Some notes from me walking through EG circ code, figuring out how it works and where the entry points for the backend javascript are.  Of course, most or all of this going to change soon with the in-db circ code. :)

For circulation behavior, there are two pertinent services, open-ils.penalty and open-ils.circ, which pull in the following backend javascript files:

<code>
open-ils.penalty -> 
    penalty/patron_penalty.js
open-ils.circ ->
    circ/circ_permit_patron.js
    circ/circ_permit_copy.js
    circ/circ_duration.js
    circ/circ_recurring_fines.js
    circ/circ_max_fines.js
    circ/circ_permit_renew.js
    circ/circ_permit_hold.js
</code>

The scripts used by the services are determined by configuration in opensrf.xml and are intended to be modified by admins for their installation.

Certain other javascript files get pulled in by these scripts:
<code>
    circ/circ_lib.js
        circ/circ_groups.js
        catalog/record_type.js
            phys_char.js
            fixed_fields.js
            JSON_V1.js
        JSON_v1.js
    circ/circ_item_config.js
        circ/circ_lib.js
</code>

The main source file for open-ils.circ is Circ.pm, but it pulls in Circ/Circulate.pm, which loads in the javascript specified in opensrf.xml.

For checkouts, there are two pertinent circ methods:
    open-ils.circ.checkout.permit
    open-ils.circ.checkout

The first determines whether the checkout is allowed, and a permit_key from that is passed to open-ils.circ.checkout to create the actual circulation.

For permit, in Circulate.pm, the program flow goes something like this, assuming no shortcutting "bail outs":
<code>
    run_method
        $circulator = new Circulator, so initialize (setup references for backend javascript)
        mk_script_runner, to build the javascript environment and fetch most of the objects needed
        do_permit
            If the requestor is not the patron, check the VIEW_PERMIT_CHECKOUT permission 
            ("Allows a user to determine of another user can checkout an item")
            check_captured_holds
                if copy status is On Holds Shelf and hold is for a different patron, push ITEM_ON_HOLDS_SHELF into the event list
            do_copy_checks
                if copy status is In Transit, then return/bail with COPY_IN_TRANSIT event
                handle_claims_returned
                    if there is an open circulation on the item with a CLAIMSRETURNED stop fines value,
                    then we'll either bail with a CIRC_CLAIMS_RETURNED event,
                    or if .permit.override, we'll check in the item
                unless renewal check for open circulation and if found, return/bail with OPEN_CIRCULATION_EXISTS event
            run_patron_permit_scripts
!               circ_permit_patron script 
                ^---- ENTRY POINT FOR CUSTOM BEHAVIOR, via circ_permit_patron.js
!               gather_penalty_request    
                ^---- ENTRY POINT FOR CUSTOM BEHAVIOR, indirectly via patron_penalty.js
                push any events from these into the event list
            run_copy_permit_scripts unless pre-cat or non-cat
!               circ_permit_copy script   
                ^---- ENTRY POINT FOR CUSTOM BEHAVIOR, via circ_permit_copy.js
                check_copy_alert    (COPY_ALERT_MESSAGE event if found)
                push any events from these into the event list
            override_events unless renewal
                If events are found here and this is not a .permit.override call, then we'll bail
                otherwise we'll test each event in the list and see if the requestor has the appropriate .override permission
                we'll bail on the first event where this is not the case, and remove the event from the event list if it is
            push SUCCESS into event list
        if multiple events, remove SUCCESS
        return $circulator->events
</code>

For checkout, in Circulate.pm, the program flow goes something like this, assuming no shortcutting "bail outs":
<code>
    run_method
        $circulator = new Circulator, so initialize (setup references for backend javascript)
        mk_script_runner, to build the javascript environment and fetch most of the objects needed
        do_checkout
            If not a renewal, check the COPY_CHECKOUT_PERMISSION
            Check the permit key, and if this is a permit override, check the CIRC_PERMIT_OVERRIDE permission
            Consider type of circ: Non-cataloged, pre-cataloged, and normal
            if A) NON-CATALOGED
                checkout_noncat (this is defined in Circ/NonCat.pm)
                return
            if B) PRE-CAT
                insert 'environment.isPrecat' into script
                make_precat_copy  (and shove it into the javascript environment for the copy checks)
            else if has a pre-cat call number then return ITEM_NOT_CATALOGED event  (this is unclear to me)

            do_copy_checks
                if copy status is In Transit, then return/bail with COPY_IN_TRANSIT event
                handle_claims_returned
                    if there is an open circulation on the item with a CLAIMSRETURNED stop fines value,
                    then we'll either bail with a CIRC_CLAIMS_RETURNED event,
                    or if .permit.override, we'll check in the item
                unless renewal check for open circulation and if found, return/bail with OPEN_CIRCULATION_EXISTS event

            run_checkout_scripts
!               circ_duration script
                ^---- ENTRY POINT FOR CUSTOM BEHAVIOR, via circ_duration.js (and circ_item_config.js)
                # This actually figures which in-db settings to use for durationRule, recurringFinesRule, and maxFine
            build_checkout_circ_object
            apply_modified_due_date
            create_action_circulation
            update the copy status to Checked Out
            handle_checkout_holds
            update_patron_penalties if permit override or renewal
            return SUCCESS event and payload
</code>