# Handling binary data in ocaml and javascript **URL:** \ **Category:** Learning\ **Tags:** web, jsoo\ **Created:** [February 1, 2018, 7:15pm UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519 "2018-02-01T19:15:04Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![martindemello](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/martindemello/32/632_2.png) [@martindemello](https://discuss.ocaml.org/u/martindemello)\ **Post date:** [February 1, 2018, 7:15pm UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519/1 "2018-02-01T19:15:04Z")
I have some ocaml code that reads in a binary file as a string, and then treats it as an array of bytes. This led to some odd errors when I tried to compile it with jsoo, which I eventually traced down to the fact that javascript strings aren’t really suited for the purpose, and I should be using an ArrayBuffer instead. Is there any nice way to create an interface that will compile to the string code in ocamlc and to Typed\_array code if compiled with jsoo? ---
**Author:** ![nojb](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/nojb/32/519_2.png) [@nojb](https://discuss.ocaml.org/u/nojb)\ **Post date:** [February 2, 2018, 12:20pm UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519/2 "2018-02-02T12:20:49Z")
Maybe you could get more help if you explain what where you trying to do and what were the errors in question . AFAIK, OCaml-strings-as-byte-buffers will work regardless of which compiler is used. ---
**Author:** ![martindemello](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/martindemello/32/632_2.png) [@martindemello](https://discuss.ocaml.org/u/martindemello)\ **Post date:** [February 3, 2018, 7:18am UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519/3 "2018-02-03T07:18:23Z")
I’m trying to load a binary file as a string, and parse it using [bitstring](https://github.com/xguerin/bitstring). when i try reading the file as a string in javascript, I see a few extra bytes inserted, which makes me think that when I call ReadAsBinaryString, javascript is using a string encoding that spills some characters into two bytes. Furthermore, when I was googling to see if any one else had had this issue, I saw that ReadAsBinaryString has been [deprecated](https://developer.mozilla.org/en-US/docs/Web/API/FileReader/readAsBinaryString) and that I’m supposed to read the file into an ArrayBuffer instead. I’ll experiment with converting the ArrayBuffer to a string and passing it to the original code, but if that doesn’t work either some sort of #ifdef js\_of\_ocaml equivalent would be useful. ---
**Author:** ![martindemello](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/martindemello/32/632_2.png) [@martindemello](https://discuss.ocaml.org/u/martindemello)\ **Post date:** [February 3, 2018, 9:11am UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519/4 "2018-02-03T09:11:16Z")
this [stackoverflow comment](https://stackoverflow.com/a/17512132) seems relevant: > The reason behind readAsBinaryString() deprecation is in my opinion the following: the standard for JavaScript strings are DOMString which only accept UTF-8 characters, NOT random binary data. So don’t use readAsBinaryString(), that’s not safe and ECMAScript-compliant at all. poking around the jsoo source I found that runtime/mlString lets me directly create an ocaml string backed by a byte array, I’m now trying to get that playing nicely with [gen\_js\_api](https://github.com/LexiFi/gen_js_api), which I believe will solve my issue. ---
**Author:** ![martindemello](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/martindemello/32/632_2.png) [@martindemello](https://discuss.ocaml.org/u/martindemello)\ **Post date:** [February 8, 2018, 3:44am UTC](https://discuss.ocaml.org/t/handling-binary-data-in-ocaml-and-javascript/1519/5 "2018-02-08T03:44:16Z")
code snippets, for anyone else facing this issue: this from the js.mli file (processed by gen\_js\_api): ``` module ArrayBuffer: sig type t val t_of_js: Ojs.t -> t val t_to_js: t -> Ojs.t val byte_length: t -> int end module Uint8Array: sig type t val t_of_js: Ojs.t -> t val t_to_js: t -> Ojs.t val new_uint8_array: ArrayBuffer.t -> t [@@js.new] val to_string: t -> string [@@js.cast] end ``` and this from the ml file: ``` external str_of_uint8Array: Js.Uint8Array.t -> string = "caml_string_of_array" let str_of_arraybuffer buf = let arr = Js.Uint8Array.new_uint8_array buf in str_of_uint8Array arr ```