How to pass value of object to new global variable in JavaScript

Question

I have written a program to be run by my Node.js server to interact with a python script (source: https://github.com/extrabacon/python-shell), which successfully sends data to the script and receieves the returned value. What is returned (message) (in pyshell.on) is a json, which only appears to be local to the function passed in, however I would like to use this data elsewhere in my program outside this function.

Being new to javascript I initially tried to just set ret_val = message; to no avail, have tried changing the type (python side) to a string (still no success), and finally converting json to string and parsing to ret_val (as it is now) ret_val = JSON.parse(JSON.stringify(message));. Unfortunately none of these methods work, as ret_val remains undefined, despite message clearly containing the data i need as an object (checked using typeof). I would like to know either the correct way to copy the contents of the message variable into ret_val to be later returned, or possibly the correct way to get the message data if I am using this module incorrectly.

SEE below for logs

UPDATE: Full script containing issue, occurs in function run_py_script when setting ret_val.

var express = require('express');
var router = express.Router();
var PythonShell = require('python-shell');

var options = {
    mode: 'json'
};

var rain_data;

function run_py_script(data, callback){
    var pyshell = new PythonShell('dummy.py', options);
    var ret_val;
    pyshell.send("dummy data"); // change to data

    pyshell.on('message', function(message){
        console.log(message);
        rev_val = message;
        //console.log(ret_val["willCollide"]);
    });

    pyshell.end(function(err){
        if (err) {
            console.log("hi");
        }
        return err ? callback(null) : callback(null, ret_val);
    });

}

/* GET rain_track data. */

router.get('/', function(req, res, next) {

    run_py_script(null, function(err, rain_data) {
        console.log(err);
        console.log(rain_data);
        if (err){ console.log("error in python script" ); return res.json(null); }
        if (rain_data == null){
            console.log("Error: data is null or undefined");
        }
        res.json(rain_data);
    })
});

module.exports = router;

Logs:

{ willCollide: 1, time: 6000, strength: 'NA' }
null
undefined
Error: data is null or undefined
GET /rain_track 200 62.778 ms - -