Apex + Terraform + AWS Lambda + API Gateway + JSON Encoded Errors + CORS

api.tf

resource "aws_api_gateway_rest_api" "myApi" {
  name = "myApi-${var.env}"
  description = "My awesome API (${var.env} environment)"
}

resource "aws_api_gateway_deployment" "myApi" {
  depends_on = [
    "aws_api_gateway_integration.myApi_myEndpoint_post",
    "aws_api_gateway_integration_response.myApi_myEndpoint_post",
    "aws_api_gateway_integration_response.myApi_myEndpoint_post_400",
    "aws_api_gateway_method_response.myApi_myEndpoint_post_200",
    "aws_api_gateway_method_response.myApi_myEndpoint_post_400",
    "aws_api_gateway_integration.myApi_myEndpoint_options"
  ]
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  stage_name = "${var.env}"
}

resource "aws_api_gateway_resource" "myApi_myEndpoint" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  parent_id = "${aws_api_gateway_rest_api.myApi.root_resource_id}"
  path_part = "myEndpoint"
}

resource "aws_api_gateway_method" "myApi_myEndpoint_post" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "POST"
  authorization = "NONE"
}

resource "aws_api_gateway_integration" "myApi_myEndpoint_post" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_post.http_method}"
  type = "AWS"
  integration_http_method = "POST" # Must be POST for invoking Lambda function
  credentials = "${var.gateway_lambda_role_arn}"
  # http://docs.aws.amazon.com/apigateway/api-reference/resource/integration/#uri
  uri = "arn:aws:apigateway:${var.aws_region}:lambda:path/2015-03-31/functions/${var.apex_function_myEndpoint}/invocations"
}

resource "aws_api_gateway_integration_response" "myApi_myEndpoint_post" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_post.http_method}"
  status_code = "${aws_api_gateway_method_response.myApi_myEndpoint_post_200.status_code}"
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Origin": "'*'"
}
PARAMS
}

resource "aws_api_gateway_integration_response" "myApi_myEndpoint_post_400" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_post.http_method}"
  status_code = "${aws_api_gateway_method_response.myApi_myEndpoint_post_400.status_code}"
  selection_pattern = ".*message.*"
  response_templates = {
    "application/json" = <<EOT
#set ($errorMessageObj = $util.parseJson($input.path('$.errorMessage')))
{
  "message" : "$errorMessageObj.message"
}
EOT
  }
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Origin": "'*'"
}
PARAMS
}

resource "aws_api_gateway_method_response" "myApi_myEndpoint_post_200" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_post.http_method}"
  status_code = "200"
  response_models = {
    "application/json" = "Empty"
  }
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Origin": true
}
PARAMS
}

resource "aws_api_gateway_method_response" "myApi_myEndpoint_post_400" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_post.http_method}"
  status_code = "400"
  response_models = {
    "application/json" = "Empty"
  }
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Origin": true
}
PARAMS
}


# CORS

resource "aws_api_gateway_method" "myApi_myEndpoint_options" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "OPTIONS"
  authorization = "NONE"
}

resource "aws_api_gateway_integration" "myApi_myEndpoint_options" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_options.http_method}"
  type = "MOCK"
  request_templates = {
    "application/json" = <<EOT
{"statusCode": 200}
EOT
  }
}

resource "aws_api_gateway_integration_response" "myApi_myEndpoint_options_200" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_options.http_method}"
  status_code = "${aws_api_gateway_method_response.myApi_myEndpoint_options_200.status_code}"
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Headers": "'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token'",
    "method.response.header.Access-Control-Allow-Methods": "'POST,OPTIONS'",
    "method.response.header.Access-Control-Allow-Origin": "'*'"
}
PARAMS
  response_templates = {
      "application/json" = <<EOT
{}
EOT
    }
}

resource "aws_api_gateway_method_response" "myApi_myEndpoint_options_200" {
  rest_api_id = "${aws_api_gateway_rest_api.myApi.id}"
  resource_id = "${aws_api_gateway_resource.myApi_myEndpoint.id}"
  http_method = "${aws_api_gateway_method.myApi_myEndpoint_options.http_method}"
  status_code = "200"
  response_models = {
    "application/json" = "Empty"
  }
  response_parameters_in_json = <<PARAMS
{
    "method.response.header.Access-Control-Allow-Headers": true,
    "method.response.header.Access-Control-Allow-Methods": true,
    "method.response.header.Access-Control-Allow-Origin": true
}
PARAMS
}

lambda.js

// Forked from https://github.com/apex/node-apex

import R from 'ramda';

function serializeError(error) {
  if (!R.is(Object, error)) {
    return error;
  }
  const copy = R.merge({}, error);
  if (typeof error.message === 'string') {
    copy.message = error.message;
  }
  return copy;
}

function wrapCb(cb) {
  return function(error, result) {
    if (!R.isNil(error)) {
      error = JSON.stringify(serializeError(error));
    }
    return cb(error, result);
  }
}

export default function λ(fn) {
  return function(e, ctx, cb) {
    cb = wrapCb(cb);
    try {
      var v = fn(e, ctx, cb);
      if (v && typeof v.then == 'function') {
        v.then(function (val) {
          cb(null, val);
        }).catch(cb);
        return;
      }

      cb(null, v);
    } catch (err) {
      cb(err);
    }
  }
};