diff --git a/src/splitInteger.test.js b/src/splitInteger.test.js
index a610317d..82117def 100644
--- a/src/splitInteger.test.js
+++ b/src/splitInteger.test.js
@@ -2,20 +2,44 @@
 
 const splitInteger = require('./splitInteger');
 
+function checkInvariants(result, value, numberOfParts) {
+  // length must equal numberOfParts
+  expect(result.length).toBe(numberOfParts);
+
+  // sum must equal value
+  expect(result.reduce((a, b) => a + b, 0)).toBe(value);
+
+  // must be sorted ascending
+  expect(result).toEqual([...result].sort((a, b) => a - b));
+
+  // max - min <= 1
+  const min = Math.min(...result);
+  const max = Math.max(...result);
+  expect(max - min).toBeLessThanOrEqual(1);
+}
+
 test(`should split a number into equal parts
   if a value is divisible by a numberOfParts`, () => {
-
+  const result = splitInteger(20, 5);
+  expect(result).toEqual([4, 4, 4, 4, 4]);
+  checkInvariants(result, 20, 5);
 });
 
 test(`should return a part equals to a value
   when splitting into 1 part`, () => {
-
+  const result = splitInteger(8, 1);
+  expect(result).toEqual([8]);
+  checkInvariants(result, 8, 1);
 });
 
 test('should sort parts ascending if they are not equal', () => {
-
+  const result = splitInteger(17, 4);
+  expect(result).toEqual([4, 4, 4, 5]);
+  checkInvariants(result, 17, 4);
 });
 
 test('should add zeros if value < numberOfParts', () => {
-
+  const result = splitInteger(3, 5);
+  expect(result).toEqual([0, 0, 1, 1, 1]); // fixed expectation
+  checkInvariants(result, 3, 5);
 });