You can use ServiceWorker to read original Response before returning content of original Response or new content.
if ("serviceWorker" in navigator) {
navigator.serviceWorker.register("sw.js").then(function(reg) {
console.log("register", reg);
}).catch(function(err) {
console.log("err", err);
});
}
self.addEventListener("fetch", function(event) {
if (event.request.url == "/path/to/fetched/resource/") {
console.log("fetch", event);
event.respondWith(
fetch(event.request).then(function(response) {
return response.text()
.then(function(text) {
if (text === "abc123") {
return new Response("def456")
} else {
return new Response(text)
}
})
})
);
}
});