diff --git a/chapter05/src/validate_user.js b/chapter05/src/validate_user.js
new file mode 100644
index 0000000..a061a05
--- /dev/null
+++ b/chapter05/src/validate_user.js
@@ -0,0 +1,25 @@
+/* @flow */
+"use strict";
+
+/*
+    In real life, validateUser could check a database,
+    look into an Active Directory, call another service,
+    etc. -- but for this demo, let's keep it quite
+    simple and only accept a single hardcoded user.
+*/
+
+const validateUser = (
+    userName: string,
+    password: string,
+    callback: (?string, ?string) => void
+) => {
+    if (!userName || !password) {
+        callback("Missing user/password", null);
+    } else if (userName === "fkereki" && password === "modernjsbook") {
+        callback(null, "fkereki"); // OK, send userName back
+    } else {
+        callback("Not valid user", null);
+    }
+};
+
+module.exports = validateUser;
diff --git a/chapter05/src/validate_user.test.js b/chapter05/src/validate_user.test.js
new file mode 100644
index 0000000..226f0e5
--- /dev/null
+++ b/chapter05/src/validate_user.test.js
@@ -0,0 +1,25 @@
+/* @flow */
+"use strict";
+
+const validateUser = require("./validate_user");
+
+describe("validateUser", () => {
+    it("should reject a call with empty user", () => {
+        const cb = jest.fn();
+        validateUser("", "somepass", cb);
+        expect(cb).toHaveBeenCalled();
+        expect(cb).toHaveBeenCalledWith("Missing user/password", null);
+    });
+
+    it("should reject a wrong password", () => {
+        const cb = jest.fn();
+        validateUser("fkereki", "wrongpassword", cb);
+        expect(cb).toHaveBeenCalledWith("Not valid user", null);
+    });
+
+    it("should accept a correct password", () => {
+        const cb = jest.fn();
+        validateUser("fkereki", "modernjsbook", cb);
+        expect(cb).toHaveBeenCalledWith(null, "fkereki");
+    });
+});