How on earth should the res.json of express be returned correctly?

in express development, we often encounter a problem:

    User.findOne({
      userName: userName
    }, function (userErr, userDoc) {
      if (userErr) {
        return res.json({
          status: "0",
          msg: userErr.message,
          result: ""
        })
      } else {
        return res.json({
          status: "1",
          msg: "",
          result: "user-error"
        })
      }
    })

it keeps reporting this error:

Error: Can"t set headers after they are sent.

could you tell me how to solve this? You can"t even add return? How to do it.

add: the complete code of this interface:

router.post("/register", function (req, res, next) {
  console.log(req.body);
  if (req.body) {
    //
    function CurrentTime() {
      var now = new Date();

      var year = now.getFullYear(); //
      var month = now.getMonth() + 1; //
      var day = now.getDate(); //

      var hh = now.getHours(); //
      var mm = now.getMinutes(); //

      var clock = year + "-";
      // var clock = "";

      if (month < 10)
        clock += "0";

      clock += month + "-";

      if (day < 10)
        clock += "0";

      clock += day + " ";

      if (hh < 10)
        clock += "0";

      clock += hh + ":";
      if (mm < 10) clock += "0";
      clock += mm;
      return (clock);
    };

    //
    let userName = req.body.name;
    User.findOne({
      userName: userName
    }, function (userErr, userDoc) {
      console.log("userDoc err")
      if (userErr) {
        console.log(":"+userName);
      } else {
        res.json({
          status: "1",
          msg: "",
          result: "user-error"
        })
        return false;
      }
    })

    //
    let userPassword = req.body.pass;
    let userCheckPassword = req.body.checkPass;
    if (userPassword == userCheckPassword) {
      //
      let md5 = crypto.createHash("md5"); //crypto,MD5
      let password = md5.update(userPassword).digest("hex"); //
    } else {
      console.log("password err")
      res.json({
        status: "0",
        msg: "",
        result: "password-error"
      });
      return false;
    }
    console.log(password);

    let param = {
      userCreatetime: CurrentTime(),
      userName: userName,
      userPassword: password,
      userHostelArea: req.body.hostelArea,
      userHostelAddress: req.body.hostelAddress,
      userSex: req.body.sex,
      userQQ: req.body.qq,
      userState: "1"
    };
    User.create(param);
    res.json({
      status: "1",
      msg: "",
      result: "success"
    });
  } else {
    console.log("req.body error")
    res.json({
      status: "0",
      msg: "",
      result: "error"
    })
    return false;
  }
})

complete error returned:

GET /users/setSessionStorage 304 8.845 ms - -
GET /hostels 304 41.645 ms - -
{ name: "",
  pass: "123456",
  checkPass: "123456",
  hostelArea: "",
  hostelAddress: "R413",
  sex: "",
  qq: "123456" }
POST /users/register 500 26.365 ms - 2258
Error: Failed to lookup view "error" in views directory "E:\\ConvenientCampus\server\views"
    at Function.render (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\application.js:580:17)
    at ServerResponse.render (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\response.js:971:7)
    at E:\\ConvenientCampus\server\app.js:89:7
    at Layer.handle_error (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\layer.js:71:5)
    at trim_prefix (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:315:13)
    at E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:284:7
    at Function.process_params (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:335:12)
    at next (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:275:10)
    at Layer.handle_error (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\layer.js:67:12)
    at trim_prefix (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:315:13)
    at E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:284:7
    at Function.process_params (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:335:12)
    at next (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:275:10)
    at E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:635:15
    at next (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\index.js:260:14)
    at next (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\router\route.js:127:14)
userDoc err
events.js:183
      throw er; // Unhandled "error" event
      ^

Error: Can"t set headers after they are sent.
    at validateHeader (_http_outgoing.js:494:11)
    at ServerResponse.setHeader (_http_outgoing.js:501:3)
    at ServerResponse.header (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\response.js:730:10)
    at ServerResponse.send (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\response.js:170:12)
    at ServerResponse.json (E:\\ConvenientCampus\node_modules\_express@4.15.5@express\lib\response.js:256:15)
    at E:\\ConvenientCampus\server\routes\users.js:80:13
    at E:\\ConvenientCampus\node_modules\_mongoose@5.0.9@mongoose\lib\model.js:3930:16
    at Immediate.<anonymous> (E:\\ConvenientCampus\node_modules\_mongoose@5.0.9@mongoose\lib\query.js:1514:14)
    at Immediate._onImmediate (E:\\ConvenientCampus\node_modules\_mquery@3.0.0@mquery\lib\utils.js:119:16)
    at runCallback (timers.js:789:20)
    at tryOnImmediate (timers.js:751:5)
    at processImmediate [as _immediateCallback] (timers.js:722:5)

Express

Feb.28,2021