Node.js trouble with callback in loop

Being new to Node, I am still having some troubles with callbacks.

In the mapBpiIfindex function I am trying to loop through all of the VLANs found by the vlans function. Once it has looped through all VLANs, creating the map, I want to output the map to the browser. But, the only output I am getting is {}. How can I send the mapping to the browser? I am not even sure if I am using my callbacks correctly.

var express = require('express');
var router = express.Router();
var snmp = require('snmp-native');


// Create a Session with explicit default host, port, and community.
let session = new snmp.Session({ host: 'AASW0120', port: 161, community: 'community' })

let Mibs = {
    hostname: [1,3,6,1,2,1,1,5,0],
    vlans: [1,3,6,1,4,1,9,9,46,1,3,1,1,2],
    dot1dBasePortIfIndex: [1,3,6,1,2,1,17,1,4,1,2]
}


/* Get all VLANs on switch */
function vlans(snmpSession, cb) {
    let vlans = []
    session.getSubtree({ oid: Mibs.vlans }, function (error, varbinds) {
        if (error) {
            console.log('Fail :(');
        } else {
            varbinds.forEach(function (varbind) {
                vlans.push(varbind.oid[varbind.oid.length -1])
            })
        }
        cb(vlans)
    })
}


/* Map BPIs to Ifindices */
function mapBpiIfindex(session, cb) {
    let map = {}
    vlans(session, function (vlans) {
        vlans.forEach(function (vlan) {
            session.getSubtree({oid: Mibs.dot1dBasePortIfIndex, community: 'community@' + vlan}, function (error, varbinds) {
                if (error) {
                    console.log('Fail :(')
                } else {
                    varbinds.forEach(function (varbind) {
                        map[varbind.oid[varbind.oid.length -1]] = {ifindex: varbind.value, vlan: vlan}
                    })
                }
            })
        })
        cb(map)
    })
}



router.get('/vlans', function (req, res, next) {
    vlans(session, function (vlans) {
        res.send(vlans)
    })
})

router.get('/bpi-ifindex', function (req, res, next) {
    mapBpiIfindex(session, function (mapping) {
        res.send(mapping)
    })
})

via rhillhouse