POST http://localhost:3000/upload 404（未找到）

Question

我正在开发一个文件上传应用程序，但遇到了一个问题。提交上传文件时，我在控制台中收到 404 后错误。完整代码为 here。我假设文件上传组件存在问题，但似乎无法理解这里的问题。

文件上传.js

import React, { useState } from "react";
import axios from "axios";

export const FileUpload = () => {
  const [file, setFile] = useState("");
  const [filename, setFilename] = useState("Choose File");
  const [uploadedFile, setUploadedFile] = useState({});

  const onChange = (e) => {
    setFile(e.target.files[0]);
    setFilename(e.target.files[0].name);
  };

  const onSubmit = async (e) => {
    e.preventDefault();
    const formData = new FormData();
    formData.append("file", file);

    try {
      const res = await axios.post("/upload", formData, {
        headers: {
          "Content-Type": "multipart/form-data",
        },
      });
      const { fileName, filePath } = res.data;

      setUploadedFile({ filename, filePath });
    } catch (err) {
      if (err.response.status === 500) {
        console.log("There was a problem with the server");
      } else {
        console.log(err.response.data.msg);
      }
    }
  };

  return (
    <>
      <form onSubmit={onSubmit}>
        <div class="custom-file">
          <input
            type="file"
            class="custom-file-input"
            id="customFile"
            onChange={onChange}
          />
          <label class="custom-file-label" for="customFile">
            {filename}
          </label>
        </div>
        <input
          type="submit"
          value="Upload"
          className="btn btn-primary btn-block mt-4"
        />
      </form>
    </>
  );
};