Fibonacci Number [JavaScript]

#### This issue is about [Algo/DS Name](link to resource for the Algo/DS).

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

The issue requests a Fibonacci Number implementation in JavaScript but names no file, test, or repository entry point. Inspect the repository's existing JavaScript algorithm implementations first, follow their organization and verification conventions, and confirm the Fibonacci implementation matches them.

Written by the indexing model from the issue text.