trying to select a .csv file from the file input and pass it to the backend and read it from the back end using remix run

I am trying to select a CSV file with some columns and data using file input. This is the content of the CSV

id,level,cvss,title,Vulnerability,Solution,reference, 963,LOW,7.8,Title - 963,Vulnerability - 963,Solution - 963,reference - 963, 964,CRITICAL,4.6,Title - 964,Vulnerability - 964,Solution - 964,reference - 964, 965,INFO,0,Title - 965,Vulnerability - 965,Solution - 965,reference - 965, 966,CRITICAL,10,Title - 966,Vulnerability - 966,Solution - 966,reference - 966, 967,HIGH,7.5,Title - 967,Vulnerability - 967,Solution - 967,reference - 967, 968,HIGH,5,Title - 968,Vulnerability - 968,Solution - 968,reference - 968, 969,MEDIUM,7.5,Title - 969,Vulnerability - 969,Solution - 969,reference - 969,

This is the code for the UI

import { Form } from "@remix-run/react";
import type { ActionArgs, UploadHandler } from "@remix-run/server-runtime";
import {
  composeUploadHandlers,
  parseMultipartFormData,
} from "@remix-run/server-runtime/dist/formData";
import { createMemoryUploadHandler } from "@remix-run/server-runtime/dist/upload/memoryUploadHandler";
import { csvUploadHandler } from "~/models/code.server";

export async function action({ request, params }: ActionArgs) {
  const uploadHandler: UploadHandler = composeUploadHandlers(
    csvUploadHandler,
    createMemoryUploadHandler()
  );

  const formData = await parseMultipartFormData(request, uploadHandler);
  const selected_csv = formData.get("selected_csv");
  console.log("========== selected csv file: ", selected_csv);
  return selected_csv;
}

export default function codeListImport() {
  return (
    <div style={{ textAlign: "center" }}>
      <h1 style={{ marginBottom: 10 }}>UPDATE CODE LIST</h1>
      <Form method="post" encType="multipart/form-data">
        <input
          type="file"
          accept=".csv"
          name="selected_csv"
        />
        <button type="submit" className="btn btn-sm">
          UPLOAD CSV
        </button>
      </Form>
    </div>
  );
}

This is the code.server.ts file

export const csvUploadHandler: UploadHandler = async ({
  name,
  filename,
  data,
  contentType,
}) => {
  if (name !== "selected_csv") {
    return undefined;
  }

  console.log("===== file name", filename);
  console.log("===== data", data);
  console.log("===== content type", contentType);
};

I was trying to get the content of the CSV file using data given by the uploadHandler. I get the correct name of the HTML input element, file name as well as the content type. But when I console log data it shows me this on the log:

What I clearly do not get is how to decode this object

I have gone through multiple tutorials and many stackoverflow posts but I am still having a hard time understanding what exactly is the Symbol.asyncIterator. I am a newbie for ES6. Please help me on this matter

In your UploadHandler, the parameter is type UploadHandlerPart

export type UploadHandlerPart = {
  name: string;
  filename?: string;
  contentType: string;
  data: AsyncIterable<Uint8Array>;
};

So data is not the CSV string, it's an AsyncIterable of bytes. You need to convert this to a string and then return that from your handler.

export const csvUploadHandler: UploadHandler = async ({
  name,
  filename,
  data,
  contentType,
}) => {
  if (name !== 'selected_csv') {
    return undefined;
  }
  // get data in chunks
  let chunks = [];
  for await (let chunk of data) {
    chunks.push(chunk);
  }
  // convert chunks to string using Blob()
  return await new Blob(chunks, { type: contentType }).text();
};

I have an example here: https://stackblitz.com/edit/node-ubpgp5?file=app%2Froutes%2Findex.tsx

Since you're not really doing anything in your csvUploadHandler, you can simply use the existing memoryUploadHandler(). Your csv file will be in formData as a File type.

const uploadHandler = createMemoryUploadHandler();
const formData = await parseMultipartFormData(request, uploadHandler);
const csvFile = formData.get('selected_csv') as File;
const csv = await csvFile.text();