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。 {代码...} 下面是输出结果： jjjj 这是@2co...

高洛峰 · Answer

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)
})

巴扎黑 · Answer

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.

天蓬老师 · Answer

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");
    } 
});