diff --git a/extension/content.js b/extension/content.js
index 572a66f..435b322 100644
--- a/extension/content.js
+++ b/extension/content.js
@@ -1,32 +1,37 @@
 function freeUrl(url) {
-  const urlObj = new URL(url)
+  const urlObj = new URL(url);
 
   urlObj.host = "freedium-mirror.cfd";
 
   return urlObj.href;
 }
 
-document.addEventListener("readystatechange", (event) => {
-  const button = document.createElement("button");
-  button.id = "free-medium__button";
-  button.textContent = "Read for free";
-  button.style.position = "fixed";
-  button.style.bottom = "20px";
-  button.style.right = "20px";
-  button.style.padding = "20px 28px";
-  button.style.backgroundColor = "#007bff";
-  button.style.color = "#fff";
-  button.style.border = "none";
-  button.style.borderRadius = "48px";
-  button.style.fontSize = "18px";
-  button.style.cursor = "pointer";
-  button.style.zIndex = "9999";
-  button.style.boxShadow = "0 2px 5px rgba(0, 0, 0, 0.2)";
+if (typeof document !== "undefined") {
+  document.addEventListener("readystatechange", (event) => {
+    const button = document.createElement("button");
+    button.id = "free-medium__button";
+    button.textContent = "Read for free";
+    button.style.position = "fixed";
+    button.style.bottom = "20px";
+    button.style.right = "20px";
+    button.style.padding = "20px 28px";
+    button.style.backgroundColor = "#007bff";
+    button.style.color = "#fff";
+    button.style.border = "none";
+    button.style.borderRadius = "48px";
+    button.style.fontSize = "18px";
+    button.style.cursor = "pointer";
+    button.style.zIndex = "9999";
+    button.style.boxShadow = "0 2px 5px rgba(0, 0, 0, 0.2)";
 
-  button.addEventListener("click", function () {
-    window.location.href = freeUrl(document.URL);
-  });
+    button.addEventListener("click", function () {
+      window.location.href = freeUrl(document.URL);
+    });
 
-  document.body.appendChild(button);
-});
+    document.body.appendChild(button);
+  });
+}
 
+if (typeof module !== "undefined" && module.exports) {
+  module.exports = { freeUrl };
+}
diff --git a/tests/unit/freeUrl.test.js b/tests/unit/freeUrl.test.js
index 5e9f433..42ed544 100644
--- a/tests/unit/freeUrl.test.js
+++ b/tests/unit/freeUrl.test.js
@@ -1,25 +1,18 @@
-function freeUrl(url) {
-  const urlObj = new URL(url)
-
-  urlObj.host = "freedium-mirror.cfd";
-
-  return urlObj.href;
-}
-
+const { freeUrl } = require("../../extension/content.js");
 
 describe("freeUrl", () => {
   it("converts the URLs correctly", () => {
     expect(freeUrl("https://medium.com/my-post")).toBe(
-      "https://freedium-mirror.cfd/my-post"
+      "https://freedium-mirror.cfd/my-post",
     );
     expect(freeUrl("https://www.medium.com/my-post")).toBe(
-      "https://freedium-mirror.cfd/my-post"
+      "https://freedium-mirror.cfd/my-post",
     );
     expect(freeUrl("https://some-subdomain.medium.com/my-post")).toBe(
-      "https://freedium-mirror.cfd/my-post"
-    );
-    expect(freeUrl("https://medium.com/hackernoon/hello-world-79436a73e443")).toBe(
-      "https://freedium-mirror.cfd/hackernoon/hello-world-79436a73e443"
+      "https://freedium-mirror.cfd/my-post",
     );
+    expect(
+      freeUrl("https://medium.com/hackernoon/hello-world-79436a73e443"),
+    ).toBe("https://freedium-mirror.cfd/hackernoon/hello-world-79436a73e443");
   });
 });