diff --git a/packages/fbjs/src/core/joinClasses.js b/packages/fbjs/src/core/joinClasses.js
index cabf40dc..4c14bccb 100644
--- a/packages/fbjs/src/core/joinClasses.js
+++ b/packages/fbjs/src/core/joinClasses.js
@@ -14,7 +14,7 @@
 /**
  * Combines multiple className strings into one.
  */
-function joinClasses(className: mixed): string {
+function _joinClasses(className: mixed): string {
   let newClassName = ((className: any): string) || '';
   const argLength = arguments.length;
 
@@ -29,4 +29,6 @@ function joinClasses(className: mixed): string {
   return newClassName;
 }
 
+const joinClasses: (...classNames: Array<mixed>) => string = (_joinClasses: any);
+
 module.exports = joinClasses;