How to scope data in express / node controller

Question

I'm trying to avoid callback hell by breaking down my Express / Kraken.js controller into smaller callback functions.

I was processing a request and had about 6 levels of nested anonymous callback functions.

so now I have my main function that looks like this:

// ugh, I know this isn't right
var globalProducts = {};

module.exports = function (server) {

    server.get('/project', function (req, res) {
        var data = req.query;
        globalData = data; 

        if(!data.projectId || !data.ruleSetId) 
            res.json({error: "Incomplete input data."});

             // pass response to products call back 
        Project.findOne({ _id: mongoose.Types.ObjectId(data.projectId) }, setUpProducts);   
    });
};

function setUpProducts(err, project){
    // get all products and pass them down the pipe
    project.findAllChildren(setUpRules);
}

function setUpRules(err, products) {
    // we need to access products in another function 
    globalProducts = products; 
    // find the rule set and build the rule Flow
    RuleSet.findOne({ _id: mongoose.Types.ObjectId(globalData.ruleSetId) }, function(err, ruleSet) { 
        ruleSet.buildFlow(processFlow);
    });
}

My question is what is the best way to pass around info between callbacks ? My solution was var globalProducts = {}; but to me, the controller contain any 'global state' .. whats the best way to handle this ?

vkurchatkin · Accepted Answer · 2013-12-20 19:11:25Z

3

Doing this is a bad idea. It will cause race condition type issue — basically it's the same as sharing data in multithreaded environment. Instead you can use req or res to store data. To do that you need them in scope, so you can define all functions inside route handler or make each function a middleware so it will have req and res as arguments. Here is an example of this approach:

function check (req, res, next) {
  if(!req.query.projectId || !req.query.ruleSetId) return res.json({error: "Incomplete input data."});  
  next()
}

function findProject (req, res, next) {
  Project.findOne({ _id: mongoose.Types.ObjectId(req.query.projectId) }, after);  

  function after (err, project) {
    if (err) return next(err);
    req.project = project;
    next();
  }
}

function findProducts (req, res, next) {
   req.project.findAllChildren(after)

  function after (err, products) {
    if (err) return next(err);
    req.products = products;
    next();
  }
}

function respond (req, res) {
  res.render('view', {
    products : req.products,
    project : req.project
  });
}


module.exports = function (server) {
   server.get('/project', check, findProject, findProducts, respond);
};

answered Dec 20, 2013 at 19:11

vkurchatkin

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2 Comments

bonez Over a year ago

FYI. I'm attaching 'globals' to res.locals .. ie. res.locals.products according to expressjs.com/api.html#res.locals

vkurchatkin Over a year ago

It's also a good way to go if you a going to use it in the view

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