Both a request and a response could have various content headers. Some of these are:

• Content-Type
• Content-Length
• Content-Encoding

Among many others. We're particularly interested in type and length. Koa has getters/setters for type and length:

• this.request.type
• this.request.length
• this.response.type
• this.response.length

Inferring this.request.type is a little difficult. For example, how do you know if the request is text? You don't want to use a regular expression or try all the possible mime types. Thus, Koa has this.request.is() for you:

this.request.is('image/*') // => image/png
this.request.is('text') // => text or false

Learn more about request.is().

Exercise

Create an app that checks the Content-Type of the request. If it's application/json, return {message: 'hi!'} with appropriate content headers. Otherwise, return ok as a string.