Bluetooth, define descriptors for characteristics ?

Bluetooth supports descriptors for characteristics.
In bluetooth.c for nrf is this:

    char_md.p_char_pf                = NULL;
    char_md.p_user_desc_md           = NULL;
    char_md.p_cccd_md                = NULL;
    char_md.p_sccd_md                = NULL; 

I can't find a way where they are used.
Can descriptors be defined in javascript ?
If yes,is this the right place to search for descriptors ?