Solved this one myself:

function dumpSessVars(session) {

                var svars = ["context","destination_number","caller_id_name","caller_id_number","network_addr","ani","aniii","rdnis","source","chan_name","uuid"];

    svars.forEach(function (v) {

                               cLog(1,"**************** SESSION VAR " + v + " : " + session.getVariable(v));

});

}

From: FreeSWITCH-users <freeswitch-users-bounces@lists.freeswitch.org> On Behalf Of Francesco Facco de Lagarda
Sent: sabato 10 marzo 2018 14:20
To: 'FreeSWITCH Users Help' <freeswitch-users@lists.freeswitch.org>
Subject: [Freeswitch-users] javascript get session extension number

Goodmorning to all.

Can anyone please tell me how I can get the extension number that originated the session in javascript?