@middy/input-output-logger/index.js

Input and output logger middleware for the middy framework

import { Transform } from 'node:stream'

const defaults = {
  logger: (message) => {
    console.log(JSON.stringify(message))
  },
  awsContext: false,
  omitPaths: [],
  mask: undefined
}

const inputOutputLoggerMiddleware = (opts = {}) => {
  const { logger, awsContext, omitPaths, mask } = {
    ...defaults,
    ...opts
  }

  if (typeof logger !== 'function') {
    throw new Error('logger must be a function', {
      cause: {
        package: '@middy/input-output-logger'
      }
    })
  }

  const omitPathTree = buildPathTree(omitPaths)
  // needs `omitPathTree`, `logger`
  const omitAndLog = (param, request) => {
    const message = { [param]: request[param] }

    if (awsContext) {
      message.context = pick(request.context, awsContextKeys)
    }

    let cloneMessage = message
    if (omitPaths.length) {
      cloneMessage = structuredClone(message) // Full clone to prevent nested mutations
      omit(cloneMessage, { [param]: omitPathTree[param] })
    }
    logger(cloneMessage)
  }

  // needs `mask`
  const omit = (obj, pathTree = {}) => {
    if (Array.isArray(obj) && pathTree['[]']) {
      for (let i = 0, l = obj.length; i < l; i++) {
        omit(obj[i], pathTree['[]'])
      }
    } else if (isObject(obj)) {
      for (const key in pathTree) {
        if (pathTree[key] === true) {
          if (mask) {
            obj[key] = mask
          } else {
            delete obj[key]
          }
        } else {
          omit(obj[key], pathTree[key])
        }
      }
    }
  }

  const inputOutputLoggerMiddlewareBefore = async (request) => {
    omitAndLog('event', request)
  }
  const inputOutputLoggerMiddlewareAfter = async (request) => {
    if (
      request.response?._readableState ??
      request.response?.body?._readableState
    ) {
      passThrough(request, omitAndLog)
    } else {
      omitAndLog('response', request)
    }
  }
  const inputOutputLoggerMiddlewareOnError = async (request) => {
    if (request.response === undefined) return
    await inputOutputLoggerMiddlewareAfter(request)
  }

  return {
    before: inputOutputLoggerMiddlewareBefore,
    after: inputOutputLoggerMiddlewareAfter,
    onError: inputOutputLoggerMiddlewareOnError
  }
}

// https://docs.aws.amazon.com/lambda/latest/dg/nodejs-context.html
const awsContextKeys = [
  'functionName',
  'functionVersion',
  'invokedFunctionArn',
  'memoryLimitInMB',
  'awsRequestId',
  'logGroupName',
  'logStreamName',
  'identity',
  'clientContext',
  'callbackWaitsForEmptyEventLoop'
]

// move to util, if ever used elsewhere
const pick = (originalObject = {}, keysToPick = []) => {
  const newObject = {}
  for (const path of keysToPick) {
    // only supports first level
    if (originalObject[path] !== undefined) {
      newObject[path] = originalObject[path]
    }
  }
  return newObject
}

const isObject = (value) =>
  value && typeof value === 'object' && value.constructor === Object

const buildPathTree = (paths) => {
  const tree = {}
  for (let path of paths.sort().reverse()) {
    // reverse to ensure conflicting paths don't cause issues
    if (!Array.isArray(path)) path = path.split('.')
    if (path.includes('__proto__')) continue
    path
      .slice(0) // clone
      .reduce((a, b, idx) => {
        if (idx < path.length - 1) {
          a[b] ??= {}
          return a[b]
        }
        a[b] = true
        return true
      }, tree)
  }
  return tree
}

const passThrough = (request, omitAndLog) => {
  // required because `core` remove body before `flush` is triggered
  const hasBody = request.response?.body
  let body = ''
  const listen = new Transform({
    objectMode: false,
    transform (chunk, encoding, callback) {
      body += chunk
      this.push(chunk, encoding)
      callback()
    },
    flush (callback) {
      if (hasBody) {
        omitAndLog('response', { response: { ...request.response, body } })
      } else {
        omitAndLog('response', { response: body })
      }
      callback()
    }
  })
  if (hasBody) {
    request.response.body = request.response.body.pipe(listen)
  } else {
    request.response = request.response.pipe(listen)
  }
}

export default inputOutputLoggerMiddleware