What are the differences between Koa and Express 4.0?
Koa and Express 4.0 are both fairly new, and from what I've read, Koa was made by the Express team.
From what I understand, Koa requires features of node that are only available in 0.11 (the unstable branch) of node, and also uses generators. Express 4.0 seems to only be the next version of the Express framework.
Are there any differences I am missing completely? Is it likely (based on what the Express team has publicly stated) that Koa and Express will merge at some point in the future?
Thanks!
Answer
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Koa vs Express
Philosophically, Koa aims to "fix and replace node", whereas Express "augments node". Koa uses co to rid apps of callback hell and simplify error handling. It exposes its own this.request and this.response objects instead of node's req and res objects.
Express, on the other hand, augments node's req and res objects with additional properties and methods and includes many other "framework" features, such as routing and templating, which Koa does not.
Thus, Koa can be viewed as an abstraction of node.js's http modules, where as Express is an application framework for node.js.
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Does Koa replace Express?
It's more like Connect, but a lot of the Express goodies were moved to the middleware level in Koa to help form a stronger foundation. This makes middleware more enjoyable and less error-prone to write, for the entire stack, not just the end application code.
Typically many middleware would re-implement similar features, or even worse incorrectly implement them, when features like signed cookie secrets among others are typically application-specific, not middleware specific.
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Why isn't Koa just Express 4.0?
Koa is a pretty large departure from what people know about Express, the design is fundamentally much different, so the migration from Express 3.0 to this Express 4.0 would effectively mean rewriting the entire application, so we thought it would be more appropriate to create a new library.