Can anyone explain to me this JavaScript code that define some variables and loop through them?

Would it be more readable for you expressed thus: var products = [ { id: 1, name : 'Mac Book Pro', description: 'Apple 13 inch Mac Book Pro notebook', price: 1000 }, { id: 2, name : 'iPad', description: 'Apple 64GB 3G iPad', price: 899 } ]; module.exports.all = products; module.exports.find = function(id) { var i, product, nProducts = products.length; id = parseInt(id, 10); for(i = 0; i < nProducts; ++i) { product = products[i]; if (product.id === id) { return product; } } return null; }

First of all, it's awful code. It could break down at the slightest change. Let's start with the first line. var products = [ Declares a variable in a global context. Try to avoid this. Everything that happens in a global context, can break down. In addition, encapsulation of data helps to increase the modularity of the code, which in turn allows you to reuse code. Let's look at the function find(): module.exports.find = function(id) { id = parseInt(id, 10); var found = null; productloop: for(product_index in products) { var product = products[product_index]; if (product.id == id) { found = product; break productloop; } }; return found; } In JavaScript, blocks do not introduce a scope. There is only function-scope. A variable introduced anywhere in a function is visible everywhere in the function. JavaScript's blocks confuse experienced programmers and lead to errors because the familiar syntax makes a false promise. So this (var product = products[product_index];) declaration does not actually declare a new variable each iteration. Next. product.id == id The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors. In case of function find() operator == does not lead an error, but this is the place where the error may appear when you change the code. Always use === operator. for(product_index in products) The for in statement allows for looping through the names of all of the properties of an object. Unfortunately, it also loops through all of the properties that were inherited through the prototype chain. This has the bad side effect of serving up method functions when the interest is in data properties. If a program is written without awareness of this situation, then it can fail. The body of every for in statement should be wrapped in an if statement that does filtering. It can select for a particular type or range of values, or it can exclude functions, or it can exclude properties from the prototype. For example, for (name in object) { if (object.hasOwnProperty(name)) { .... } } In your code 'products' is an array, so you can be sure that it has no extra properties. But! Products is global variable, so in reality it can be anything. I'm a little rewrite the function and explain what this code does. module.exports.find = function(id) { var found = null, product; id = parseInt(id, 10); for (product_index in products) { if (products.hasOwnProperty(product_index)){ product = products[product_index]; if (product.id === id) { found = product; break; } } }; return found; } It's simple. The function takes a product ID as an argument (function(id)), turns it into a number (id = parseInt(id, 10);). Then runs through an array of products one by one with a loop (for(product_index in products)), checking the equality of passed to the function identifier and the identifier of the current product (if (product.id == id)). If they are equal, stops iterate (break;), assign the value of current product to the variable "found" (found = product;) and return the result to the caller of the function (return found;). P.S. I really hardly know English, so please forgive me for possible mistakes.

Products is an array containing two objects. Some people might call them "associative arrays" but they are just objects. They have 'properties' (like an id) and 'values': var products = [{ id: 1, name : 'Mac Book Pro', description: 'Apple 13 inch Mac Book Pro notebook', price: 1000 },{ id: 2, name : 'iPad', description: 'Apple 64GB 3G iPad', price: 899 }]; module.exports is the object that your node program will actually see when you do require('./products.js'): module.exports = { // just pass along the whole array all: products, // method to find a product by id find: function(id){ return products.filter(function(product){ return product.id == id })[0] // we return the first element, at index zero } } I replaced the for in loop with the https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/filter, should make things clearer here. It will loop over the products array and execute the function for each of them - if the result is true, the item is added to the result, otherwise it's ignored. This is what using this module should look like using Express: var products = require('./products.js') app.get('/products', function(req, res){ // get the whole products array products.all // render page... }) app.get('/products/:id', function(req, res){ // get product with id ':id' from URL (http://mysite.com/products/25) var p = products.find(req.params.id) // render page... })