Socket.io connection refused

I'm currently working on a simple game using WebSockets (first time!). However, my console is filled with net::ERR_CONNECTION_REFUSED errors, and I can't get anything to work.

I've been trying to fix the errors for hours and have seen a few other questions online with similar problems, but nothing has worked for me so far.

Here is my code:

server.js:

const { createGameState, gameLoop } = require("./game")
const { FRAME_RATE } = require("./constants")

const io = require("socket.io")(httpServer, {
  cors: {
    origin: "http://localhost:3000",
    methods: ["GET", "POST"]
  }
});

io.on("connection", client => {
  const state = createGameState()

  startGameInterval(client, state)
});

function startGameInterval(client, state) {
  const intervalId = setInterval(() => {
      const winner = gameLoop(state);
      console.log("interval");

      if(!winner) {
        client.emit("gameOver");
        clearInterval(intervalId);
      } else {
        client.emit("gameover");
        clearInterval(intervalId);
      }
  }, 1000 / FRAME_RATE); // 1000
}

io.listen(3000);

index.js (top part):

const backgroundColor = "#1c1315";
const snakeColor = "#7d7778";
const foodColor = "#f55872";

const socket = io("http://localhost:3000")

socket.on("init", handleInit());
// socket.on("gamestate", handleGameState());

const gameScreen = document.getElementById("gameScreen");

let canvas, ctx;

const gameState = {
  player: {
    pos: {
      x: 3,
      y: 10,
    },
    vel: {
      x: 1,
      y: 0,
    },
    snake: [
      {x: 1, y: 10},
      {x: 2, y: 10},
      {x: 3, y: 10},
    ]
  },
  food: {
    x: 7,
    y: 7,
  },
  gridsize: 20,
};

Here is the error that I'm getting; it gets printed in the console every 5 seconds or so. polling-xhr.js:206 GET http://localhost:3000/socket.io/?EIO=4&transport=polling&t=NW6GI07 net::ERR_CONNECTION_REFUSED

I'm very new to socket.io/websockets in general, so I don't know what this error means or what I could do to fix the issue. Any help would be massively appreciated!