index.ts

import { ProxyIntegrationConfig, ProxyIntegrationEvent } from './lib/proxyIntegration'
import { SnsConfig, SnsEvent } from './lib/sns'
import { SqsConfig, SqsEvent } from './lib/sqs'
import { S3Config, S3Event } from './lib/s3'
import { Context } from 'aws-lambda'
import { EventProcessor } from './lib/EventProcessor'

export interface RouteConfig {
  proxyIntegration?: ProxyIntegrationConfig
  sns?: SnsConfig
  sqs?: SqsConfig
  s3?: S3Config
  debug?: boolean
}

export type RouterEvent = ProxyIntegrationEvent | SnsEvent | SqsEvent | S3Event

export const handler = (routeConfig: RouteConfig) => {
  const eventProcessorMapping = extractEventProcessorMapping(routeConfig)

  return async <TContext extends Context> (event: RouterEvent, context: TContext) => {
    if (routeConfig.debug) {
      console.log('Lambda invoked with request:', event)
      console.log('Lambda invoked with context:', context)
    }

    for (const [eventProcessorName, eventProcessor] of eventProcessorMapping.entries()) {

      try {
        // the contract of 'processors' is as follows:
        // - their method 'process' is called with (config, event)
        // - the method...
        //   - returns null: the processor does not feel responsible for the event
        //   - throws Error: the 'error.toString()' is taken as the error message of processing the event
        //   - returns object: this is taken as the result of processing the event
        //   - returns promise: when the promise is resolved, this is taken as the result of processing the event
        const result = eventProcessor.process((routeConfig as any)[eventProcessorName], event, context)
        if (result) {
          // be resilient against a processor returning a value instead of a promise:
          return await result
        } else {
          if (routeConfig.debug) {
            console.log('Event processor couldn\'t handle request.')
          }
        }
      } catch (error) {
        if (error.stack) {
          console.log(error.stack)
        }
        throw error.toString()
      }
    }
    throw 'No event processor found to handle this kind of event!'
  }
}

const extractEventProcessorMapping = (routeConfig: RouteConfig) => {
  const processorMap = new Map<string, EventProcessor>()
  for (const key of Object.keys(routeConfig)) {
    if (key === 'debug') {
      continue
    }
    try {
      processorMap.set(key, require(`./lib/${key}`))
    } catch (error) {
      throw new Error(`The event processor '${key}', that is mentioned in the routerConfig, cannot be instantiated (${error.toString()})`)
    }
  }
  return processorMap
}