diff --git a/packages/gatsby/src/commands/build-javascript.js b/packages/gatsby/src/commands/build-javascript.ts
similarity index 59%
rename from packages/gatsby/src/commands/build-javascript.js
rename to packages/gatsby/src/commands/build-javascript.ts
index 885ad0c0736f5..21bf971741ff4 100644
--- a/packages/gatsby/src/commands/build-javascript.js
+++ b/packages/gatsby/src/commands/build-javascript.ts
@@ -1,10 +1,14 @@
-/* @flow */
-const webpack = require(`webpack`)
-const webpackConfig = require(`../utils/webpack.config`)
+import webpack from "webpack"
+import webpackConfig from "../utils/webpack.config"
+import { IProgram } from "./types"
+import { Span } from "opentracing"
 
-const { reportWebpackWarnings } = require(`../utils/webpack-error-utils`)
+import { reportWebpackWarnings } from "../utils/webpack-error-utils"
 
-module.exports = async (program, { parentSpan }) => {
+export const buildProductionBundle = async (
+  program: IProgram,
+  parentSpan: Span
+): Promise<webpack.Stats> => {
   const { directory } = program
 
   const compilerConfig = await webpackConfig(
diff --git a/packages/gatsby/src/commands/build.js b/packages/gatsby/src/commands/build.js
index aa39e7dd43f81..1ce3e58c3f677 100644
--- a/packages/gatsby/src/commands/build.js
+++ b/packages/gatsby/src/commands/build.js
@@ -4,7 +4,7 @@ const path = require(`path`)
 const report = require(`gatsby-cli/lib/reporter`)
 const fs = require(`fs-extra`)
 import { buildHTML } from "./build-html"
-const buildProductionBundle = require(`./build-javascript`)
+import { buildProductionBundle } from "./build-javascript"
 const bootstrap = require(`../bootstrap`)
 const apiRunnerNode = require(`../utils/api-runner-node`)
 const { copyStaticDirs } = require(`../utils/get-static-dir`)
@@ -111,11 +111,11 @@ module.exports = async function build(program: BuildArgs) {
     { parentSpan: buildSpan }
   )
   activity.start()
-  const stats = await buildProductionBundle(program, {
-    parentSpan: activity.span,
-  }).catch(err => {
-    activity.panic(structureWebpackErrors(`build-javascript`, err))
-  })
+  const stats = await buildProductionBundle(program, activity.span).catch(
+    err => {
+      activity.panic(structureWebpackErrors(`build-javascript`, err))
+    }
+  )
   activity.end()
 
   const workerPool = WorkerPool.create()