What I want to do
I'm trying to intercept a third party website's fetch events and modify its request body in a Chrome extension. Modifying the request body is not allowed by the chrome.webRequest.onBeforeRequest event handler. But it looks like regular service workers do have the ability to listen for and modify fetch events and manually respond to the request using my own response, which means I should be able to intercept the request, modify the body, send the modified request to the API, and then respond to the original request with my modified request's response.
The problem
It looks like neither of these event handlers ever get triggered, despite plenty of fetch events being triggered by the website, as I can see in the Network panel.
// background.js
self.onfetch = (event) => console.log(event.request); // never shows
// or
self.addEventListener("fetch", (event) => {
console.log(event.request); // never shows
});
I can verify that the service worker is running by seeing other console.logs appearing in the service worker console, both top-level logs as well as logs triggered by the "install" event
// background.js
console.log(self); // works
self.addEventListener("install", (event) => {
console.log(event); // works
});
Hypothesis
Do the fetch event handlers not get triggered because extension service workers are not allowed access to these for security reasons? That would make sense, I just haven't seen this documented anywhere explicitly so it would be good to know if this is indeed a platform limitation or if I'm doing something wrong.
Alternate solutions?
If this is indeed a limitation of the extensions platform, is there any way other way I can use a Chrome extension to modify request bodies on a third party website?
