How do we sent data to local variable in node js async with DB Calls

I am new to javascript async programming and i have a basic issue where I have a Set of code where i am doing two separate DB calls on basis of request body params . These are two methods which does a DB Call and returns a Promise

1. validateExam
2. validateUserExists

I want to store resullts from async call to this variable and then return it in response .

getExam: function(req, res) {
    var myExam = {};
    var coupon = req.body.coupon;
    var email = req.body.email;


    async.series([
        function(callback) {
            validateExam(coupon)
                .then(function(success) {
                    assign(success);
                    callback(null, success);
                });
        },
        function(callback) {
            validateUserExists(email)
                .then(function(result) {
                    callback(null, result);
                })
        }
    ], function(error, results) {
        myExam.res = results;

    });


    res.json({
        "status": 400,
        "message": myExam
    });
},

via Sanjay-Dev