diff --git a/packages/system/src/unit.js b/packages/system/src/unit.js
deleted file mode 100644
index 91d8a147c3..0000000000
--- a/packages/system/src/unit.js
+++ /dev/null
@@ -1,7 +0,0 @@
-const toUnit = (unit = 'px') => {
-  return value => (Number(value) && value !== 0 ? `${value}${unit}` : value);
-};
-
-const toPx = toUnit('px');
-
-export { toUnit, toPx };
diff --git a/packages/system/src/unit.test.ts b/packages/system/src/unit.test.ts
new file mode 100644
index 0000000000..65dd5f0032
--- /dev/null
+++ b/packages/system/src/unit.test.ts
@@ -0,0 +1,22 @@
+import { toPx } from './unit';
+
+describe('Unit', () => {
+  describe('toPx', () => {
+    it('should add "px" to the number', () => {
+      expect(toPx(10)).toBe('10px');
+    });
+
+    it('should return 0 without adding unit', () => {
+      expect(toPx(0)).toBe(0);
+    });
+
+    it('should return a numeric string with "px"', () => {
+      expect(toPx('20')).toBe('20px');
+    });
+
+    it('should add "px" to a decimal numeric string or number', () => {
+      expect(toPx(0.5)).toBe('0.5px');
+      expect(toPx('0.5')).toBe('0.5px');
+    });
+  });
+});
diff --git a/packages/system/src/unit.ts b/packages/system/src/unit.ts
new file mode 100644
index 0000000000..4ac5ff4eb9
--- /dev/null
+++ b/packages/system/src/unit.ts
@@ -0,0 +1,7 @@
+const toUnit = (unit = 'px') =>
+  (value: string | number) =>
+    Number(value) && value !== 0 ? `${value}${unit}` : value;
+
+const toPx = toUnit('px');
+
+export { toUnit, toPx };