Vite + React + Flask not proxying frontend to backend

I am trying to set a proxy in my frontend so that I can fetch() apis from my backend.

Here is my setup:

vite.config.ts:

  server: {
    proxy: { "/api":  {
                      target: 'http://localhost:3001',
                      changeOrigin: true,
                      secure: false      
                      }
             }
  },

frontend.tsx:

const Test: React.FC = () => {

  useEffect(()=>{
    fetch(`api/hello`)
    .then(res => console.log(res))
  }, [])
  return(
  <div>
    hi
  </div>
  )
}

Backend:

@app.route('/api/hello', methods=['GET'])
def test():
    return jsonify({"hi":"there"})

I get the error: GET http://localhost:5173/api/hello net::ERR_ABORTED 500 (Internal Server Error)

It is using port 5173 for the fetch() and not 3001 which is what my server is listening on.

I have tried solutions such as: How to configure proxy in Vite? and it does not work.

I added some event listeners to the vite.config.ts file as follows:

proxy: { "/api":  {
                      target: 'http://localhost:3001',
                      changeOrigin: true,
                      secure: false,
                      configure: (proxy, _options) => {
                        proxy.on('error', (err, _req, _res) => {
                          console.log('proxy error', err);
                        });
                        proxy.on('proxyReq', (proxyReq, req, _res) => {
                          console.log('Sending Request to the Target:', req.method, req.url);
                        });
                        proxy.on('proxyRes', (proxyRes, req, _res) => {
                          console.log('Received Response from the Target:', proxyRes.statusCode, req.url);
                        });
                      },   
                      }
             }

I got the following errors when I refreshed:

Sending Request to the Target: GET /api/hello
proxy error Error: connect ECONNREFUSED ::1:3001
    at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1494:16) {
  errno: -4078,
  code: 'ECONNREFUSED',
  syscall: 'connect',
  address: '::1',
  port: 3001
}
3:45:38 p.m. [vite] http proxy error at /api/hello:
Error: connect ECONNREFUSED ::1:3001
    at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1494:16) (x3)
    ```

I solved this by changing localhost to the localhost ip address

  server: {
    proxy: { "/api":  {
                      target: 'http://127.0.0.1:3001',
                      changeOrigin: true,
                      secure: false      
                      }
             }
  },

Why does this work and not localhost? We may never know...