如何使用`js_of_ocaml`从`bytes`生成`ArrayBuffer`

Question

I am building a JavaScript library that is implemented in Ocaml and compiled to JavaScript using js_of_ocaml .

One of my Ocaml function returns a string with binary data. How can I expose that using js_of_ocaml as a ArrayBuffer ?

Answer 1

When you compile to javascript, manipulating binary data in string s is extremely bug prone!

The underlying reason is questionable choice of js_of_ocaml : Because javascript strings are encoded in UTF16 whereas OCaml ones are (implicitly) encoded in UTF8, js_of_ocaml tries to navigate in between the 2. Therefore, when it encounters a "character" whose code is > 127, js_of_ocaml converts it which is a disaster if it is, in fact, raw binary data!

The solution is to manipulate bigstring s instead of string s. Bigstrings are (char, Bigarray.int8_unsigned_elt, Bigarray.c_layout) Bigarray.Array1.t in raw OCaml but more and more libraries aliases them. Especially, they are Typed_array.​Bigstring.t in js_of_ocaml (where you can see functions to convert from and to ArrayBuffer )

If your function does work by magic on string once compiled in javascript, there are translation function in between bigstrings and strings in several places. For example the bigstring library: http://c-cube.github.io/ocaml-bigstring/ but these functions are also available in Lwt_bytes of lwt

You can see an other question on the same subject (including ways to manipulate OCaml string in javascript while not touching them at all using gen_js_api ) at https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519