elm-serverless-cors

This is CORS middleware for elm-serverless.

There are two ways to use it.

1. Set the headers that you need, individually.
2. Set the headers using externally provided configuration.

Method 1

Set the headers that you need, individually.

import Serverless.Conn.Request exposing (Method(..))
import Serverless.Cors exposing (..)
import Serverless.Plug exposing (pipeline, plug)


myPipeline =
    pipeline
        -- Sets access-control-allow-origin
        -- the same as the origin request header
        -- or if that is not found then "*"
        |>
            plug (allowOrigin ReflectRequest)
        -- Sets access-control-allow-methods
        |>
            plug (allowMethods [ GET, OPTIONS ])

Method 2

Set the headers using externally provided configuration (i.e. AWS Environment variables), and decode them into your app's config record. This method is demonstrated in elm-serverless-demo. In summary, you need to do the following.

First add CORS configuration to your Serverless app's Config type. And decode it with the provided Cors.configDecoder.

import Json.Decode exposing (Decoder, list, string)
import Json.Decode.Pipeline exposing (required, decode, hardcoded)
import Serverless.Cors as Cors


type alias Config =
    { cors : Cors.Config
    -- ...
    }


configDecoder : Json.Decode.Decoder Config
configDecoder =
    decode Config
        |> required "cors" Cors.configDecoder
        -- ...

Then you can access the config from your Conn and use it to configure CORS.

import Serverless.Conn exposing (config)
import Serverless.Plug exposing (pipeline, plug)
import Serverless.Cors exposing (cors)

myPipeline =
    pipeline
        |> plug (Cors.fromConfig .cors)

On the JavaScript side, you can do something like this to map AWS Lambda environment variables to a JavaScript object.

const elmServerless = require('elm-serverless');
const rc = require('strip-debug!shebang!rc');

const elm = require('./API.elm');

// Use AWS Lambda environment variables to override these values
// See the npm rc package README for more details
const config = rc('myApi', {
  cors: {
    origin: '*',
    methods: 'get,post,options',
  },
});

module.exports.handler = elmServerless.httpApi({
  handler: elm.API,
  config,
});

So in the AWS Lambda console, you can set environment variables, which will get loaded by rc into a JavaScript configuration object, and passed into elm for decoding. In this example, you could set

• myApi_cors__origin=foo.com,bar.com: decodes to Exactly ["foo.com", "bar.com"] for origin
• myApi_cors__credentials=true: decodes to True for credentials