koa-lite/docs/faq.md

# Frequently Asked Questions

## 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.

## Does Koa replace Connect?

  No, just a different take on similar functionality
  now that generators allow us to write code with less
  callbacks. Connect is equally capable, and some may still prefer it,
  it's up to what you prefer.

## Do generators decrease performance?

  Barely - check out the benchmarks in our readme, the numbers
  are more than fine, and there's no substitute for proper
  horizontal scaling.

## Does Koa include routing?

  No - out of the box Koa has no form of routing, however
  many routing middleware are available: https://github.com/koajs/koa/wiki

## 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.

## What custom properties do the Koa objects have?

  Koa uses its own custom objects: `this`, `this.request`, and `this.response`.
  These objects abstract node's `req` and `res` objects with convenience methods and getters/setters.
  Generally, properties added to these objects must obey the following rules:

  - They must be either very commonly used and/or must do something useful
  - If a property exists as a setter, then it will also exist as a getter, but not vice versa

  Many of `this.request` and `this.response`'s properties are delegated to `this`.
  If it's a getter/setter, then both the getter and the setter will strictly
  correspond to either `this.request` or `this.response`.

  Please think about these rules before suggesting additional properties.