Using async in express - express

Question

Below is my rest API endpoint /signup. The problem I'm having now is that the endpoint does not stop after validateEmail. Even after it failed email form-validation and res.send() is done, the endpoint continues. So I'm keep getting the error 'Error: Can't set headers after they are sent.'. I would like to be able to finish the endpoint inside its functions like validateEmail , checkEmailInUse, makeUser, and so on.
router.post("/signup", async (req, res, next) => {
const { email, password } = req.body;
const users = req.app.get("users");
validateEmail(res, email);
await checkEmailInUse(res, users, email);
const user = await makeUser(res, users, email, password);
res.send({ message: "POST signup request OK", user });
});
function validateEmail(res, email) {
const isEmail = emailFilter.test(email);
if (!isEmail) {
res.status(400).send({
error: {
message: "Requested email is not email type",
type: "FormatValidation",
location: "validateEmail"
}
});
return;
}
}
async function checkEmailInUse(res, users, email) {
const query = { email };
try {
const user = await users.findOne(query);
if (user) {
res.send({ message: "The email is already used" });
}
} catch (err) {
res.status(400).send({
error: {
message: "Failed to find user",
type: "DatabaseError",
location: "checkEmailInUse"
}
});
return;
}
}

Answer 1

The code keeps going after a failed validate because you call:
validateEmail(res, email);
and then your code just keeps going. This is normal flow of control in Javascript. Your function keeps executing lines of code until you return in the function. The same issue is true for checkEmailInUse(). If you want to sometimes send the response inside those functions and be done, then you need a return value from those functions that you can check and then use if statements to determine whether your code should do more or not.
Following your style of sending the error response inside the validation functions (which is not how I would probably structure things), you could return values from those functions and test those return values in the request handler like this:
router.post("/signup", async (req, res, next) => {
const { email, password } = req.body;
const users = req.app.get("users");
if (validateEmail(res, email)) {
if (await checkEmailInUse(res, users, email)) {
const user = await makeUser(res, users, email, password);
res.send({ message: "POST signup request OK", user });
}
}
});
function validateEmail(res, email) {
const isEmail = emailFilter.test(email);
if (!isEmail) {
res.status(400).send({
error: {
message: "Requested email is not email type",
type: "FormatValidation",
location: "validateEmail"
}
});
return false;
}
return true;
}
async function checkEmailInUse(res, users, email) {
const query = { email };
try {
const user = await users.findOne(query);
if (user) {
res.send({ message: "The email is already used" });
return false;
} else {
return true;
}
} catch (err) {
res.status(400).send({
error: {
message: "Failed to find user",
type: "DatabaseError",
location: "checkEmailInUse"
}
});
return false;
}
}
}
But, I think you might find this is simpler if you get rid of the local functions because then when you send a response, you can just directly return from the main function and be done. Here's how that could look:
router.post("/signup", async (req, res, next) => {
function err(res, message, type, location) {
res.status(400).send({error: {message, type, location}});
}
const { email, password } = req.body;
if (!emailFilter.test(email)) {
err(res, "Requested email is not email type", "FormatValidation", "validateEmail");
return;
}
const users = req.app.get("users");
try {
const user = await users.findOne({email});
if (user) {
res.send({ message: "The email is already used" });
return;
}
} catch(e) {
err(res, "Failed to find user", "DatabaseError", "checkEmailInUse");
return;
}
try {
const user = await makeUser(res, users, email, password);
res.send({ message: "POST signup request OK", user });
} catch(e) {
err(res, "Failed to make user", "DatabaseError", "makeUser");
}
}