CODEWITHSUNDEEP
javascriptnode.jsmongodbexpressmongoose
Brad Ross
Brad Ross

Reputation: 461

Node.js Making a Variable Wait to be Assigned Until Callback Function is Done

I am using Node.js, Express, MongoDB, and Mongoose. I have a function that fetches the largest id number of a document in my MongoDB database and returns it to the program. I have begun modularizing my code, and have migrated that function to another module. I have successfully accessed the function in my main module, but it involves an asynchronous database query. As the function returns a value, I want to assign it to a variable. Unfortunately, When the returned value is assigned to the variable, the variable is actually set to undefined. I was thinking about using event emitters to signal that the query is finished, but that presents two issues as well:

1) I don't think you can do anything in a program AFTER a return statement, which would be what is required.

2) Event Emitters between modules seem very finicky.

Please help me get the variable to be assigned to the correct value. Code for both the main function and the module is below:

(main file) app.js:

//requires and start up app
var express = require('express');
var mongoose = require('mongoose')
  , dbURI = 'localhost/test';
var app = express();
var postmodel = require('./models/post').postmodel;

//configures app for general stuff needed such as bodyParser and static file directory
app.configure(function () {
    app.use(express.bodyParser());
    app.use(express.static(__dirname + '/static'));
});

//configures app for production, connects to mongoLab databse rather than localhost
app.configure('production', function () {
    dbURI = 'mongodb://brad.ross.35:[email protected]:37387/heroku_app6901832';
});

//tries to connect to database.
mongoose.connect(dbURI);
//once connection to database is open, then rest of app runs
mongoose.connection.on('open', function () {
    var PostModel = new postmodel();
    var Post = PostModel.setupPostSchema();
    var largest_id = PostModel.findLargestID(Post);

(module) post.js:

var mongoose = require('mongoose');

module.exports.postmodel = function () {
    this.setupPostSchema = function () {
        var postSchema = new mongoose.Schema({
                title: String,
                body: String,
                id: Number,
                date_created: String
            });

        var Post = mongoose.model('Post', postSchema);
        return Post;
    };

    this.findLargestID = function (Post) {
        Post.find(function (err, posts) {
            if (err) {
                console.log("error finding largest ID!");
            } else {
                var largest_id = 0;
                for (var post in posts) {
                    if (posts[post].id >= largest_id) largest_id = posts[post].id;
                }
                console.log(largest_id);
                return largest_id;
            }
        });
    };
};

Upvotes: 2

Views: 1736

Answers (1)

JohnnyHK
JohnnyHK

Reputation: 311895

You need to have findLargestID accept a callback parameter that it will call once largest_id is available:

this.findLargestID = function (Post, callback) {
    Post.find(function (err, posts) {
        if (err) {
            console.log("error finding largest ID!");
            callback(err);
        } else {
            var largest_id = 0;
            for (var post in posts) {
                if (posts[post].id >= largest_id) largest_id = posts[post].id;
            }
            console.log(largest_id);
            callback(null, largest_id);
        }
    });
};

Upvotes: 3

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