Mock function return in node.js test - node.js

Question

I'm new to testing in node.js and I would like to mock the return of a specific function call in a process that looks like the following.
doSomething(function(err, res){
callAnotherOne(res, function(err, result){
getDataFromDB(result, function(err, docs){
//some logic over the docs here
})
})
})
The function that I want to mock is the getDataFromDB() and specifically the documents (using MongoDB) that it returns.
How could I do something like this with mocha?
Part of the code, strip from the logic in between, is the following:
filterTweets(item, input, function(err, item) {
//Some filtering and logging here
db.getTwitterReplies(item, function(err, result) {
if(err) {
return callback('Failed to retrieve tweet replies');
}
//Do some work here on the item using the result (tweet replies)
/***** Here I want to test that the result is the expected ****/
db.storeTweets(item function (err, result){
//error checks, logging
callback();
});
});
});
Based on the amount of twitter replies (function call "getTwitterReplies"), I will modify my object accordingly (didn't include that code). I want to see if based on different replies result, my object is constructed as expected.
p.s. I also checked into sinon.js after some searching and I managed to mock the return of a callback (by writing some testing code outside my project) but not the return of a callback of a nested function call.

Answer 1

Here's how I would approach this category of problem:
First create a "config.js" that wraps the dependencies that you'd like to inject. This will become your container.
var db = {
doSomeDbWork : function(callback){
callback("db data");
}
};
module.exports = {
db: db
};
From there, you can call config dependencies like so:
var config = require('./index/config');
config.db.doSomeDbWork(function(data){
res.render('index', { title: 'Express' , data:data});
});
And in your tests, inject a mock/spy easily:
var config = require('../routes/index/config');
config.db = {
doSomeDbWork : function(callback){
callback("fake db data");
}
};
var indexRouter = require('../routes/index');
indexRouter.get('/');
Because the require call refers to the same config module exports, the changes made to the config in the spec will be reflected where ever they are imported via require()