When comparing Iris vs Koa, the Slant community recommends Koa for most people. In the question“What are the best backend web frameworks?” Koa is ranked 33rd while Iris is ranked 56th. The most important reason people chose Koa is:
Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
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Pros
Pro No need to find and install external libraries
Iris is a "batteries included" web framework, which means that a lot of features already come out of the box. This way you don't have to spend time and find third-party libraries to integrate to the framework for most of the tasks you need to complete.
Pro Efficiency
Efficiency by using a light engine.
Pro Development
Pro API
If you're familiar with expressjs or other modern web frameworks, Iris will make you feel at home.
Pro More than feature complete
Pro More than 250 examples
More than 250 examples for implementing web application.
Pro Low amount of bugs
Most issues are questions, few bug reports and if a bug is found it's quickly patched.
Pro Good for big projects
Iris is a good framework for big projects and the author helps solving problems.
Pro Generator support from ground up
Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
Pro Extremely lightweight
Koa is very lightweight with just 550 lines of code.
Pro async/await keywords are supported and has transcended beyond generator functions
Generators functions are of course a huge plus, but at the time Koa team has transcended generations functions and shifted towards async/await style programming. It has made the Koa best framework available in the market.
Pro Development team has a proven track record
Koa is developed by the team behind a widely used node.js framework (express.js).
Pro Built for ES6
Cons
Con Previously experience with net/http
Previously experience with the Go's standard net/http package is necessary for you to move forward with Iris.
Con Author copies code
Con Community is relatively small
Con Not compatible with express style middleware
Koa uses generators which are not compatible with any other type of Node.js framework middleware.