somewhat fixed test.re case

test_fix.re

open Belt;

[@bs.deriving abstract]
type file = {
  .
  "lastModified": int,
  "lastModifiedDate": string,
  "name": string,
  "size": int,
  "type_": string,
  "webkitRelativePath": string,
};

type fileList = array(file);

type state = {
  noFile: bool,
  files: fileList,
};

type action =
  | HandleFileChange(fileList)
  | UploadFile;

module Encode = {
  let toJson = file =>
    Json.Encode.(
      object_([
        ("lastModified", file##lastModified |> int),
        ("lastModifiedDate", file##lastModifiedDate |> string),
        ("name", file##name |> string),
        ("size", file##size |> int),
        ("type", file##type_ |> string),
        ("webkitRelativePath", file##webkitRelativePath |> string),
      ])
    );
};

let component = ReasonReact.reducerComponent("Uploader");

let make = _children => {
  ...component,
  initialState: _state => {noFile: true, files: [||]},
  reducer: (action, _state) =>
    switch (action) {
    | HandleFileChange(fileList) =>
      ReasonReact.Update({noFile: false, files: fileList})
    | UploadFile =>
      let payload = Belt.Array.getExn(_state.files, 0);
      Js.Promise.(
        Fetch.fetchWithInit(
          "/api/hello",
          Fetch.RequestInit.make(
            ~method_=Post,
            ~body=
              Fetch.BodyInit.make(
                Js.Json.stringify(Encode.toJson(payload)),
              ),
            ~headers=
              Fetch.HeadersInit.make({"Content-Type": "application/json"}),
            (),
          ),
        )
        |> then_(Fetch.Response.json)
        |> ignore
      );
      ReasonReact.NoUpdate;
    },
  render: _self =>
    <div>
      <input
        onChange=(
          _event =>
            _self.send(
              HandleFileChange(
                ReactDOMRe.domElementToObj(ReactEventRe.Form.target(_event))##files,
              ),
            )
        )
        _type="file"
        name="importing_file"
      />
      <div>
        <button _type="button" onClick=((_) => _self.send(UploadFile))>
          (ReasonReact.string("Importieren"))
        </button>
      </div>
    </div>,
};