This is not true. From the App Store guidelines:

    > 2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript.

and also

    > 2.5.2 Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code, including other iOS, watchOS, macOS, or tvOS apps.

So you can't download and execute your own JavaScript, much less anything your user points your browser towards.