diff --git a/packages/gatsby-cli/src/structured-errors/__tests__/error-schema.ts b/packages/gatsby-cli/src/structured-errors/__tests__/error-schema.ts
index d49787a1dc45a..a01e785f294b5 100644
--- a/packages/gatsby-cli/src/structured-errors/__tests__/error-schema.ts
+++ b/packages/gatsby-cli/src/structured-errors/__tests__/error-schema.ts
@@ -1,12 +1,12 @@
-import schema from "../error-schema"
+import { errorSchema } from "../error-schema"
 
 test(`throws invalid on an invalid error`, () => {
-  expect(schema.validate({ lol: `true` })).rejects.toBeDefined()
+  expect(errorSchema.validate({ lol: `true` })).rejects.toBeDefined()
 })
 
 test(`does not throw on a valid schema`, () => {
   expect(
-    schema.validate({
+    errorSchema.validate({
       context: {},
     })
   ).resolves.toEqual(expect.any(Object))
diff --git a/packages/gatsby-cli/src/structured-errors/construct-error.ts b/packages/gatsby-cli/src/structured-errors/construct-error.ts
index 20f1db865b959..a53249b47c5fc 100644
--- a/packages/gatsby-cli/src/structured-errors/construct-error.ts
+++ b/packages/gatsby-cli/src/structured-errors/construct-error.ts
@@ -1,44 +1,8 @@
-import Joi from "@hapi/joi"
 import stackTrace from "stack-trace"
-import errorSchema from "./error-schema"
-import { errorMap, defaultError, IErrorMapEntry, ErrorId } from "./error-map"
+import { errorSchema } from "./error-schema"
+import { errorMap, defaultError, IErrorMapEntry } from "./error-map"
 import { sanitizeStructuredStackTrace } from "../reporter/errors"
-
-interface IConstructError {
-  details: {
-    id?: ErrorId
-    context?: Record<string, string>
-    error?: Error
-    [key: string]: unknown
-  }
-}
-
-interface ILocationPosition {
-  line: number
-  column: number
-}
-
-interface IStructuredError {
-  code?: string
-  text: string
-  stack: {
-    fileName: string
-    functionName?: string
-    lineNumber?: number
-    columnNumber?: number
-  }[]
-  filePath?: string
-  location?: {
-    start: ILocationPosition
-    end?: ILocationPosition
-  }
-  error?: unknown
-  group?: string
-  level: IErrorMapEntry["level"]
-  type?: IErrorMapEntry["type"]
-  docsUrl?: string
-}
-
+import { IConstructError, IStructuredError } from "./types"
 // Merge partial error details with information from the errorMap
 // Validate the constructed object against an error schema
 const constructError = ({
@@ -63,7 +27,7 @@ const constructError = ({
   }
 
   // validate
-  const { error } = Joi.validate(structuredError, errorSchema)
+  const { error } = errorSchema.validate(structuredError)
   if (error !== null) {
     console.log(`Failed to validate error`, error)
     process.exit(1)
diff --git a/packages/gatsby-cli/src/structured-errors/error-map.ts b/packages/gatsby-cli/src/structured-errors/error-map.ts
index bc6c917ee7555..a3a56cc07a934 100644
--- a/packages/gatsby-cli/src/structured-errors/error-map.ts
+++ b/packages/gatsby-cli/src/structured-errors/error-map.ts
@@ -1,25 +1,5 @@
 import { stripIndent, stripIndents } from "common-tags"
-
-interface IOptionalGraphQLInfoContext {
-  codeFrame?: string
-  filePath?: string
-  urlPath?: string
-  plugin?: string
-}
-
-enum Level {
-  ERROR = `ERROR`,
-  WARNING = `WARNING`,
-  INFO = `INFO`,
-  DEBUG = `DEBUG`,
-}
-
-enum Type {
-  GRAPHQL = `GRAPHQL`,
-  CONFIG = `CONFIG`,
-  WEBPACK = `WEBPACK`,
-  PLUGIN = `PLUGIN`,
-}
+import { IOptionalGraphQLInfoContext, Level, Type } from "./types"
 
 const optionalGraphQLInfo = (context: IOptionalGraphQLInfoContext): string =>
   `${context.codeFrame ? `\n\n${context.codeFrame}` : ``}${
diff --git a/packages/gatsby-cli/src/structured-errors/error-schema.ts b/packages/gatsby-cli/src/structured-errors/error-schema.ts
index a5de534971e91..ec14cfd6f65be 100644
--- a/packages/gatsby-cli/src/structured-errors/error-schema.ts
+++ b/packages/gatsby-cli/src/structured-errors/error-schema.ts
@@ -1,38 +1,39 @@
 import Joi from "@hapi/joi"
+import { ILocationPosition, IStructuredError } from "./types"
 
-const Position = Joi.object().keys({
+export const Position: Joi.ObjectSchema<ILocationPosition> = Joi.object().keys({
   line: Joi.number(),
   column: Joi.number(),
 })
 
-const errorSchema = Joi.object().keys({
-  code: Joi.string(),
-  text: Joi.string(),
-  stack: Joi.array()
-    .items(
-      Joi.object().keys({
-        fileName: Joi.string(),
-        functionName: Joi.string().allow(null),
-        lineNumber: Joi.number().allow(null),
-        columnNumber: Joi.number().allow(null),
-      })
-    )
-    .allow(null),
-  level: Joi.string().valid([`ERROR`, `WARNING`, `INFO`, `DEBUG`]),
-  type: Joi.string().valid([`GRAPHQL`, `CONFIG`, `WEBPACK`, `PLUGIN`]),
-  filePath: Joi.string(),
-  location: Joi.object({
-    start: Position.required(),
-    end: Position,
-  }),
-  docsUrl: Joi.string().uri({
-    allowRelative: false,
-    relativeOnly: false,
-  }),
-  error: Joi.object({}).unknown(),
-  context: Joi.object({}).unknown(),
-  group: Joi.string(),
-  panicOnBuild: Joi.boolean(),
-})
-
-export default errorSchema
+export const errorSchema: Joi.ObjectSchema<IStructuredError> = Joi.object().keys(
+  {
+    code: Joi.string(),
+    text: Joi.string(),
+    stack: Joi.array()
+      .items(
+        Joi.object().keys({
+          fileName: Joi.string(),
+          functionName: Joi.string().allow(null),
+          lineNumber: Joi.number().allow(null),
+          columnNumber: Joi.number().allow(null),
+        })
+      )
+      .allow(null),
+    level: Joi.string().valid([`ERROR`, `WARNING`, `INFO`, `DEBUG`]),
+    type: Joi.string().valid([`GRAPHQL`, `CONFIG`, `WEBPACK`, `PLUGIN`]),
+    filePath: Joi.string(),
+    location: Joi.object({
+      start: Position.required(),
+      end: Position,
+    }),
+    docsUrl: Joi.string().uri({
+      allowRelative: false,
+      relativeOnly: false,
+    }),
+    error: Joi.object({}).unknown(),
+    context: Joi.object({}).unknown(),
+    group: Joi.string(),
+    panicOnBuild: Joi.boolean(),
+  }
+)
diff --git a/packages/gatsby-cli/src/structured-errors/types.ts b/packages/gatsby-cli/src/structured-errors/types.ts
new file mode 100644
index 0000000000000..428620550433d
--- /dev/null
+++ b/packages/gatsby-cli/src/structured-errors/types.ts
@@ -0,0 +1,57 @@
+import { IErrorMapEntry, ErrorId } from "./error-map"
+
+export interface IConstructError {
+  details: {
+    id?: ErrorId
+    context?: Record<string, string>
+    error?: Error
+    [key: string]: unknown
+  }
+}
+
+export interface ILocationPosition {
+  line: number
+  column: number
+}
+
+export interface IStructuredError {
+  code?: string
+  text: string
+  stack: {
+    fileName: string
+    functionName?: string
+    lineNumber?: number
+    columnNumber?: number
+  }[]
+  filePath?: string
+  location?: {
+    start: ILocationPosition
+    end?: ILocationPosition
+  }
+  error?: unknown
+  group?: string
+  level: IErrorMapEntry["level"]
+  type?: IErrorMapEntry["type"]
+  docsUrl?: string
+}
+
+export interface IOptionalGraphQLInfoContext {
+  codeFrame?: string
+  filePath?: string
+  urlPath?: string
+  plugin?: string
+}
+
+export enum Level {
+  ERROR = `ERROR`,
+  WARNING = `WARNING`,
+  INFO = `INFO`,
+  DEBUG = `DEBUG`,
+}
+
+export enum Type {
+  GRAPHQL = `GRAPHQL`,
+  CONFIG = `CONFIG`,
+  WEBPACK = `WEBPACK`,
+  PLUGIN = `PLUGIN`,
+}
diff --git a/packages/gatsby/index.d.ts b/packages/gatsby/index.d.ts
index ffcdbdde7478b..020f0c8fa9a50 100644
--- a/packages/gatsby/index.d.ts
+++ b/packages/gatsby/index.d.ts
@@ -176,6 +176,8 @@ export interface GatsbyConfig {
   >
   /** It’s common for sites to be hosted somewhere other than the root of their domain. Say we have a Gatsby site at `example.com/blog/`. In this case, we would need a prefix (`/blog`) added to all paths on the site. */
   pathPrefix?: string
+  /** In some circumstances you may want to deploy assets (non-HTML resources such as JavaScript, CSS, etc.) to a separate domain. `assetPrefix` allows you to use Gatsby with assets hosted from a separate domain */
+  assetPrefix?: string
   /** Gatsby uses the ES6 Promise API. Because some browsers don't support this, Gatsby includes a Promise polyfill by default. If you'd like to provide your own Promise polyfill, you can set `polyfill` to false.*/
   polyfill?: boolean
   mapping?: Record<string, string>
diff --git a/packages/gatsby/package.json b/packages/gatsby/package.json
index 5f7df108d504e..28365f1f23c85 100644
--- a/packages/gatsby/package.json
+++ b/packages/gatsby/package.json
@@ -150,6 +150,7 @@
   "devDependencies": {
     "@babel/cli": "^7.8.4",
     "@babel/runtime": "^7.8.7",
+    "@types/hapi__joi": "^16.0.12",
     "@types/socket.io": "^2.1.4",
     "babel-preset-gatsby-package": "^0.3.1",
     "cross-env": "^5.2.1",
diff --git a/packages/gatsby/src/joi-schemas/__tests__/__snapshots__/joi.js.snap b/packages/gatsby/src/joi-schemas/__tests__/__snapshots__/joi.ts.snap
similarity index 100%
rename from packages/gatsby/src/joi-schemas/__tests__/__snapshots__/joi.js.snap
rename to packages/gatsby/src/joi-schemas/__tests__/__snapshots__/joi.ts.snap
diff --git a/packages/gatsby/src/joi-schemas/__tests__/joi.js b/packages/gatsby/src/joi-schemas/__tests__/joi.ts
similarity index 92%
rename from packages/gatsby/src/joi-schemas/__tests__/joi.js
rename to packages/gatsby/src/joi-schemas/__tests__/joi.ts
index c4588d6502f67..a2480b7c1e009 100644
--- a/packages/gatsby/src/joi-schemas/__tests__/joi.js
+++ b/packages/gatsby/src/joi-schemas/__tests__/joi.ts
@@ -1,4 +1,4 @@
-const { gatsbyConfigSchema, nodeSchema } = require(`../joi`)
+import { gatsbyConfigSchema, nodeSchema } from "../joi"
 
 describe(`gatsby config`, () => {
   it(`returns empty pathPrefix when not set`, async () => {
@@ -133,7 +133,8 @@ describe(`node schema`, () => {
 
     const { error } = nodeSchema.validate(node)
     expect(error).toBeTruthy()
-    expect(error.message).toMatchSnapshot()
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    expect(error!.message).toMatchSnapshot()
   })
 
   it(`doesn't allow unknown internal fields`, async () => {
@@ -152,6 +153,7 @@ describe(`node schema`, () => {
 
     const { error } = nodeSchema.validate(node)
     expect(error).toBeTruthy()
-    expect(error.message).toMatchSnapshot()
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    expect(error!.message).toMatchSnapshot()
   })
 })
diff --git a/packages/gatsby/src/joi-schemas/joi.js b/packages/gatsby/src/joi-schemas/joi.ts
similarity index 83%
rename from packages/gatsby/src/joi-schemas/joi.js
rename to packages/gatsby/src/joi-schemas/joi.ts
index d211184251ee8..9f626f19dfb42 100644
--- a/packages/gatsby/src/joi-schemas/joi.js
+++ b/packages/gatsby/src/joi-schemas/joi.ts
@@ -1,13 +1,16 @@
-const Joi = require(`@hapi/joi`)
+import Joi from "@hapi/joi"
+import { IGatsbyConfig, IGatsbyPage, IGatsbyNode } from "../redux/types"
+
+const stripTrailingSlash = (chain: Joi.StringSchema): Joi.StringSchema =>
+  chain.replace(/(\w)\/+$/, `$1`)
 
-const stripTrailingSlash = chain => chain.replace(/(\w)\/+$/, `$1`)
 // only add leading slash on relative urls
-const addLeadingSlash = chain =>
+const addLeadingSlash = (chain: Joi.StringSchema): Joi.StringSchema =>
   chain.when(Joi.string().uri({ relativeOnly: true }), {
     then: chain.replace(/^([^/])/, `/$1`),
   })
 
-export const gatsbyConfigSchema = Joi.object()
+export const gatsbyConfigSchema: Joi.ObjectSchema<IGatsbyConfig> = Joi.object()
   .keys({
     __experimentalThemes: Joi.array(),
     polyfill: Joi.boolean().default(true),
@@ -73,7 +76,7 @@ export const gatsbyConfigSchema = Joi.object()
     }
   )
 
-export const pageSchema = Joi.object()
+export const pageSchema: Joi.ObjectSchema<IGatsbyPage> = Joi.object()
   .keys({
     path: Joi.string().required(),
     matchPath: Joi.string(),
@@ -85,7 +88,7 @@ export const pageSchema = Joi.object()
   })
   .unknown()
 
-export const nodeSchema = Joi.object()
+export const nodeSchema: Joi.ObjectSchema<IGatsbyNode> = Joi.object()
   .keys({
     id: Joi.string().required(),
     children: Joi.array().items(Joi.string(), Joi.object().forbidden()),
diff --git a/packages/gatsby/src/redux/actions/public.js b/packages/gatsby/src/redux/actions/public.js
index 5a6db72d66266..ec5061d2170bb 100644
--- a/packages/gatsby/src/redux/actions/public.js
+++ b/packages/gatsby/src/redux/actions/public.js
@@ -14,7 +14,7 @@ const { hasNodeChanged, getNode } = require(`../../db/nodes`)
 const sanitizeNode = require(`../../db/sanitize-node`)
 const { store } = require(`..`)
 const fileExistsSync = require(`fs-exists-cached`).sync
-const joiSchemas = require(`../../joi-schemas/joi`)
+import { nodeSchema } from "../../joi-schemas/joi"
 const { generateComponentChunkName } = require(`../../utils/js-chunk-names`)
 const {
   getCommonDir,
@@ -744,7 +744,7 @@ const createNode = (
 
   trackCli(`CREATE_NODE`, trackParams, { debounce: true })
 
-  const result = Joi.validate(node, joiSchemas.nodeSchema)
+  const result = Joi.validate(node, nodeSchema)
   if (result.error) {
     if (!hasErroredBecauseOfNodeValidation.has(result.error.message)) {
       const errorObj = {
diff --git a/yarn.lock b/yarn.lock
index 15423b1e043c8..2409b681cd75c 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3927,6 +3927,11 @@
     "@types/tough-cookie" "*"
     form-data "^2.5.0"
 
+"@types/hapi__joi@^16.0.12":
+  version "16.0.12"
+  resolved "https://registry.yarnpkg.com/@types/hapi__joi/-/hapi__joi-16.0.12.tgz#fb9113f17cf5764d6b3586ae9817d1606cc7c90c"
+  integrity sha512-xJYifuz59jXdWY5JMS15uvA3ycS3nQYOGqoIIE0+fwQ0qI3/4CxBc6RHsOTp6wk9M0NWEdpcTl02lOQOKMifbQ==
+
 "@types/history@*":
   version "4.7.0"
   resolved "https://registry.yarnpkg.com/@types/history/-/history-4.7.0.tgz#2fac51050c68f7d6f96c5aafc631132522f4aa3f"