Returning value of function containing a Promise - node.js

Question

I want to be informed about a user's login status. For example, if a user attempts to sign up using an existing email in the database, I want to get a return value of STATUS_DUPLICATE_EMAIL (a string). Another possibility is if a user attempts to sign in with an email with a previously logged in social media account, I want to get a return value of STATUS_SNS_EMAIL.
Code snippet:
app.post("/api/signup", (req, res) => {
const status = checkEmail(req, res);
// some sign up logic...
}
...
checkEmail = (req, res) => {
Database.findOne({
where: {
email: req.body.email
}
}).then((user) => {
if (user) {
res.status(400).send({ message: "Email already used." });
}
return STATUS_DUPLICATE_EMAIL;
}
When I try to console.log status, it is undefined, and I'm not sure why so. What is the general way to get a return value from a function like checkEmail?

Answer 1

The value of status is the value the function checkEmail() returns - which is currently nothing.
What you should do is return your promise (Database.findOne) from your checkEmail.
Now the value of status will be the promise your returned.
Now you should call it's method then again to get the value.
The promise is resolved but the Javascript engine reads your code before it is. Therefore, the method then of a promise returns a promise as well.
app.post("/api/signup", (req, res) => {
checkEmail(req, res).then(desired_value => {
const status = desired_value;
}
}
...
checkEmail = (req, res) => {
return Database.findOne({
where: {
email: req.body.email
}
}).then((user) => {
if (user) {
res.status(400).send({ message: "Email already used." });
}
return STATUS_DUPLICATE_EMAIL;
}
}
Alternatively
You can use async/await pattern. When declaring a function as async you can prefix promises inside it with await which will block the code execution until it's resolved.
app.post("/api/signup", async (req, res) => {
const status = await checkEmail(req, res);
}
...
checkEmail = (req, res) => {
return Database.findOne({
where: {
email: req.body.email
}
}).then((user) => {
if (user) {
res.status(400).send({ message: "Email already used." });
}
return STATUS_DUPLICATE_EMAIL;
}
}

Answer 2

You should return Database.findOne().
Also use async await to get the result of checkMail().
app.post("/api/signup", async (req, res) => {
const status = await checkEmail(req, res);
// some sign up logic...
}
...
app.post("/api/signup", (req, res) => {
const status = checkEmail(req, res);
// some sign up logic...
}
...
checkEmail = (req, res) => {
return Database.findOne({
where: {
email: req.body.email
}
}).then((user) => {
if (user) {
res.status(400).send({ message: "Email already used." });
}
return STATUS_DUPLICATE_EMAIL;
}
}