content-length 不为0时，才缓存。旧版本学生端取不到 content-length

serviceWorkerAppend.js

@@ -29,6 +29,10 @@ self.addEventListener("fetch", event => {
 
         // Step 2: get total length
         const contentLength = +res.headers.get("Content-Length");
+
+        if (contentLength === 0) {
+          return fetch(event.request);
+        }
         // console.log("content-length", contentLength);
         // self.sessionStorage.setItem("test-sw", "1");
         // self.postMessage({