File and folder structure for Node-Express applications

Alberto Basalo
6 min readOct 23, 2023

I’ve seen as many code organization systems as there are applications. Everyone does it in their own way. Some are clearly over-complicated, others too simplistic to scale as development demands. And with their differences, they also have things in common.

Express, don’t let the trees prevent you from seeing the forest

Without An Official Style Guide for Express, I will compile some good practices and tips for creating file and folder structures for Node-Express applications. Many of the ideas and advice are heavily inspired by Angular, which does have an official guide, and its LIFT principles, which is what I’m going to start with:

Locate: Group coherently

Identify: Name to indicate the content

Flat: Create subfolders if necessary

Try-DRY: Some redundancy can be beneficial

File naming convention.

Single Responsibility

By following good coding practices and clean architecture, you’ll likely have many classes or functional modules dedicated to very specific things. The idea is that each of those things ends up in a file. And that the file’s name describes what that thing does in the best possible way.

Indeed, this code responds to some business requirements (for example, reservations, payments…) or system requirements (for example, log, security…). Therefore, the file must contain, in fact, it should begin with, the name of that functionality.

Following any software architecture, concepts associated with it will soon appear, such as controller, service, repository… Or more general concepts such as utilities, class, model, etc…

Well, this is also important, so we already have the first rule:

The file name combines the functional and technical domains of a single element:

functional-name.technological-type.js|ts

Group in folders

Easy peasy if you only have a dozen of files. However, this is not the case for real-world solutions. We will face hundreds or maybe thousands of small files. How do we keep them under control?

You may expect me to enter the famous folder tree dichotomy: group by function or type. For sure, this is a legitimate question based on the two dimensions that we applied to name our files.

Let’s try to find a solution.

A universal base agreement

As I said, there is a pattern that, with slight variations, I find in almost all the Node-Express projects that I review. The basic functionality of Express is to build a web server, and its main contribution is the concept of middleware to process requests and issue responses.

On the other hand, sooner or later, you start to find common things you intend to reuse. And the need arises to have a mixed bag to store those things that don’t belong to anyone but that everyone wants.

So, it’s natural that a few folders appear for these features. And these are my proposals for top-level folders.

The first level folders should reflect that this is an Express server:

/api

/middleware

/shared

Flat or deep, a matter of size.

Now we know how to name the files and the three folders to save them. It’s time to determine whether this is enough or if we need more subfolders. From here on, there is no longer a fixed rule but rather something that depends on the number of files (size of the application) and the architectural style applied.

Complying with LIFT’s Flat principle, I will keep the folder structure as little nested as possible; some advice may arise from there.

Rule 5–15:

A folder with less than 5 elements does not need to be subdivided

A folder with more than 15 elements has to be subdivided

Folders between 6 and 14 items are divided if an obvious criterion arises

Let’s apply this tip to the main folders.

The API is a navigation tree.

Following the REST methodology, we will find names of entities or resources (activities, reservations, users, payments, credentials…). Following a layered architecture, technological concepts appear (router, controller, service, and repository). The first gives us functional names, and the second gives us technical types, and with that, we can name files as we have seen previously, also complying with the LIFT principle of Identification.

If, at this point, your application does not contain 15 files… stop reading this article and dedicate yourself to more productive things or things that make you happier. No one worries about how to organize a dozen things.

But the normal case is that you surpass it widely. So you will consider how to subdivide the API folder, and here, now, the famous question appears: divide by functionality or divide by type.

Both have their advantages. Grouping by type allows you to establish at root the three or four necessary folders (controllers, services…, etc.) that will be filled with the functionalities as they arise (reservations, users…).

But, my recommendation, and the most used, is that you choose a functional grouping based on the names of the resources exposed in the API. In this way, two advantages are achieved:

  • Keep together the things that usually change together (functional changes affecting the router and the service…).
  • Reflect in the folder system what the application does and not so much the technical details (Screaming architecture)

So, our rule for the API would look like this:

The API folder must reflect the route tree with the first-level end-points grouping the files by their business functionality.

Middlewares are plain

A key success of Express is its ability to dynamically add functionality to the requests it processes. It is a brilliant application of the Chain of Responsibility behavior design pattern. These intermediate functions are known as middlewares, and you can create your own or adapt those offered by the framework itself or the community.

Following the principle of single responsibility, I allocate a file to each functional need that the middleware resolves. In most applications, I don’t see more than a dozen of them, and I prefer to leave them in a flat structure. Only when there are really many do I decide to classify them into two or three folders that scream their original purpose: log security validation

The middleware folder should be kept flat until the need arises due to obvious quantity and grouping criteria.

The Shared catch-all

Following clean code principles, we should keep our code DRY. That is, avoid duplication by sharing code. Many of these shared functions end up in middleware, but others are completely unrelated to the Express request-response processing pattern. They are simple utility functions, data model definitions, or the crown jewel of this drawer: database access.

Since this folder is prone to growing, and it is good that it does, sooner rather than later, it will far exceed the limit of 15 items and up to 10 times more. Your common sense and knowledge of the project will help you to categorize it appropriately. I can only remind you of the general principles:

The shared folder grows more than one level of nesting. Try to comply with the principles that we have proposed:

Between 5 and 15 entries per folder.

The lowest level of nesting possible to facilitate localization.

Clear names that facilitate identification.

Summary

Here, you have an example with all those tips applied.

Folders/files naming and structure for a node-Express App

Sample code :

  • LIFT principles: Locate, Identify, Flat, and Try-DRY. These principles help to organize the code coherently, clearly, and concisely.
  • File naming convention: Use a combination of functional and technical domains to name the files, such as `functional-name.technological-type.js|ts`
  • First-level folders: Three main folders to reflect the basic functionality of Express: /api /middleware /shared
  • Flat over deep structure: Keep the folder structure as flat as possible, and only create subfolders when there are more than 15 files or when there is an obvious criterion.

If you are an Angular developer, I have you covered with the Angular version of this guide:

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Alberto Basalo

Advisor and instructor for developers. Keeping on top of modern technologies while applying proven patterns learned over the last 25 years. Angular, Node, Tests