diff --git a/src/index.js b/src/index.ts
similarity index 85%
rename from src/index.js
rename to src/index.ts
index 856b3af..d1fdfec 100644
--- a/src/index.js
+++ b/src/index.ts
@@ -22,11 +22,13 @@
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-function compare(v1, v2) {
+export type CompareFunction = (v1: number, v2: number) => Boolean;
+
+function compare(v1: number, v2: number) {
     return v1 < v2;
 }
 
-function upperBound(array, value, comp = compare) {
+function upperBound(array: number[], value: number, comp: CompareFunction = compare) {
     let len = array.length;
     let i = 0;
 
@@ -43,7 +45,7 @@ function upperBound(array, value, comp = compare) {
     return i;
 }
 
-function lowerBound(array, value, comp = compare) {
+function lowerBound(array: number[], value: number, comp: CompareFunction = compare) {
     let len = array.length;
     let i = 0;
 
@@ -60,7 +62,7 @@ function lowerBound(array, value, comp = compare) {
     return i;
 }
 
-function binarySearch(array, value, comp = compare) {
+function binarySearch(array: number[], value: number, comp: CompareFunction = compare) {
     let cursor = lowerBound(array, value, comp);
     return cursor !== array.length && !comp(value, array[cursor]);
 }
diff --git a/test/binary-search.js b/test/binary-search.js
index 0d10186..7d54459 100644
--- a/test/binary-search.js
+++ b/test/binary-search.js
@@ -23,7 +23,7 @@
 */
 
 import assert from 'assert'
-import { binarySearch } from '../src/index.js'
+import { binarySearch } from '../src/index'
 
 describe('binarySearch', () => {
     it('less', () => {
diff --git a/test/lower-bound.js b/test/lower-bound.js
index df8f58a..b4b1a00 100644
--- a/test/lower-bound.js
+++ b/test/lower-bound.js
@@ -23,7 +23,7 @@
 */
 
 import assert from 'assert'
-import { lowerBound } from '../src/index.js'
+import { lowerBound } from '../src/index'
 
 describe('lower-bound', () => {
     it('less', () => {
diff --git a/test/tsconfig.json b/test/tsconfig.json
index cb359a9..aba9c36 100644
--- a/test/tsconfig.json
+++ b/test/tsconfig.json
@@ -2,10 +2,11 @@
     "extends": "../tsconfig.json",
     "compilerOptions": {
         "declaration": false,
-        "noEmit": true
+        "noEmit": true,
+        "allowJs": true
     },
     "include": [
         "../src/**/*",
         "./**/*"
     ]
-}
\ No newline at end of file
+}
diff --git a/test/upper-bound.js b/test/upper-bound.js
index f2419c7..a70bc2c 100644
--- a/test/upper-bound.js
+++ b/test/upper-bound.js
@@ -23,7 +23,7 @@
 */
 
 import assert from 'assert'
-import { upperBound } from '../src/index.js'
+import { upperBound } from '../src/index'
 
 describe('upper-bound', () => {
     it('less', () => {
diff --git a/tsconfig.json b/tsconfig.json
index 6e59769..2ecd1be 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -20,7 +20,6 @@
     "noUnusedParameters": true,
     "noImplicitReturns": true,
     "noFallthroughCasesInSwitch": true,
-    "allowJs": true
   },
   "include": [
     "src/**/*"