.css';
 
 type Props = {
-  +onClick: (event: SyntheticEvent<HTMLButtonElement>) => void,
+  +onClick: OnClick,
   +children: React.Node,
   +variant?: 'primary' | 'secondary' | 'danger' | 'round',
   +type?: string,