如何在 javascript 中使用 defer/promise 进行数据库调用

Question

I am having db.js with db related functions, I want to make call to db.js and wait until it returns the query result. But the result is returned after the execution of the db call. Can anyone please help how to solve this.

Code sample:

var Q = require('q');

db= require("./dbaccess.js");

function waitfor(ms){

     var deferred = Q.defer();

     setTimeout(function() {

        deferred.resolve(db);
     }, 5000);
     return deferred.promise;
}

waitfor(2000).done(function(dbcall) {

console.log('contrived example '+ dbcall.query1()); 

});

dbacess.js:

var sql = require('mssql');

var config = {

user: 'xx',

    password: 'xxx',

    server: 'aaa', 

    database: 'RequestCenter',

    stream: true,  

}

this.query1=function() {

sql.connect(config, function(err) {

    var result;
    var request = new sql.Request();
    request.query("select * from dbo.AcAccount where Name like 'AutomationCli%' ");  
    request.on('row', function(row) {
        console.log(row.Name);
        result = row.Name;
    });

    request.on('error', function(err) {
        console.log("err : "+err); 
    });

    request.on('done', function(returnValue) {
        console.log("done"); 
    });
    return result;
});



sql.on('error', function(err) {

console.log("sql err : "+err);

});

}

Output:

contrived example undefined

in db: AutomationClient

Expected output:

in db: AutomationClient

contrived example AutomationClient

Answer 1

Not sure why your main code passes in 2000 for the ms argument and then does a 5000ms timeout, in fact, why are you doing a timeout at all, if that was some attempt to wait for the db function to complete, then you don't need it

If you must use promises - personally I'd use a simple callback for such simple code, however, I get that you want to learn how to use Promises

Your original code looked like it was attempting to return the value of the LAST row.name

This code returns an array of row.name

Not knowing the type of data you'd be getting, I don't know which is correct

dbacess.js

var Q = require('q');
var sql = require('mssql');

var config = {
    user: 'xx',
    password: 'xxx',
    server: 'aaa',
    database: 'RequestCenter',
    stream: true,

}

this.query1 = function() {
    var deferred = Q.defer();
    sql.connect(config, function(err) {
        var result = []; // return all rows - modify as required
        var request = new sql.Request();
        request.query("select * from dbo.AcAccount where Name like 'AutomationCli%' ");
        request.on('row', function(row) {
            console.log(row.Name);
            result.push(row.Name);
        });

        request.on('error', function(err) {
            console.log("err : " + err);
            deferred.reject(err);
        });

        request.on('done', function(returnValue) {
            deferred.resolve(result);
        });
    });

    sql.on('error', function(err) {
        console.log("sql err : " + err);
        deferred.reject(err);
    });
    return deferred.promise;
}

Code sample:

db = require("./dbaccess.js");

db.query1().then(function(result) {
    console.log('contrived example ' + result);
});