diff --git a/packages/next/build/generate-build-id.ts b/packages/next/build/generate-build-id.ts
index 0f9de31c1ddb4..884ed5e062219 100644
--- a/packages/next/build/generate-build-id.ts
+++ b/packages/next/build/generate-build-id.ts
@@ -1,5 +1,5 @@
 export async function generateBuildId(
-  generate: () => string | null,
+  generate: () => string | null | Promise<string>,
   fallback: () => string
 ): Promise<string> {
   let buildId = await generate()
diff --git a/packages/next/next-server/server/config.ts b/packages/next/next-server/server/config.ts
index 9e0487e29c5ab..8980711d9d7a8 100644
--- a/packages/next/next-server/server/config.ts
+++ b/packages/next/next-server/server/config.ts
@@ -6,11 +6,73 @@ import { basename, extname } from 'path'
 import { CONFIG_FILE } from '../lib/constants'
 import { execOnce } from '../lib/utils'
 import * as Log from '../../build/output/log'
+import webpack from 'webpack'
 
 const targets = ['server', 'serverless', 'experimental-serverless-trace']
 const reactModes = ['legacy', 'blocking', 'concurrent']
 
-const defaultConfig: { [key: string]: any } = {
+type ExportPathMap = {
+  [page: string]: { page: string; query?: { [key: string]: string } }
+}
+
+type Webpack = (
+  config: webpack.Configuration,
+  {
+    dir,
+    dev,
+    isServer,
+    buildId,
+  }: { dir: string; dev: boolean; isServer: boolean; buildId: boolean }
+) => webpack.Configuration | Promise<webpack.Configuration>
+type WebpackDevMiddleware = (
+  config: webpack.Configuration
+) => webpack.Configuration
+
+export interface IIndexable {
+  [key: string]: any
+}
+
+interface Config extends IIndexable {
+  env: { [key: string]: any }
+  webpack: Webpack | null
+  webpackDevMiddleware: WebpackDevMiddleware | null
+  distDir: string
+  assetPrefix: string
+  configOrigin: string
+  useFileSystemPublicRoutes: boolean
+  generateBuildId: () => string | null | Promise<string>
+  generateEtags: boolean
+  pageExtensions: string[]
+  target: 'server' | 'serverless'
+  poweredByHeader: boolean
+  compress: boolean
+  devIndicators: {
+    buildActivity: boolean
+    autoPrerender: boolean
+  }
+  onDemandEntries: {
+    maxInactiveAge: number
+    pagesBufferLength: number
+  }
+  amp: {
+    canonicalBase: string
+  }
+  exportTrailingSlash: boolean
+  sassOptions: object
+  experimental: { [key: string]: any }
+  future: {
+    excludeDefaultMomentLocales: boolean
+  }
+  serverRuntimeConfig: object
+  publicRuntimeConfig: object
+  reactStrictMode: boolean
+  exportPathMap?: (defaultMap: ExportPathMap) => ExportPathMap
+  typescript?: {
+    ignoreBuildErrors: boolean
+  }
+}
+
+const defaultConfig: Config = {
   env: [],
   webpack: null,
   webpackDevMiddleware: null,
@@ -194,7 +256,7 @@ function assignDefaults(userConfig: { [key: string]: any }) {
   return result
 }
 
-export function normalizeConfig(phase: string, config: any) {
+export function normalizeConfig(phase: string, config: Config | any): Config {
   if (typeof config === 'function') {
     config = config(phase, { defaultConfig })
 
@@ -210,8 +272,8 @@ export function normalizeConfig(phase: string, config: any) {
 export default function loadConfig(
   phase: string,
   dir: string,
-  customConfig?: object | null
-) {
+  customConfig?: Config | null
+): Config {
   if (customConfig) {
     return assignDefaults({ configOrigin: 'server', ...customConfig })
   }
@@ -286,7 +348,7 @@ export default function loadConfig(
   return defaultConfig
 }
 
-export function isTargetLikeServerless(target: string) {
+export function isTargetLikeServerless(target: string): boolean {
   const isServerless = target === 'serverless'
   const isServerlessTrace = target === 'experimental-serverless-trace'
   return isServerless || isServerlessTrace