As of [sept 2022, Fastly's Compute@Edge used
Spidermonkey.wasm](https://news.ycombinator.com/item?id=32916019).
> These are good questions! Here's some answers from the corner of the world I
> know best as a Wasmtime contributor at Fastly:
>
> 1. Spidermonkey.wasm is the basis of Fastly's JavaScript on Compute@Edge
> support. We have found it to be faster than QuickJS. The source code is here:
> https://github.com/fastly/js-compute-runtime.
>
> 2. Fastly Compute@Edge is built on wasmtime. You can develop web services for
> it in Rust, JS, and Go: https://developer.fastly.com/learning/compute/
>
> 3. Fastly's multi-tenant platform is closed source, but our single-tenant
> local development platform, which also uses wasmtime under the hood as well,
> is open source: https://github.com/fastly/viceroy. It isn't a big leap to
> make viceroy multi-tenant: Wasmtime provides everything you need, and all
> Viceroy would have to do is dispatch on e.g. HTTP host header to the correct
> tenant. Our multi-tenant platform is closed source because it is very
> specialized for use on Fastly's edge, not because the multi-tenant aspect is
> special.
Fastly's https://github.com/fastly/spidermonkey-wasi-embedding