node.js - nodejs怎么同步从一个数据库查询函数中返回一个值？

Question

我想把从数据库中查询得到的值储存到一个变量中然后打印出来，但数据库查询是异步的，打印函数总是先一步调用，查询函数后一步调用，打印出来的值是空的。下面是代码。

var sqlite3 = require('sqlite3').verbose();


var async = require('async');


var str = "";

var db = new sqlite3.Database('score.db', function () {
    db.all("select * from score", function (err, res) {
        if (!err) {
           // console.log(JSON.stringify(res));
            str = JSON.stringify(res);
            console.log(str);//@1 这里输出的是有值的str
        }else {
            console.log(err);
        }
    });
});

console.log(str+"jjjj");//@2 这里输出的Str，是没有值的空的

下面是输出结果：
jjjj 这是@2console打印的，Str是空的

[{"id":1,"name":"jim","score":"100"},{"id":2,"name":"tom","score":"200"}]
这是@1 console打印的，str是有值的。

请问怎样让查询回调先执行，让str获得值，传递出来，然后再执行最后一句@2的打印语句。javascript的异步回调真令人头痛啊，一直不知道怎么解决。

Answer 1

Congratulations on entering the callback pit

var sqlite3 = require('sqlite3').verbose();


var async = require('async');


var str = "";

var get_result = function(callback) {
    var db = new sqlite3.Database('score.db', function () {
        db.all("select * from score", function (err, res) {
            if (!err) {
                callback(JSON.stringify(res));
                console.log(str);//@1 这里输出的是有值的str
            }else {
                console.log(err);
            }
        });
    });
}

get_result(function(data){
    console.log(data)
})

Answer 2

In your example, you can only put the sentence that prints the results into the callback function. In short, you don’t want the code to execute linearly in Node.js.

Recommend a book about JavaScript asynchronous process control: JavaScript Asynchronous Programming.

• There are some libraries that can make the code look more linear, such as async, promise, co. For details, see the appendix of the book above.
• There are some database drivers that implement synchronous database queries, but this kind of library is basically impossible to use in a production environment because it will block other events.

Answer 3

You can try this method:

var sqlite3 = require('sqlite3').verbose();

function selectFromDB(dbName, operation, callback) {
    var db = new sqlite3.Database(dbName, function () {
        db.all(operation, function (err, res) {
            if (!err) {
                str = JSON.stringify(res);
                return callback(null, str);
            } else {
                return callback(err);
            }
        });
    }); }

selectFromDB("score.db", "select * from score", function (err, result){
    if (err) {
        console.log(err);
    }
    else {
        console.log(result + "jjjj");
    } 
});