Add test for next greater element

src/_Problems_/next-greater-element/index.js

@@ -4,7 +4,7 @@
   To keep it simple, the next greater element for the last or maximum value in the array is -1.
 
   Input: [4, 6, 3, 2, 8, 1]
-  Output: {6, 8, 8, 8, -1, -1}
+  Output: [6, 8, 8, 8, -1, -1]
  */
 
 const Stack = require('../../_DataStructures_/Stack');
@@ -42,5 +42,4 @@ function nextGreaterElement(arr) {
   return nextGreater;
 }
 
-// eslint-disable-next-line no-console
-console.log(nextGreaterElement([4, 6, 3, 2, 8, 1]));
+module.exports = { nextGreaterElement };

src/_Problems_/next-greater-element/next-greater-element.test.js

@@ -0,0 +1,38 @@
+const { nextGreaterElement } = require('.');
+
+describe('Next greater element', () => {
+  it('returns next greater elements collection', () => {
+    const input = [4, 6, 3, 2, 8, 1]
+    const greaterElements = [6, 8, 8, 8, -1, -1]
+
+    expect(nextGreaterElement(input)).toEqual(greaterElements);
+  });
+
+  it('returns and empty collection for an empty array', () => {
+    expect(nextGreaterElement([])).toEqual([]);
+  });
+
+  it('returns an array with -1 if the input has only one element', () => {
+    expect(nextGreaterElement([0])).toEqual([-1]);
+  });
+
+  it('returns a collection of -1 if there is no greater element', () => {
+    const input = [90, 40, 15, 7, -1, -10]
+    const greaterElements = [-1, -1, -1, -1, -1, -1]
+
+    expect(nextGreaterElement(input)).toEqual(greaterElements);
+  });
+
+  it('uses -1 if the numbers are the same', () => {
+    const input = [90, 90]
+    const greaterElements = [-1, -1]
+
+    expect(nextGreaterElement(input)).toEqual(greaterElements);
+  });
+
+  it('throws an error if the input is not an array', () => {
+    expect(() => {
+      nextGreaterElement('xunda')
+    }).toThrowError('Invalid Argument');
+  });
+});