不使用回调时的未定义结果。 Nodejs、Express 和 SQL Server Express

Question

以下问题由 mssql、Nodejs、Gulp、Express 和 SQL Server Express 组成。我能够成功登录 SQL Server Express。但是，当我使用没有回调的bookRoute.js代码片段时，返回值是undefined。然而，当我使用回调时，我得到了数据。但是，我不明白为什么。

app.js 代码片段：

var config = {
    user: 'user',
    password: 'password',
    server: 'localhost',
    database: 'Books',
    options: {
       instance: 'SQLEXPRESS'
    }
};

sql.connect(config, function(err){
    console.log(err);
});

bookRoute.js 没有回调的代码片段：

bookRouter.route('/')
.get(function (req, res) {
    console.log('book router');
    var request = new sql.Request();
    request.query('select * from books').then(
        function (err, recordset) {
            console.log(recordset);
        })
    .catch(function(err){  console.log(err)});
    });

bookRoute.js 带回调的代码片段：

bookRouter.route('/')
.get(function (req, res) {
    console.log('book router');
    var request = new sql.Request();
    request.query('select * from books',
        function (err, recordset) {
            console.log(recordset);
        });
});

用户访问网页后，控制台应显示结果。不幸的是，当不使用回调时，唯一显示的结果是 undefined。

控制台输出：

P:\ub\lic\library>gulp serve
[11:08:28] Using gulpfile P:\ub\lic\library\gulpfile.js
[11:08:28] Starting 'style'...
[11:08:28] Starting 'inject'...
[11:08:53] Finished 'inject' after 808 ms
[11:08:53] Finished 'style' after 25 s
[11:08:53] Starting 'serve'...
[11:08:53] Finished 'serve' after 5.31 ms
[11:08:53] [nodemon] 1.9.2

[11:08:53] [nodemon] to restart at any time, enter `rs`
[11:08:53] [nodemon] watching: *.js src/**/*.js
[11:08:53] [nodemon] starting `node app.js`

running server on port 3000
null
book router
undefined
[11:09:21] [nodemon] restarting due to changes...
Restarting the server.....beep boop beep beep
[11:09:21] [nodemon] restarting due to changes...
Restarting the server.....beep boop beep beep
[11:09:21] [nodemon] starting `node app.js`
running server on port 3000
null
book router
[ { id: 1,
    title: 'A,B,C with Big Bird           ',
    author: 'Michael Jacob ' },
  { id: 2,
    title: 'Peter and his Petunias        ',
    author: 'Jess Holiday  ' },
  { id: 3,
    title: 'The Amazing Average Guy       ',
    author: 'Don Dillon    ' } ]

Answer 1

bookRoute.js code snippet without callback:

bookRouter.route('/').get(function (req, res) {
console.log('book router');
var request = new sql.Request();
request.query('select * from books')
    .then(function (recordset) {
        console.log(recordset);
    })
    .catch(function (err) {
        console.log(err);
    });
});

当使用不带 callback.The then 函数的 bookRoute.js 代码片段时，应该只有一个参数，即查询结果，即 documentation.当出现错误时，调用 catch 函数。