fixes #31

Adds tests for custom heaps and comparators

Author: ignlg

src/Heap.ts

@@ -690,7 +690,7 @@ export class Heap<T> implements Iterable<T> {
     const self = this.heapArray[i];
 
     const getPotentialParent = (best: number, j: number) => {
-      if (this.compare(this.heapArray[j], this.heapArray[best]) < 0) {
+      if (this.heapArray.length > j && this.compare(this.heapArray[j], this.heapArray[best]) < 0) {
         best = j;
       }
       return best;

tests/heap-private-methods.test.ts

@@ -1,7 +1,5 @@
 import Heap from '../src/Heap';
-
-const someValues = [3, 15, 2, 300, 16, 4, 1, 8, 50, 21, 58, 7, 4, 9, 78, 88];
-const otherValues = [12, 1, 2, 30, 116, 42, 12, 18, 1, 1, 1, 1];
+import { someValues } from './test-helpers';
 
 describe('Heap private', function () {
   const heaps = [