diff --git a/src/cache.ts b/src/cache.ts
index be48e19..180e049 100644
--- a/src/cache.ts
+++ b/src/cache.ts
@@ -26,9 +26,9 @@
  * cachedFunctionWithParam('figure', str => str.small())
  * // => '<big>figure</big>'
  */
-export default function cache (fn: (str: any) => any) {
+export default function cache (fn: (str: any) => any): (str: any) => any {
   const cache: { [propName: string]: any } = {}
-  return function cachedFn () {
+  return function cachedFn (this: any) {
     const str = arguments[0]
     const hit = cache[str]
     return cache.hasOwnProperty(str) ? hit : (cache[str] = fn.apply(this, arguments))
diff --git a/src/debounce.ts b/src/debounce.ts
index d2a6f5e..0fcc726 100644
--- a/src/debounce.ts
+++ b/src/debounce.ts
@@ -16,7 +16,7 @@ export default function debounce (
 ): (...args: any[]) => void {
   let timer: any = null
 
-  return function (...args) {
+  return function (this: any, ...args) {
     const later = () => {
       timer = null
       func.call(this, ...args)