The value of a --custom-header is not placed in the begining of the file [BUG]

[modlfo]

Describe the bug

The value passed using the --custom-header is not placed in the beginning of the file. For example:

js_of_ocaml --custom-header="#!/usr/bin/env node" main.bc

This produces the following code where the header is in the third line

// Generated by js_of_ocaml
//# buildInfo:effects=false, kind=exe, use-js-string=true, version=5.1.1
#!/usr/bin/env node
(function(a){typeof .......

Expected behavior
That the first line of the file is the header and any other text comes after.

#!/usr/bin/env node
// Generated by js_of_ocaml
//# buildInfo:effects=false, kind=exe, use-js-string=true, version=5.1.1

This produces the following error when running the code:

SyntaxError: Invalid or unexpected token
    at wrapSafe (internal/modules/cjs/loader.js:915:16)
    at Module._compile (internal/modules/cjs/loader.js:963:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)
    at Module.load (internal/modules/cjs/loader.js:863:32)
    at Function.Module._load (internal/modules/cjs/loader.js:708:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:60:12)
    at internal/main/run_main_module.js:17:47
F/home/leonardo/main.js:3
#!/usr/bin/env node
^

Versions
Version of packages used to reproduce the bug
Ocaml 4.14.0
js_of_ocaml version=5.1.1

[hhugo]

Thanks, should be relatively easy to fix. The custom header is handled in driver.ml, the other header is generated in bin-js_of_ocaml/compile.ml. I think we should move the custom header handling to compile.ml. @modlfo, would you be interested by trying to fix this yourself?

[modlfo]

Hi @hhugo I gave it a look and I'm not familiar at all with the js_of_ocaml code. It does not look very straightforward for me.

[hhugo]

@modlfo, I can provide guidance if you want to spend time on this.
The code inserting the custom header can be removed from

js_of_ocaml/compiler/lib/driver.ml

Line 190 in 8ba6297

let header formatter ~custom_header =

and added around that line

js_of_ocaml/compiler/bin-js_of_ocaml/compile.ml

Line 39 in 8ba6297

if Config.Flag.header () then jsoo_header fmt build_info;

.

[modlfo]

Thank you for fixing it. I was going to give it a try last weekend but you were faster.