diff --git a/spec/internal-spec.js b/spec-main/internal-spec.ts
similarity index 75%
rename from spec/internal-spec.js
rename to spec-main/internal-spec.ts
index 823d2f8afcff..320f241789b3 100644
--- a/spec/internal-spec.js
+++ b/spec-main/internal-spec.ts
@@ -1,12 +1,11 @@
-const chai = require('chai')
-const { expect } = chai
+import { expect } from 'chai'
 
 describe('feature-string parsing', () => {
   it('is indifferent to whitespace around keys and values', () => {
     const parseFeaturesString = require('../lib/common/parse-features-string')
-    const checkParse = (string, parsed) => {
-      const features = {}
-      parseFeaturesString(string, (k, v) => { features[k] = v })
+    const checkParse = (string: string, parsed: Record<string, string | boolean>) => {
+      const features: Record<string, string | boolean> = {}
+      parseFeaturesString(string, (k: string, v: any) => { features[k] = v })
       expect(features).to.deep.equal(parsed)
     }
     checkParse('a=yes,c=d', { a: true, c: 'd' })