diff --git a/js/mailto.js b/js/mailto.js
new file mode 100644
index 0000000..d353119
--- /dev/null
+++ b/js/mailto.js
@@ -0,0 +1,15 @@
+/**
+ * Build a `mailto:` URL from an address, subject, and body.
+ * Subject and body are URL-encoded so the visitor's mail client
+ * opens pre-filled with the submitted values.
+ *
+ * @param {{to?: string, subject?: string, body?: string}} options
+ * @returns {string}
+ */
+export function buildMailtoUrl({ to = "", subject = "", body = "" } = {}) {
+  const params = new URLSearchParams();
+  if (subject) params.set("subject", subject);
+  if (body) params.set("body", body);
+  const query = params.toString();
+  return query ? `mailto:${to}?${query}` : `mailto:${to}`;
+}