节点js返回在回调函数中设置的全局变量

Question

I have 2 functions in javascript and I want to set a global variable in a callback and then return it from my main function. I know javascript is async by nature so what would be the best way to do this? I have the code below:

var results="";

function getData() {

var sql = require("mssql");
var dbConfig={
        server:"server",
        database: "db",
        user:"user",
        password: "pw"
}




        var conn = new sql.Connection(dbConfig);
        var req = new sql.Request(conn);
        conn.connect(function (err){
                if (err) {
                console.log(err);
                return;
                }


                req.query("SELECT * FROM table",resultsCallback)

                conn.close();
        });

    return results;
}

function resultsCallback (err, recordset) {

        var tableify = require('tableify');

        if (err) {
                console.log(err);
        }
        else {

                var html = tableify(recordset);
                html = html.replace('<table>','');
                html = html.replace('</table>',');
                results=html;

        }
};

Answer 1

There is no right way to do this, because what you want to do is not right. You are trying to return a value before it was fetched. And you are using global variable, which can be dangerous for your code.

I understand why you are trying to use global variable to get the results from your callback, but it can't work.

When your code return results; executes, the SQL query has not even started, so it returns "" .

Instead, use Promise :

function getData() {
    return new Promise((resolve, reject) => {
        var sql = require("mssql");
        var dbConfig = {
            server: "server",
            database: "db",
            user: "user",
            password: "pw"
        }
        var conn = new sql.Connection(dbConfig);
        var req = new sql.Request(conn);
        conn.connect(function (err) {
            if (err) {
                console.log(err);
                reject(err);
                return;
            }
            req.query("SELECT * FROM table",
                (err, recordset) => {
                    // Here we call the resolve/reject for the promise
                    try {
                        // If the results callback throws exception, it will be caught in 
                        // the catch block
                        resolve(resultsCallback(err, recordset));
                    }
                    catch (e) {
                        reject(e);
                    }
                }
            );

            conn.close();
        });
    })
}

function resultsCallback(err, recordset) {
    var tableify = require('tableify');
    if (err) {
        console.log(err);
        throw err;
    }
    else {
        var html = tableify(recordset);
        html = html.replace('<table>', '');
        html = html.replace('</table>', '');
        return html;
    }
};

Global variables can be dangerous, you don't need them. Promise solves your problem. You can have as many callbacks nested, but you always return value from Promise using resolve . This is the standard modern way to do this in JavaScript.

Usage:

getData().then((data)=>{console.log("Table data:",data);})
         .catch((error)=>{console.log("ERROR LOADING SQL:",error);})

Or, in an async function:

async function doSomethingWithData() {
    console.log("Start loading data.");
    const data = await getData();
    console.log("Done loading data.");
    // your results are in the data variable now
}