index.js

import { normalizeHttpResponse } from '@middy/util'

const defaults = {
  disableBeforePreflightResponse: true,
  getOrigin: undefined, // default inserted below
  credentials: undefined,
  headers: undefined,
  methods: undefined,
  origin: undefined,
  origins: [],
  exposeHeaders: undefined,
  maxAge: undefined,
  cacheControl: undefined,
  vary: undefined
}

const httpCorsMiddleware = (opts = {}) => {
  const getOrigin = (incomingOrigin, options = {}) => {
    if (options.origins.length > 0) {
      if (originStatic[incomingOrigin]) {
        return incomingOrigin
      }
      if (originAny) {
        if (options.credentials) {
          return incomingOrigin
        } else {
          return '*'
        }
      }
      if (originDynamic.some((regExp) => regExp.test(incomingOrigin))) {
        return incomingOrigin
      }
      // TODO deprecate `else` in v6
    } else {
      if (incomingOrigin && options.credentials && options.origin === '*') {
        return incomingOrigin
      }
      return options.origin
    }
    return null
  }
  const options = {
    ...defaults,
    getOrigin,
    ...opts
  }

  let originAny = false
  let originMany = options.origins.length > 1
  const originStatic = {}
  const originDynamic = []

  for (const origin of [options.origin, ...options.origins]) {
    if (!origin) {
      continue
    }
    // All
    if (origin === '*') {
      originAny = true
      continue
    }
    // Static
    if (!origin.includes('*')) {
      originStatic[origin] = true
      continue
    }
    originMany = true
    // Dynamic
    // TODO: IDN -> puncycode not handled, add in if requested
    const regExpStr = origin.replaceAll('.', '\\.').replaceAll('*', '[^.]*')
    originDynamic.push(new RegExp(`^${regExpStr}$`))
  }

  const modifyHeaders = (headers, options, request) => {
    const existingHeaders = Object.keys(headers)
    if (existingHeaders.includes('Access-Control-Allow-Credentials')) {
      options.credentials =
        headers['Access-Control-Allow-Credentials'] === 'true'
    }
    if (options.credentials) {
      headers['Access-Control-Allow-Credentials'] = String(options.credentials)
    }
    if (
      options.headers &&
      !existingHeaders.includes('Access-Control-Allow-Headers')
    ) {
      headers['Access-Control-Allow-Headers'] = options.headers
    }
    if (
      options.methods &&
      !existingHeaders.includes('Access-Control-Allow-Methods')
    ) {
      headers['Access-Control-Allow-Methods'] = options.methods
    }

    let newOrigin
    if (!existingHeaders.includes('Access-Control-Allow-Origin')) {
      const eventHeaders = request.event.headers ?? {}
      const incomingOrigin = eventHeaders.Origin ?? eventHeaders.origin
      newOrigin = options.getOrigin(incomingOrigin, options)
      if (newOrigin) {
        headers['Access-Control-Allow-Origin'] = newOrigin
      }
    }

    if (!headers.Vary) {
      addHeaderPart(headers, 'Vary', options.vary)
    }

    if (
      originMany ||
      (originAny && newOrigin !== '*') ||
      (newOrigin === '*' && options.credentials)
    ) {
      addHeaderPart(headers, 'Vary', 'Origin')
    }

    if (
      options.exposeHeaders &&
      !existingHeaders.includes('Access-Control-Expose-Headers')
    ) {
      headers['Access-Control-Expose-Headers'] = options.exposeHeaders
    }
    if (options.maxAge && !existingHeaders.includes('Access-Control-Max-Age')) {
      headers['Access-Control-Max-Age'] = String(options.maxAge)
    }
    const httpMethod = getVersionHttpMethod[request.event.version ?? '1.0']?.(
      request.event
    )
    if (
      httpMethod === 'OPTIONS' &&
      options.cacheControl &&
      !existingHeaders.includes('Cache-Control')
    ) {
      headers['Cache-Control'] = options.cacheControl
    }
  }

  const httpCorsMiddlewareBefore = async (request) => {
    if (options.disableBeforePreflightResponse) return

    const method = getVersionHttpMethod[request.event.version ?? '1.0']?.(
      request.event
    )
    if (method === 'OPTIONS') {
      normalizeHttpResponse(request)
      const headers = {}
      modifyHeaders(headers, options, request)
      request.response.headers = headers
      request.response.statusCode = 204
      return request.response
    }
  }

  const httpCorsMiddlewareAfter = async (request) => {
    normalizeHttpResponse(request)
    const headers = structuredClone(request.response.headers)
    modifyHeaders(headers, options, request)
    request.response.headers = headers
  }
  const httpCorsMiddlewareOnError = async (request) => {
    if (request.response === undefined) return
    await httpCorsMiddlewareAfter(request)
  }
  return {
    before: httpCorsMiddlewareBefore,
    after: httpCorsMiddlewareAfter,
    onError: httpCorsMiddlewareOnError
  }
}
const getVersionHttpMethod = {
  '1.0': (event) => event.httpMethod,
  '2.0': (event) => event.requestContext.http.method
}

// header in offical name, lowercase varient handeled
const addHeaderPart = (headers, header, value) => {
  if (!value) return
  const headerLower = header.toLowerCase()
  header = headers[headerLower] ? headerLower : header
  headers[header] ??= ''
  headers[header] &&= headers[header] + ', '
  headers[header] += value
}

export default httpCorsMiddleware