It is possible to use gRPC directly from JavaScript using the gRPC-Web [1] project by Improbable. More accurately, it uses TypeScript, which is generated from the proto file in a similar way to other languages. You still need a proxy to transform requests from HTTP/1.1 to HTTP/2.0.

I've actually only used the gRPC JSON gateway mentioned in the other replies so I'm not sure how it compares, but it looks interesting.

[1] https://github.com/improbable-eng/grpc-web