Fix TypeError: "fetch" is read-only

[PleasantMachine9]

It seems a twitch (p.js?) script redefines the window.fetch with Object.defineProperty which makes it read-only (not over-writable with regular assignment).

Only using Object.defineProperty will be able to overwrite it.

[pixeltris]

Sounds good. I guess this is something they're A/B testing? I'm currently not seeing this on either Chrome/Firefox.

video-swap-new also uses the same hook so it'd be good to fix there too.

[PleasantMachine9]

Probably. The load trace has kasada stuff in it

722956/t.prototype.loadKPSDK/b</</this.loadPromise<
https://static.twitchcdn.net/assets/minimal-ebf9adeffef061974e24.js:1:745537
722956/t.prototype.loadKPSDK/b</<
https://static.twitchcdn.net/assets/minimal-ebf9adeffef061974e24.js:1:744877
...
722956/t.prototype.loadKPSDK
https://static.twitchcdn.net/assets/minimal-ebf9adeffef061974e24.js:1:744741

[pixeltris]

It looks like you need to pass in a descriptor structure rather than just passing in a function. Example:

var realFetch = window.fetch;
Object.defineProperty(window, "fetch", { value: function(url, init, ...args) { console.log('hi'); realFetch.apply(this, arguments); } });
fetch();

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

Do they do anything with the worker fetch?

[PleasantMachine9]

It looks like window.fetch is not really used by the twitch vendor.js files after the page is loaded. Overwriting it with anything does nothing.

Based on some traces it looks like p.js or some other obfuscated scriptlet stores the overwritten fetch in some closures and doesn't use the window-global one.

[pixeltris]

That could be problematic. I think the simplest solution would be to ensure the scripts run before their code.

I'll take a look when they have pushed this change out.

[PleasantMachine9]

This PR doesn't fix the issue