AWS-SDK for js exposed as a REST service

index.js

/*
 * Requires Lambda Proxy Integration
 *
 * Example usage
 *
 * curl -s https://SomeGatewayID.execute-api.us-east-1.amazonaws.com/test/aws/DynamoDB.DocumentClient/default/query -X PUT -H 'X-api-key:TheKey' -d '{"TableName":"SomeTable","KeyConditionExpression":"id = :id","ExpressionAttributeValues":{":id":"SomeID"}}' -H 'Content-Type: application/json' | jq .
 * curl -s https://SomeGatewayID.execute-api.us-east-1.amazonaws.com/test/aws/SimpleDB/2009-04-15/listDomains -X PUT -H 'X-api-key:TheKey' | jq .
 */
const API_KEY = '' + process.env['API_KEY']

const AWS = require('aws-sdk')

const MESSAGES = {
  ENV_API_KEY_MISSING: 'Internal Error: env API_KEY is missing',
  CLI_API_KEY_MISSING: 'Bad Request: X-Api-Key header is MANDATORY',
  CLI_API_KEY_INVALID: 'Please UPDATE your client X-Api-Key',
  SRV_INVALID_SERVICE_METHOD: 'Invalid AWS service method',
  SRV_INVALID_SERVICE: 'Invalid AWS service',
  SRV_INSTANTIATION_ERROR: 'Error creating instance',
  SRV_UNEXPECTED_ERROR: 'Unexpected Error'
}

function _r (statusCode, headers, body) {
  return {
    statusCode: '' + statusCode,
    headers: headers,
    body: JSON.stringify(body)
  }
}

exports.handler = (event, context, callback) => {
  if (!API_KEY) {
    callback(null, _r(500, null, MESSAGES.ENV_API_KEY_MISSING))
  }

  let headers = /** @type {Object.<string,?>} */ event.headers
  let params = /** @type {Object.<string,?>} */ event.pathParameters
  let body = /** @type {Object.<string,?>} */ event.body

  /* Note that X-api-key etc will get automatically
    lowercased by execution environment
  */
  let clientApiKey = headers['x-api-key']
  if (!clientApiKey) {
    callback(null, _r(400, null, MESSAGES.CLI_API_KEY_MISSING))
  } else {
    if (API_KEY === clientApiKey) {
      let version = params['version']
      let service = params['service']
      let method = params['method']
      let svcParts = service.split('.')
      try {
        let NS = AWS
        while (svcParts.length) {
          let part = svcParts.shift()
          NS = NS[part]
          if (!NS) {
            let errMsg = { message: MESSAGES.SRV_INVALID_SERVICE + `: [${part}]` }
            callback(null, _r(400, null, errMsg))
            return
          }
        }
        let options = version === 'default' ? undefined : { apiVersion: version }
        let svc
        svc = new NS(options)
        if (typeof svc[method] !== 'function') {
          throw new Error(`method ${method} is undefined on ${service}`)
        }
        svc[method](JSON.parse(body)).promise()
        .then(function (response) {
          let httpResp = response.$response.httpResponse
          callback(null, _r(httpResp.statusCode, null, {
            statusCode: httpResp.statusCode,
            body: response,
            headers: httpResp.headers
          }))
        })
        .catch(function (awsErr) {
          let errMsg = { message: awsErr.message, code: awsErr.code, errors: awsErr.errors }
          callback(null, _r(awsErr.statusCode || 500, null, errMsg))
        })
      } catch (err) {
        let errMsg = {message: MESSAGES.SRV_UNEXPECTED_ERROR + ': ' + err.message}
        callback(null, _r(500, null, errMsg))
      }
    } else {
      let errMsg = {message: MESSAGES.CLI_API_KEY_INVALID}
      callback(null, _r(401, null, errMsg))
    }
  }
}