Content Negotiation

Content negotiation is the mechanism that is used for serving different representations of a resource from the same URL. The content negotiation is done using the Accept HTTP header and in this guide you will learn how content negotiation works in AdonisJS.

When to use content negotiation?

Content negotiation is helpful, when you (the webservice provider) decides to support different representations of a single resources. For example: Creating an API sever, that can respond both with XML and JSON.

The client sets the appropriate Accept header and server uses its value to respond with the correct response type. In AdonisJS, you can use the request.accepts method to find the best possible response type for a given HTTP request.

import { HttpContextContract } from '@ioc:Adonis/Core/HttpContext'

export default class PostsController {
  public async index ({ request, view }: HttpContextContract) {
    const posts = [{
      title: 'Adonis 101',

    switch (request.accepts(['html', 'json'])) {      case 'html':        return view.render('posts/index', { posts })      case 'json':        return posts      default:        return view.render('posts/index', { posts })    }  }

Content Negotiation in Exception Handler

The base exception handler of AdonisJS also uses content negotiation for returning errors in the correct format.

  • If HTML: Render status pages or return error as an HTML string.
  • If JSON: Render error as an error object
  • If JSONAPI: Render error in the format as per the JSONAPI spec.

Content Negotiation in Validation Exception

The AdonisJS validator also use content negotiation for returning validation error messages in the correct format.

  • If HTML: Use session flash messages for passing errors to the form and redirect the request.
  • If JSON: Return errors as an array of objects.
  • If JSONAPI: Returns errors as per the JSONAPI spec.