How to tackle a JavaScript function that returns a JS Promise?

[jbhoot]

From what I understand, while lwt provides a counterpart to JS Promise, it doesn't have a mechanism to mix those two (like translating js Promise into lwt).

[TyOverby]

Js_of_ocaml doesn't have official support for the promise API. You'll need to use a library like https://github.com/mnxn/promise_jsoo

[jbhoot]

Thank you. Does Promise support seem likely via #944 ?

[jbhoot]

Update: I opted for promise_jsoo, but the following commands (as recommended in the manual):

>  ocamlfind ocamlc -package js_of_ocaml,js_of_ocaml-ppx,promise_jsoo -linkpkg -o bg.byte bg.ml

>  js_of_ocaml bg.byte

fail to include promise_jsoo in the compiled file (mnxn/promise_jsoo#2).

Dune works, however.

Am I missing anything in the above commands?

[TyOverby]

you need to pass the path to indirect_promise.js to the js_of_ocaml invocation.

if i were you, id stick to using Dune though

[jbhoot]

Your suggestion worked!

js_of_ocaml +promise_jsoo/indirect-promise.js bg.byte

compiled bg.js with its dependencies.

dune, even with release profile, produces a js file of 2.2MB. The above command generated an output of 40KB. That’s a drastic difference. Unless there is a way to drastically cut down the output size of dune, I see much value in the latter solution.

[TyOverby]

dune, even with release profile, produces a js file of 2.2MB. The above command generated an output of 40KB

I think that's because Dune includes the "--sourcemap-inline" command in it's invocation of js_of_ocaml. I don't know how you turn that off, but you can check to see what the difference would be by deleting the last line (it should be a comment) in the .js file

[jbhoot]

Sorry I got time to inspect further today. Anyway, quoting myself from ocaml/dune#4030 (comment):

That's not the case. The last line is not a sourcemap comment in bg.bc.js file.

That inspired me to try another command:

ocamlfind ocamlc -g -package js_of_ocaml,js_of_ocaml-ppx,promise_jsoo -linkpkg -o bg.byte bg.ml

js_of_ocaml --source-map-inline +promise_jsoo/indirect_promise.js bg.byte

With --source-map-inline, js_of_ocaml adds a sourcemap comment starting with //# sourceMappingURL=data:application/json;base64,...., and so produces a larger file, but only with a size of 544K (still smaller than dune's 2.2M).

[hhugo]

The size issue has been resolved by mnxn/promise_jsoo@174d490

The answer to this issue is to use promise_jsoo. I'll create an issue to document this fact and close this issue.