diff --git a/src/rx-custom/add/filterUndefined.ts b/src/rx-custom/add/filterUndefined.ts
new file mode 100644
index 0000000..40c7257
--- /dev/null
+++ b/src/rx-custom/add/filterUndefined.ts
@@ -0,0 +1,10 @@
+import { Observable } from "rxjs"
+import { filterUndefined } from "../operator/filterUndefined"
+
+declare module "rxjs/Observable" {
+    interface Observable<T> {
+        filterUndefined: typeof filterUndefined;
+    }
+}
+
+Observable.prototype.filterUndefined = filterUndefined
diff --git a/src/rx-custom/operator/filterUndefined.ts b/src/rx-custom/operator/filterUndefined.ts
new file mode 100644
index 0000000..774fe74
--- /dev/null
+++ b/src/rx-custom/operator/filterUndefined.ts
@@ -0,0 +1,5 @@
+import { Observable } from "rxjs"
+
+export function filterUndefined<T>(this: Observable<T | undefined>): Observable<T> {
+  return this.filter(x => typeof x !== "undefined")
+}
diff --git a/src/setup.ts b/src/setup.ts
index 7320b26..679aa4f 100644
--- a/src/setup.ts
+++ b/src/setup.ts
@@ -8,4 +8,6 @@ import "rxjs/add/operator/delay"
 import "rxjs/add/operator/catch"
 import "rxjs/add/observable/of"
 
+import "rx-custom/add/filterUndefined"
+
 // const isDev = (process && process.env && process.env.NODE_ENV) !== "production"