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Pro Development team has a proven track record
Koa is developed by the team behind a widely used node.js framework (express.js).
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 Built for ES6
Con Has no routes separated by HTTP method or URL pattern
Con Not compatible with express style middleware
Koa uses generators which are not compatible with any other type of Node.js framework middleware.