How do I create Javascript wrappers? about jsil

[kg]

The attributes from the JSIL.Meta assembly are the secret here. For example:
https://github.com/kevingadd/JSIL/blob/master/Proxies/Array.cs makes heavy use of JSReplacement to replace method calls with arbitrary JS expression. The dollar-sign expressions are used to reference arguments. (Using just plain '$.' in this case should still work.)

You can also use the methods and objects from https://github.com/kevingadd/JSIL/blob/master/Meta/Verbatim.cs to simplify producing the JavaScript you want. For example, you could write the method you've provided like this:

void csharp_ajax (string url) {
    (JSIL.Builtins.Local["$"] as dynamic).ajax(url);
}

Note that in this example, there would still be a JS function generated named csharp_ajax, containing the actual call to $.ajax.