What are the differences between Koa and Express 4.0?
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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 callbackhell and simplify error handling. It exposes its own this.request andthis.response objects instead of node's req and res objects.
Express, on the other hand, augments node's req and res objects withadditional 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 tothe middleware level in Koa to help form a stronger foundation. Thismakes middleware more enjoyable and less error-prone to write, for theentire stack, not just the end application code.
Typically many middleware would re-implement similar features, or evenworse incorrectly implement them, when features like signed cookiesecrets among others are typically application-specific, notmiddleware 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 fromExpress 3.0 to this Express 4.0 would effectively mean rewriting theentire application, so we thought it would be more appropriate tocreate a new library.
- Koa does not provides functionalities like Routing, Templating, Sending files and JSONP while the express does.
- koa exposes its own ctx.request and ctx.response objects instead of node's req and res objects i.e. functions of the form (req, res, next), are incompatible with Koa.
- Koa can be viewed as an abstraction of node.js's http modules, where as Express is an application framework for node.js.
For more detailed answer you can visit official documents on this link :https://github.com/koajs/koa/blob/master/docs/koa-vs-express.md