diff --git a/package.json b/package.json
index 14ec09d..2933962 100644
--- a/package.json
+++ b/package.json
@@ -18,7 +18,7 @@
   "type": "module",
   "types": "dist/hyparquet.d.ts",
   "scripts": {
-    "build": "tsc",
+    "build": "tsc && cp src/*.js dist",
     "coverage": "vitest run --coverage",
     "demo": "http-server -o",
     "lint": "eslint . --ext .ts",
diff --git a/src/metadata.ts b/src/metadata.ts
index 073c84d..e0ecf09 100644
--- a/src/metadata.ts
+++ b/src/metadata.ts
@@ -123,13 +123,14 @@ export function schemaElement(schema: SchemaElement[], name: string[]): SchemaEl
  * @param {unknown} obj object to convert
  * @returns {unknown} converted object
  */
-export function toJson(obj: unknown): unknown {
+export function toJson(obj: any): unknown {
   if (typeof obj === 'bigint') {
     return Number(obj)
   } else if (Array.isArray(obj)) {
     return obj.map(toJson)
-  } else if (typeof obj === 'object') {
-    const newObj = {}
+  } else if (obj instanceof Object) {
+    /** @type {Record<string, unknown>} */
+    const newObj: Record<string, unknown> = {}
     for (const key of Object.keys(obj)) {
       newObj[key] = toJson(obj[key])
     }
diff --git a/src/snappy.ts b/src/snappy.js
similarity index 92%
rename from src/snappy.ts
rename to src/snappy.js
index 20e49f3..aedccce 100644
--- a/src/snappy.ts
+++ b/src/snappy.js
@@ -10,7 +10,7 @@ const WORD_MASK = [0, 0xff, 0xffff, 0xffffff, 0xffffffff]
  * @param {number} length number of bytes to copy
  * @returns {void}
  */
-function copyBytes(fromArray: Uint8Array, fromPos: number, toArray: Uint8Array, toPos: number, length: number) {
+function copyBytes(fromArray, fromPos, toArray, toPos, length) {
   for (let i = 0; i < length; i++) {
     toArray[toPos + i] = fromArray[fromPos + i]
   }
@@ -25,7 +25,7 @@ function copyBytes(fromArray: Uint8Array, fromPos: number, toArray: Uint8Array,
  * @param {number} length number of bytes to copy
  * @returns {void}
  */
-function selfCopyBytes(array: Uint8Array, pos: number, offset: number, length: number) {
+function selfCopyBytes(array, pos, offset, length) {
   for (let i = 0; i < length; i++) {
     array[pos + i] = array[pos - offset + i]
   }
@@ -39,7 +39,7 @@ function selfCopyBytes(array: Uint8Array, pos: number, offset: number, length: n
  * @param {Uint8Array} outputArray output buffer
  * @returns {boolean} true if successful
  */
-export function snappyUncompress(inputArray: Uint8Array, outputArray: Uint8Array): boolean {
+export function snappyUncompress(inputArray, outputArray) {
   const inputLength = inputArray.byteLength
 
   let pos = 0
diff --git a/src/thrift.ts b/src/thrift.ts
index 61ea46d..62cdbb9 100644
--- a/src/thrift.ts
+++ b/src/thrift.ts
@@ -28,7 +28,8 @@ export function deserializeTCompactProtocol(arrayBuffer: ArrayBuffer): Decoded<R
   const view = new DataView(arrayBuffer)
   let byteLength = 0
   let lastFid = 0
-  const value = {}
+  /** @type {Record<string, any>} */
+  const value: Record<string, any> = {}
 
   while (byteLength < arrayBuffer.byteLength) {
     // Parse each field based on its type and add to the result object
@@ -91,7 +92,8 @@ function readElement(view: DataView, type: number, index: number): [any, number]
     return [listValues, index]
   }
   case CompactType.STRUCT: {
-    const structValues = {}
+    /** @type {Record<string, any>} */
+    const structValues: Record<string, any> = {}
     let structLastFid = 0
     while (true) {
       let structFieldType, structFid, structIndex
diff --git a/tsconfig.json b/tsconfig.json
index 4b50dee..36a613c 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,10 +1,13 @@
 {
   "compilerOptions": {
-    "declaration": true,
+    "allowJs": true,
+    "checkJs": true,
     "lib": ["esnext", "dom"],
     "module": "nodenext",
     "outDir": "dist",
+    "skipLibCheck": true,
     "sourceMap": true,
+    "strict": true,
     "target": "esnext",
   },
   "include": ["src"]