diff --git a/lib/use.injection.js b/lib/use.injection.js
index e80be0d..e9f7741 100644
--- a/lib/use.injection.js
+++ b/lib/use.injection.js
@@ -72,8 +72,7 @@ module.exports.useInjection = (fn, params) => async function (...args) {
 /**
  * A more precisely type checker than Javascript built-in typeof.
  *
- * @param { * } o
- * @returns { string }
+ * @type { import('../types/use.injection').typeOf }
  */
 module.exports.typeOf = function (o) {
   /**
diff --git a/types/use.injection.d.ts b/types/use.injection.d.ts
index cb16bb7..cdbffb3 100644
--- a/types/use.injection.d.ts
+++ b/types/use.injection.d.ts
@@ -44,3 +44,5 @@ export type InjectionOptions = {
 export type TwilioFunction = (context: Context, event: Event, callback: import('@twilio-labs/serverless-runtime-types/types').ServerlessCallback) => Promise<void>
 
 export function useInjection<T>(action: Action<T>, options: InjectionOptions): TwilioFunction
+
+export function typeOf(value: any): string
\ No newline at end of file