Let's add a few more useful methods to our linked list class. Wouldn't it be useful if we could tell if our list was empty or not, as with our <code>Stack</code> and <code>Queue</code> classes?
We should also be able to find specific elements in our linked list. Traversing through data structures is something you'll want to get a lot of practice with! Let's create an <code>indexOf</code> method that takes an <code>element</code> as an argument, and returns that element's <code>index</code> in the linked list. If the element is not found in the linked list, return <code>-1</code>.
Let's also implement a method that does the opposite: an <code>elementAt</code> method that takes an <code>index</code> as an argument and returns the <code>element</code> at the given <code>index</code>. If no <code>element</code> is found, return <code>undefined</code>.
</section>
## Instructions
<sectionid='instructions'>
Write an <code>isEmpty</code> method that checks if the linked list is empty, an <code>indexOf</code> method that returns the <code>index</code> of a given element, and an <code>elementAt</code> that returns an <code>element</code> at a given <code>index.
</section>
## Tests
<sectionid='tests'>
```yml
- text: Your <code>LinkedList</code> class should have a <code>indexOf</code> method.
testString: 'assert((function(){var test = new LinkedList(); return (typeof test.indexOf === ''function'')}()), ''Your <code>LinkedList</code> class should have a <code>indexOf</code> method.'');'
testString: 'assert((function(){var test = new LinkedList(); return (typeof test.elementAt === ''function'')}()), ''Your <code>LinkedList</code> class should have a <code>elementAt</code> method.'');'
testString: 'assert((function(){var test = new LinkedList(); test.add(''cat''); test.add(''dog''); test.add(''kitten''); return test.size() === 3}()), ''Your <code>size</code> method should return the length of the linked list'');'
testString: 'assert((function(){var test = new LinkedList(); test.add(''cat''); test.add(''dog''); test.add(''kitten''); return test.indexOf(''kitten'') === 2}()), ''Your <code>indexOf</code> method should return the index of the given element.'');'
testString: 'assert((function(){var test = new LinkedList(); test.add(''cat''); test.add(''dog''); test.add(''kitten''); return test.elementAt(1) === ''dog''}()), ''Your <code>elementAt</code> method should return at element at a given index.'');'