Koa vs Express: How Is Koa Middleware Different from Express Middleware?

Koa vs Express: How Is Koa Middleware Different from Express Middleware?

2 of the best Node.js web frameworks, each one with its own philosophy, load of clear benefits and set of drawbacks. So, how would the results of a Koa vs Express comparison look like?

How are they different when it comes to the middleware implementation approach?

What are the obvious benefits of using Koa over Express? And what project requirements would make Express a better candidate?

Let's dig into the 2 piles of framework-specific features, advantages, limitations, and use cases and dig out the arguments to base your final choice on:

1. Koa.js: Expressive, Minimalist and Modular, Ditching the “Callback Hell”

The “mission” that Koa's been assigned with?

To fix and replace Node. 

Take it as anabstraction of Node.js's http modules”.

And here I'm sure you're thinking what everyone's thinking when it comes to Node.js' major drawback: “the callback hell”.

Using promises and async functions, Koa.js ditches the callback hell. And it simplifies error handling...

It comes as a modular, minimalistic, highly expressive and more modern version of Express.js. A lighter version, yet robust enough to power your APIs and web app development projects.

And by “modern” I do refer to:

  • the async/await paradigm
  • the generator functions
  • developers being empowered to build out the middleware that best suits their use cases

And speaking of “building out your own middleware”, stay assured: 

Even though there's no Koa middleware in core, it still provides a suite for a fast and simple server-side rendering.

Also, despite its smaller size, it can still extend its functionality to perfectly fit your needs by adding plugins and custom-built middleware.

Now, let's sum up the main Koa vs Express differences by putting Koa into the spotlight:

  • no (more) callback hell thanks to its async/await features
  • it does not have a middleware
  • it's modular (you're free to include only what you need): component-based building blocks
  • it doesn't provide many of the usual convenience utilities (e.g. sending files)
  • it doesn't have built-in routing or helmet of security, yet it provides corresponding third-party libraries 
  • better error handling (try/catch)
  • it's less dependent on the middleware: for instance, you'd use a body parsing function instead of the corresponding middleware
  • it's lighter
  • it has a more minimalist and manageable code footprint

Main drawbacks: 

  • being newer than Express.js, its community is still significantly smaller
  • it's incompatible with the Express-style middleware
  • its generators are not compatible with Node.js's middleware

2. Express.js: The Standard, Feature-Rich Framework for Node.js 

Compared to Koa's “fix and replace node” philosophy, Express is built to “augment node”. It's an application framework of Node.js itself.

An unopinionated web framework providing you with all the standard “framework”  features for building your web/mobile apps and their APIs.

And here I refer to:

  • templating
  • routing

… and all the other usual features that minimalist Koa doesn't include.

In short: take Express.js as your reliable tooling for HTTP servers; a go-to full-featured solution for building single page applications, public HTTP APIs, websites, hybrids...

And now, summing up the Koa vs Express differences by highlighting the latter's key features:

  • it's single threaded
  • it speeds up the app development cycle
  • it's easy to learn (and backed up by a thriving, large open-source community)
  • event loop
  • I/O request handling/non-blocking I/O

Main Drawbacks:

  • its event-driven architecture
  • its code organization (not as easily manageable as Koa's)

3. How Is Koa Middleware Different from Express Middleware?

A predictable and legitimate “dilemma” considering that:

  • working with middleware is one of the key concepts that Node.js developers “juggle with”
  • the middleware mechanism is also one of the fundamental Koa vs Express differences

“What are middleware functions?”

Functions that intermediate requests and responses.

Koa Middleware 

Koa “surprises” you, so to say, with the option of cascading middleware.

A pattern which makes implementing the whole middleware flow a lot more intuitive:

  • the “logger” middleware putting the execution of its code “on hold”
  • the x-response-time
  • the “response”
  • the “x-response-time”
  • the “logger”

With this cascading pattern of the middleware, “next()” returns the result of the next middleware in the flow.

Basically, there are 2 ways that the Koa middlewares can interact:

It's not necessary for the last middleware in the flow to send a response. Instead, the previous middleware in the stack could be assigned to send a response.

With no “res.send()” paradigm in the Koa middleware, the response could be sent after the middleware stack execution itself. 

Koa leverages the async/await paradigm instead of the request/response objects, specific to Express.js:

Complete middleware 1... await middleware 2 from inside middleware 1... complete middleware 2 and so on.

In other words, you can delegate other middlewares in the flow to send it. And you won't get the usual Express message: “headers were already sent.” 

Express Middleware 

Express.js uses event-driven callbacks, which means that its middlewares flow could get roughly reduced to this “formula”:

Middleware 1 has completed, then middleware 2 has completed and so on.

You have the standard “request and response” structure.

Meaning that middlewares in Express.js interact in one way only: the last triggered middleware in the flow is “assigned” with sending a request to the next middleware.

Basically, in order to register a middleware in Express.js, all you need to do is bind it to the app object triggering the “app.use()” function.

4. Koa vs Express: When to Use One Over the Other

Drawing the line now, here are some of the Koa-specific and the Express-specific use cases and scenarios.

Use Koa if:

  • its modularity — the freedom to include only what you need, to use an external middleware or to build your own — appeals to you 
  • you “dread” callbacks
  • high performance is on top of your project requirements list
  • it's a large-sized app project that you have in mind; its minimalist, lighter and therefore more manageable code footprint is more suitable for such scenarios
  • you're in for a more futuristic approach, based on the use of generators, the async/await paradigm and the freedom to build your own middleware

Use Express if:

  • you're looking for a “plug and play” framework and you don't want to get tangled up in plugins and third-party libraries
  • you want to stick to the familiar Node.js-style coding
  • it's a small/medium-sized app that you're building
  • you depend on the support of a large, thriving open-source community

In short: in a Koa vs Express “debate”, the winner depends exclusively on your specific project requirements and on your expectations in terms of flexibility:

Would you prefer a light, minimalist Node.js web framework that grants you more freedom (which usually involves a bit more work, as well)? Or a more “conventional” one that provides you with all the needed robust features right out of the box?