Js_of_ocaml : missing primitive when calling ocurl and str
Some OCaml libraries use primitives written in plain C. For obvious reasons js_of_ocaml cannot link them to Javascript. In this case you need to implement them manually, which is mentioned in the manual.
Functions from Str module have equivalents in ECMAScript library, so probably you need to use them (if somebody didn't create helper module which has the same interface as Str and call Javascript functions under the hood)
To do http requests in js_of_ocaml, you will need to use that module: http://ocsigen.org/js_of_ocaml/2.5/api/XmlHttpRequest