Update hash table methods.

Author: trekhleb

src/data-structures/hash-table/HashTable.js

@@ -78,7 +78,7 @@ export default class HashTable {
     const bucketLinkedList = this.buckets[this.hash(key)];
     const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
 
-    return node ? node.value.value : null;
+    return node ? node.value.value : undefined;
   }
 
   /**

src/data-structures/hash-table/__test__/HashTable.test.js

@@ -43,12 +43,13 @@ describe('HashTable', () => {
 
     expect(hashTable.get('a')).toBe('sky');
     expect(hashTable.get('d')).toBe('ocean');
+    expect(hashTable.get('x')).not.toBeDefined();
 
     hashTable.delete('a');
 
     expect(hashTable.delete('not-existing')).toBeNull();
 
-    expect(hashTable.get('a')).toBeNull();
+    expect(hashTable.get('a')).not.toBeDefined();
     expect(hashTable.get('d')).toBe('ocean');
 
     hashTable.set('d', 'ocean-new');