Variables not being updated in node.js asynchronously

Question

I'm new to node.js and I have some code that isn't working properly. I think the issue is in asynchronous functions. This code is pretty simple. Read some xml, assign the values to variables, assign those variables as properties that are used to connect to a database in another node.js file. Problem is the variables don't update after reading the xml.

Here's the code,

//set some variables
userXML = "bgdf";
passwordXML = "";
serverXML = "";
databaseXML = "";

//parse the xml
var fs = require('fs'),
xml2js = require('xml2js');
var parser = new xml2js.Parser({explicitArray : false});
fs.readFile(__dirname + '/variables.xml', function(err, data) {
parser.parseString(data, function (err, result) {
    userXML = result.variables.user;
    passwordXML = result.variables.password;
    serverXML = result.variables.server;
    databaseXML = result.variables.database;

    //What's in this variables? The right values!
    console.log(userXML);
    console.log(passwordXML);
    console.log(serverXML);
    console.log(databaseXML);

})
});

    exports.dev = {
            db: {
            //What's in these variables? The wrong values! Values from top of file - never updated
                user: userXML,
                password: passwordXML,
                server: "myservername.net",
                database: databaseXML       
            }
        };

I tried adding a .then() promise after the readFile but then my other file is saying that "dev" is undefined.

By the time you've loaded your file, you've already exported your object. If you put all of your async code into promises and chain them properly, at the end of all of that is where you want to do your export, etc. And you don't even need promises - just the final callback. — Tad Donaghe
– Tad Donaghe, Commented Aug 17, 2016 at 22:22

dvlsg · Accepted Answer · 2016-08-18 16:11:13Z

1

Loading configuration files is probably the best use-case for blocking file I/O in node (IMHO). Just use fs.readFileSync() instead.

Otherwise, if you must keep async file reading (which I doubt is the case, when you're trying to load your configuration values for startup), don't try exporting the values directly. Instead, export a function which will provide those values to either a callback or a promise, then use that function instead. You can add some sort of caching mechanism if you need these config values in multiple locations and you want to make sure the file is only read once.

Truth be told (and I'm not sure if this is an option for you), the simplest solution would be ditch the idea of a configuration file altogether, and then to provide the configuration you are looking for (user, server, password, etc) with environment variables, and then reading them from process.env.

edited Aug 18, 2016 at 16:11

answered Aug 18, 2016 at 0:00

dvlsg

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1 Comment

Andrew K. Over a year ago

That (readFileSync) worked thanks! I agree, there are better ways to do this but I don't have the option to reconfigure. Maybe someday

Wake · Accepted Answer · 2016-08-17 23:51:49Z

0

You just need to move your code that accesses the newly assigned variables into the your callback:

//set some variables
userXML = "bgdf";
passwordXML = "";
serverXML = "";
databaseXML = "";

//parse the xml
var fs = require('fs'),
xml2js = require('xml2js');
var parser = new xml2js.Parser({explicitArray : false});
fs.readFile(__dirname + '/variables.xml', function(err, data) {
    parser.parseString(data, function (err, result) {
            userXML = result.variables.user;
            passwordXML = result.variables.password;
            serverXML = result.variables.server;
            databaseXML = result.variables.database;

            //What's in this variables? The right values!
            console.log(userXML);
            console.log(passwordXML);
            console.log(serverXML);
            console.log(databaseXML);
            exports.dev = {
                db: {
                    //What's in these variables? The wrong values! Values from top of file - never updated
                    user: userXML,
                    password: passwordXML,
                    server: "myservername.net",
                    database: databaseXML       
                }
            };
    })
});

answered Aug 17, 2016 at 23:51

Wake

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1 Comment

Andrew K. Over a year ago

This doesn't work because "db" becomes undefined, the scope is lost

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