Plug.ErrorHandler: how to use to return JSON errors?

Please bear in mind that all what I am about to suggest is from the top of my mind, therefore not tested, and may be wrong, but hey it doesn’t hurt for you to give it a try

  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    send_resp(conn, conn.status, "Something went wrong")
  end

If I am not mistake the above will override the call to the Error view at lib/yourapp_web/views/error_view.ex.

I think that this will return it in json:

# You may need to use the Jason library if returning only the map doesn't work.
# You also want to put the `application/json` header in the `conn`
send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})

Maybe you can simplify the above to use in your router:

# file: lib/yourapp_web/router.ex
  pipeline :api do
    plug :accepts, ["json"],
    plug :handle_errors
  end

  # you may need to pattern match in `conn.status` to make this work
  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    # You may need to use the Jason library if returning only the map doesn't work.
    # You also want to put the `application/json` header in the `conn`
    send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})
  end

EDIT: On a second though I think this isn’t reflecting the intended use from the Plug.ErrorHandler

An Alternative that you can try is to change the Error view to always return a JSON object:

defmodule YourAppWeb.ErrorView do
  use YourAppWeb, :view

  require Logger

  # If you want to customize a particular status code
  # for a certain format, you may uncomment below.
  def render(template, assigns) do
    _json_error(template, assigns)
  end

  # By default, Phoenix returns the status message from
  # the template name. For example, "404.html" becomes
  # "Not Found".
  def template_not_found(template, assigns) do
    _json_error(template, assigns)
  end

  defp _json_error(template, assigns) do

    Logger.info("Error template: #{template}")
    # render("500.html", assigns)
    
    # I am not sure, but I think you can just return a Map, otherwise just try to use the Jason library.
    %{
      error: 400, # You need to extract it from assigns or template name
      maessage: "Whoops!"
    }

  end
  
end

I still don’t understand where to put this copy-pasted module code in my Phoenix project, and how do I register it to be used – in the router, the endpoint, somewhere else?

The docs kind of assume certain knowledge that I found I don’t have.

I update my previous post while you made this one to make it clear, but here it his again:

# file: lib/yourapp_web/router.ex
  pipeline :api do
    plug :accepts, ["json"],
    plug :handle_errors
  end

  # you may need to pattern match in `conn.status` to make this work
  def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    # You may need to use the Jason library if returning only the map doesn't work.
    # You also want to put the `application/json` header in the `conn`
    send_resp(conn, conn.status,%{error: conn.status, message:  "Something went wrong"})
  end

This is a guess from me, not actually tried it.

EDIT: On a second though I think this isn’t reflecting the intended use from the Plug.ErrorHandler

Nope, that didn’t work.

Hitting a non-existent endpoint with curl:

HTTP/1.1 404 Not Found
cache-control: max-age=0, private, must-revalidate
content-length: 1333
content-type: text/markdown; charset=utf-8
date: Mon, 12 Apr 2021 16:33:02 GMT
server: Cowboy

# Phoenix.Router.NoRouteError at POST /does_not_exist

Exception:

    ** (Phoenix.Router.NoRouteError) no route found for POST /does_not_exist (MyAppWeb.Router)
# <snip>

And it’s returning markdown text? Why?

I am still trying to understand how do I just intercept ALL errors and wrap them in keys in a resulting JSON object.

Sorry for not having given you a good solution

But did you try the Error view approach? I think that one should work

Because you are in development mode. Go to config/dev.exs and set debug_errors to false.

No idea what to do exactly there either.

It’s been an absolutely awful day and I’d love complete code examples. At least today. If not, I’ll try formulating a solution on a fresh head tomorrow.

I showed it to you in my first post:

This file can be found in your app web views folder.

Basically ti’s overriding the render/2 function called to render the error templates for each error status code that is defined in the templates folder.

It also customizes the template_not_found/2 to return the Json error.