javascript - Why is my module function returning undefined in node.js?

Question

So, I wanted to move all my mysql query functions to another file and import it as a module, but I'm having trouble in that the function is returning undefined.

index.js:

const express = require('express')
const https = require('https');
const fs = require('fs');
const dbx = require('./databaseconn');

var options = {
    key: fs.readFileSync('/etc/letsencrypt/live/example.com/privkey.pem'),
    cert: fs.readFileSync('/etc/letsencrypt/live/example.com/cert.pem'),
    ca: fs.readFileSync('/etc/letsencrypt/live/example.com/chain.pem')
};

const app = express();

app.use(express.json());

app.use(function(req,res,next) {
    if (req.body.action) {
        switch (req.body.action) {
            case "login":
                var user = req.body.username;
                var pass = req.body.password;
                console.log(dbx.checklogin(user, pass));  // This prints undefined
                break;
            default:
                break;
        }
    }
});

https.createServer(options,app).listen(8000);

databaseconn.js:

const mysql = require('mysql');

exports.checklogin = function(username, password) {
    var connection = mysql.createConnection({
        host     : 'localhost',
        user     : 'root',
        password : 'xxxxxxxxxxxxxx',
        database : 'users',
    });

    connection.query('select * from logins where username = ? and password = ?;',
    [
        username,
        password
    ], function (err, rows, fields) {
        if (err) throw err;
        if (!rows.length) {
            console.log('0');
            return {"success":0};
        } else {
            console.log('1');      // THIS PRINTS
            return {"success":1};  // So this should return the JSON
        }
    })
}

So I see a "1" from the console.log in the databaseconn.js, but the console.log in the index.js file is printing undefined. What am I doing wrong? I also did a super barebones test that works fine so I can't see the difference.

test.js:

const express = require('express');
const test2 = require('./test2');
var t = test2.test();
console.log(t);

test2.js:

exports.test = function() {
    return {"success":1};
}

标签： javascriptnode.jsjsonfunctionmodule

Answer 1

console.log模块内的语句index.js打印undefined，因为模块内的checklogin函数databaseconn.js不返回任何内容（{"success":1}是内部匿名函数的返回值checklogin，但它不是checklogin自身的返回值）。

如果你想在模块内部处理查询结果，你可以从模块index.js传递一个回调，它可能如下所示：index.jsdatabaseconn.js

index.js：

app.use(function(req, res, next) {
  if (req.body.action) {
    switch (req.body.action) {
      case "login":
        var user = req.body.username;
        var pass = req.body.password;
        dbx.checklogin(user, pass, function(err, rows, fields) {
          console.log('Here you can handle the query result')
        });
        break;
      default:
        break;
    }
  }
});

数据库连接.js：

exports.checklogin = function(username, password, callback) {
  var connection = mysql.createConnection({
    host     : 'localhost',
    user     : 'root',
    password : 'xxxxxxxxxxxxxx',
    database : 'users',
  });

  connection.query('select * from logins where username = ? and password = ?;',
    [
      username,
      password
    ], callback)
}