enable conditionnal loading for polyfill

[krichprollsch]

No description provided.

[karlseguin]

Is the reason this is working is that V8 first checks to see if globalThis["fetch"] exists, which executes this callback. We say "NO, it doesn't" (missing always returns false, so we always return v8.Intercepted.No), but then there's something else (what??) v8 is checking, and at that point, fetch has been defined?

Put differently, i understand why this code works the 2nd time if fetch is called twice. But I don't understand why it works the first time.

[krichprollsch]

It seems v8 first checks for native implementation and then looks at the pure JS value.
So the callback is called even if the property exists in pure JS.

return Intercepted::kYes. If the interceptor does not handle the request it must return Intercepted::kNo and it must not produce side effects.
https://v8.github.io/api/head/namespacev8.html#a35b89945995c331fb2e21d5db4e873a7

When returning "no", v8 continues to search for the value, falling back to pure JS. Since we inject a polyfill, we define the property in pure JS, so it works...

I know this is fragile, if the order change, it won't work anymore. But it was really easier than trying to inject the value manually after loading the polyfill.

[krichprollsch]

I will add a comment, but it's important to set the polyfill done immediately.
Indeed, if your polyfill calls the property during its execution, the missing handler will be called too, resulting to an infinite recursive load...

[karlseguin]

These are going to be called a lot, there could be value in optimizing the string comparison to be a single int equality check. Something like this:

switch (name.len) {
    5 => switch (@as(u40, @bitCast(domain[0..5].*))) {
        asUint(u40, "fetch") => if (!self.done.fetch) {
            self.done.fetch = true;
            return load(fetch_src);
        },
        else => {},
    },
    else => {},
};
return false;


fn asUint(comptime T: type, comptime string: []const u8) T {
    return @bitCast(string[0..string.len].*);
}

[karlseguin]

These are going to be called a lot, there could be value in optimizing the string comparison to be a single int equality check. Something like this:

switch (name.len) {
    5 => switch (@as(u40, @bitCast(domain[0..5].*))) {
        asUint(u40, "fetch") => if (!self.done.fetch) {
            self.done.fetch = true;
            return load(fetch_src);
        },
        else => {},
    },
    else => {},
};
return false;


fn asUint(comptime T: type, comptime string: []const u8) T {
    return @bitCast(string[0..string.len].*);
}

[karlseguin]

I guess the BrowserContext will need its own loader to track the isolate world polyfills?