Snippets "Rewrite links on HTML pages" example is missing headers validation

[dungahk]

Existing documentation URL(s)

• https://developers.cloudflare.com/rules/snippets/examples/rewrite-site-links/

What changes are you suggesting?

Add checks on whether the header exists before using it, as the get method can return null and the code calls the startsWith String method.

diff --git a/rewrite.js b/rewrite.js
index e34e765..63b4554 100644
--- a/rewrite.js
+++ b/rewrite.js
@@ -25,7 +25,13 @@ export default {
             .on("img", new AttributeRewriter("src"));
 
         const res = await fetch(request);
+        if (!res.headers.has("Content-Type")) {
+          return res;
+        }
         const contentType = res.headers.get("Content-Type");
+        if (typeof contentType !== "string") {
+          return res;
+        }
 
         // If the response is HTML, it can be transformed with
         // HTMLRewriter -- otherwise, it should pass through

Additional information

Return value of the get method: https://developer.mozilla.org/en-US/docs/Web/API/Headers/get#return_value

[dungahk]

I can't tell why exactly, but in some situations, that header doesn't exist, and the code throws an exception responding with a 500 status code with "Please enable cookies" and 1101 error message.