In Nodejs, how wait process until response back from function - node.js

Question

I am new to Nodejs. Process is not waiting until response back from function. Because of asynchronous calling of Nodejs. how to make synchronous. Please help me.
Here is my code
module.exports.signup = function(req, res){
console.log('signup');
User.findOne({'emails.email' : req.body.email}, function(err, doc) {
if (doc === null) {
var vr_token= genRandomString(16);
var ex_date = Date.now();
var user = new User();
user.emails.push({
email : req.body.email,
email_verification_token : vr_token,
verify_key_expire : ex_date });
user.save(function(err2,user1) {
if (!err2) {
var result = send_email.sync(vr_token);//process not waiting
if(result) {
res.json ({
status: 200,
message:"mail sent successfully",
data:user1
})
}
}
});
}
})
}
here is my function
function send_email(vr_token){
var mailOpts = {
from: process.env.Mail_From_Addr,
to: 'xxxxxxxxxxxxx',
subject: 'Verify Your New Account Email',
html:'<p>Hello,</p>code : '+vr_token
};
mailgun.messages().send(mailOpts, function (err, response){
if (!err){
return true;
}else{
return false;
}
})
}

Answer 1

Add a callback to it:
function send_email(vr_token, callback){
var mailOpts = {
from: process.env.Mail_From_Addr,
to: 'xxxxxxxxxxxxx',
subject: 'Verify Your New Account Email',
html:'<p>Hello,</p>code : '+vr_token
};
mailgun.messages().send(mailOpts, function (err, response){
callback(null, !err);
})
}
The Code:
module.exports.signup = function(req, res){
console.log('signup');
User.findOne({'emails.email' : req.body.email}, function(err, doc) {
if (doc === null) {
var vr_token= genRandomString(16);
var ex_date = Date.now();
var user = new User();
user.emails.push({
email : req.body.email,
email_verification_token : vr_token,
verify_key_expire : ex_date });
user.save(function(err2,user1) {
if (!err2) {
send_email.sync(vr_token, function(err, result){
if(result) {
res.json ({
status: 200,
message:"mail sent successfully",
data:user1
})
}
});
}
});
}
})
}