javascript - How does this callback call work in Node.js with mongo .find call -

models/category.js

var mongoose = require('mongoose');  // category schema var categoryschema = mongoose.schema({   title: {     type: string   },   description: {     type: string   },   created_at: {     type: date,     default: date.now   } });  var category = module.exports = mongoose.model('category', categoryschema);  // categories module.exports.getcategories = function(callback, limit) {   category.find(callback).limit(limit).sort([['title', 'ascending']]); } 

routes/categories.js

var express = require('express'); var router = express.router();  category = require('../models/category.js');  router.get('/', function(req, res, next) {   category.getcategories(function(err, categories) {     if (err) res.send(err);     res.render('categories',        {          title: 'categories',         categories: categories       });   }); });  router.post('/add', function(req,res) {   res.send('form submitted'); });  module.exports = router; 

i got few questions code

a) how callback mechanism work routes/categories.js when pass callback function models/category.js in category.find(callback). seems bizarre me since doing whole res.render becomes part of category.find() ?

b) limit specified?

c) why isn't there var in front of category = require('../models/category.js');

a) indeed happens, , good: res.render not called until find() operation executes on database , result sent mongoose code return you. want run callback function after result query, , calling res.render before more bizarre.

b) in documentation. http://mongoosejs.com/docs/api.html#model_model.find yields query object, may synchronously (i.e. before query made resolve @ database) further specified where, limit, etc.

c) because got lazy. in example doesn't make difference because without var (or const or let in modern js) variable declaration tacked onto local context, in file routes/categories.js module context, , because categories declared @ top of scope, var doesn't change variable ends being bound. it's lazy, , example code, should have var in front of it.