diff --git a/packages/babel-helper-create-class-features-plugin/src/decorators.js b/packages/babel-helper-create-class-features-plugin/src/decorators.ts
similarity index 89%
rename from packages/babel-helper-create-class-features-plugin/src/decorators.js
rename to packages/babel-helper-create-class-features-plugin/src/decorators.ts
index b5e90ad6e7..ae1f58f17e 100644
--- a/packages/babel-helper-create-class-features-plugin/src/decorators.js
+++ b/packages/babel-helper-create-class-features-plugin/src/decorators.ts
@@ -59,21 +59,15 @@ function extractElementDescriptor(/* this: File, */ classRef, superRef, path) {
     );
   }
 
-  new ReplaceSupers(
-    {
-      methodPath: path,
-      methodNode: node,
-      objectRef: classRef,
-      isStatic: node.static,
-      superRef,
-      scope,
-      file: this,
-      refToPreserve: classRef,
-    },
-    true,
-  ).replace();
+  new ReplaceSupers({
+    methodPath: path,
+    objectRef: classRef,
+    superRef,
+    file: this,
+    refToPreserve: classRef,
+  }).replace();
 
-  const properties = [
+  const properties: t.ObjectExpression["properties"] = [
     prop("kind", t.stringLiteral(isMethod ? node.kind : "field")),
     prop("decorators", takeDecorators(node)),
     prop("static", node.static && t.booleanLiteral(true)),
@@ -135,7 +129,7 @@ export function buildDecoratedClass(ref, path, elements, file) {
       .map(extractElementDescriptor.bind(file, node.id, superId)),
   );
 
-  let replacement = template.expression.ast`
+  const wrapperCall = template.expression.ast`
     ${addDecorateHelper(file)}(
       ${classDecorators || t.nullLiteral()},
       function (${initializeId}, ${superClass ? t.cloneNode(superId) : null}) {
@@ -144,17 +138,18 @@ export function buildDecoratedClass(ref, path, elements, file) {
       },
       ${superClass}
     )
-  `;
-  let classPathDesc = "arguments.1.body.body.0";
+  ` as t.CallExpression & { arguments: [unknown, t.FunctionExpression] };
 
   if (!isStrict) {
-    replacement.arguments[1].body.directives.push(
+    wrapperCall.arguments[1].body.directives.push(
       t.directive(t.directiveLiteral("use strict")),
     );
   }
 
+  let replacement: t.Node = wrapperCall;
+  let classPathDesc = "arguments.1.body.body.0";
   if (isDeclaration) {
-    replacement = template.ast`let ${ref} = ${replacement}`;
+    replacement = template.statement.ast`let ${ref} = ${wrapperCall}`;
     classPathDesc = "declarations.0.init." + classPathDesc;
   }
 
diff --git a/packages/babel-helper-create-class-features-plugin/src/features.js b/packages/babel-helper-create-class-features-plugin/src/features.ts
similarity index 100%
rename from packages/babel-helper-create-class-features-plugin/src/features.js
rename to packages/babel-helper-create-class-features-plugin/src/features.ts
diff --git a/packages/babel-helper-create-class-features-plugin/src/fields.js b/packages/babel-helper-create-class-features-plugin/src/fields.ts
similarity index 100%
rename from packages/babel-helper-create-class-features-plugin/src/fields.js
rename to packages/babel-helper-create-class-features-plugin/src/fields.ts
diff --git a/packages/babel-helper-create-class-features-plugin/src/index.js b/packages/babel-helper-create-class-features-plugin/src/index.ts
similarity index 98%
rename from packages/babel-helper-create-class-features-plugin/src/index.js
rename to packages/babel-helper-create-class-features-plugin/src/index.ts
index 89394ba8f1..e071aa2845 100644
--- a/packages/babel-helper-create-class-features-plugin/src/index.js
+++ b/packages/babel-helper-create-class-features-plugin/src/index.ts
@@ -22,6 +22,8 @@ import {
 
 export { FEATURES, enableFeature, injectInitialization };
 
+declare const PACKAGE_JSON: { name: string; version: string };
+
 // Note: Versions are represented as an integer. e.g. 7.1.5 is represented
 //       as 70000100005. This method is easier than using a semver-parsing
 //       package, but it breaks if we release x.y.z where x, y or z are
@@ -38,6 +40,12 @@ export function createClassFeaturePlugin({
   manipulateOptions,
   // TODO(Babel 8): Remove the default falue
   api = { assumption: () => {} },
+}: {
+  name;
+  feature;
+  loose?;
+  manipulateOptions;
+  api?;
 }) {
   const setPublicClassFields = api.assumption("setPublicClassFields");
   const privateFieldsAsProperties = api.assumption("privateFieldsAsProperties");
diff --git a/packages/babel-helper-create-class-features-plugin/src/misc.js b/packages/babel-helper-create-class-features-plugin/src/misc.ts
similarity index 99%
rename from packages/babel-helper-create-class-features-plugin/src/misc.js
rename to packages/babel-helper-create-class-features-plugin/src/misc.ts
index 8bb57a85df..6bdf0b645d 100644
--- a/packages/babel-helper-create-class-features-plugin/src/misc.js
+++ b/packages/babel-helper-create-class-features-plugin/src/misc.ts
@@ -44,7 +44,7 @@ const classFieldDefinitionEvaluationTDZVisitor = {
   ReferencedIdentifier: handleClassTDZ,
 };
 
-export function injectInitialization(path, constructor, nodes, renamer) {
+export function injectInitialization(path, constructor, nodes, renamer?) {
   if (!nodes.length) return;
 
   const isDerived = !!path.node.superClass;
diff --git a/packages/babel-helper-create-class-features-plugin/src/typescript.js b/packages/babel-helper-create-class-features-plugin/src/typescript.ts
similarity index 84%
rename from packages/babel-helper-create-class-features-plugin/src/typescript.js
rename to packages/babel-helper-create-class-features-plugin/src/typescript.ts
index c18f0ea173..364ccd15e9 100644
--- a/packages/babel-helper-create-class-features-plugin/src/typescript.js
+++ b/packages/babel-helper-create-class-features-plugin/src/typescript.ts
@@ -1,8 +1,7 @@
-// @flow
-
 import type { NodePath } from "@babel/traverse";
+import type * as t from "@babel/types";
 
-export function assertFieldTransformed(path: NodePath) {
+export function assertFieldTransformed(path: NodePath<t.ClassProperty>) {
   // TODO (Babel 8): Also check path.node.definite
 
   if (path.node.declare) {
diff --git a/packages/babel-helper-replace-supers/src/index.ts b/packages/babel-helper-replace-supers/src/index.ts
index 5fd6d4bc10..617c6c7746 100644
--- a/packages/babel-helper-replace-supers/src/index.ts
+++ b/packages/babel-helper-replace-supers/src/index.ts
@@ -254,7 +254,6 @@ const looseHandlers = {
 
 type ReplaceSupersOptionsBase = {
   methodPath: NodePath<any>;
-  superRef: any;
   constantSuper?: boolean;
   file: any;
   // objectRef might have been shadowed in child scopes,
diff --git a/tsconfig.json b/tsconfig.json
index a6a886b9df..7fc7051507 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -7,6 +7,7 @@
     "./packages/babel-generator/src/**/*.ts",
     "./packages/babel-helper-annotate-as-pure/src/**/*.ts",
     "./packages/babel-helper-compilation-targets/src/**/*.ts",
+    "./packages/babel-helper-create-class-features-plugin/src/**/*.ts",
     "./packages/babel-helper-create-regexp-features-plugin/src/**/*.ts",
     "./packages/babel-helper-explode-assignable-expression/src/**/*.ts",
     "./packages/babel-helper-fixtures/src/**/*.ts",
@@ -53,6 +54,9 @@
       "@babel/helper-compilation-targets": [
         "./packages/babel-helper-compilation-targets/src"
       ],
+      "@babel/helper-create-class-features-plugin": [
+        "./packages/babel-helper-create-class-features-plugin/src"
+      ],
       "@babel/helper-create-regexp-features-plugin": [
         "./packages/babel-helper-create-regexp-features-plugin/src"
       ],