Can anyone tell me how can I access or edit my js (javascript) files from Wordpress dashboard?? I can get access in css/.php files from Appearence-> Editor option. Is there any way to get access / edit / add js file without getting cpanel access, just from WP Dashboard??
No. You have to actually get to the files you have, but fortunatially this is easy. Two ways: 1) open your cpanel and open file manager. Browse to the files (check the path of the URL). 2) Access the files via ftp. You have to set up ftp access via the cpanel if you haven't done it before. It's more work to setup but faster once you do.
Better create some child theme. Here's a tutorial for that http://wordpressinaction.com/creating-child-theme-wordpress/
There are some plugins that will let you edit any file I believe. Here's one https://wordpress.org/plugins/wpide/ (not a recommendation, haven't used it - but it is possible)
Yeah just use FTP client and have the your local directory and access to the remote directory to easily make changes in your preferred HTML/text editor/client and upload/overwrite with Filezilla. Best and fastest way if you do a lot of editing and making a lot of changes on the go all the time. Or bookmark the /wp-admin/themes.php?page=wpeditor_themes page page. If you use WP-Editor plugin it gives you more access to the themes files.