How to run function with this function - node.js

Question

How to run function within a function
If the first function throw any error then 2nd function should run
server.post('/User', (req, res,next) =>
server.post('/submit', (req, res) =>
server.post('/User', (req, res,next) => {
// Some Task
if(){
//Do something
}
else {
server.post('/submit', (req, res) => {

Answer 1

If the first function returns a promise you could run those functions after catching the error like this:
firstFunction()
.catch(e => {
secondFunction();
});
Otherwise you could use a try-catch statement like this:
try {
firstFunction();
} catch (e) {
secondFunction();
}

Answer 2

In Express, the next parameter allows a route/middleware to pass processing to the next route/middleware (actually, all routes are merely middlewares that don't call next). Therefore you can implement passing your route processing by just calling next():
function submitHandler (req, res) {
// ...
}
server.post('/User',
(req, res, next) => {
if(/* all ok */){
//Do something
}
else {
next(); // continue processing
return;
}
},
submitHandler // continue with this function
)
server.post('/submit', submitHandler); // also have a separate endpoint where
// submitHandler can be accessed directly

Answer 3

Remember that server.post(...) doesn't actually carry out that operation in that route. It just registers a route handler for some future route. So, you don't generally do things like:
server.post("/firstRoute", function(req, res, next) {
if (some condition) {
server.post("/secondRoute", ...);
}
});
This would configure a second route handler for all users based on what one user did in their /firstRoute request.
Remember a server contains a set of route handlers that work for ALL users that will use the server now and in the future. What your code proposes is that based on some data in one particular incoming request, you would change the routes that the server supports for all users. That's not how you would code a server.
If you need to maintain server-side state for a particular user such that data from one request influences how some future request works, then you would typically create a user session object, store the state for that user in the session and then one some future request, you can consult the data in the session to help you respond to the future request.
A simple example would be a shopping cart. Each time you add something to the cart, the data is added to the session object. Then, when the user asks to view the cart, you render a page by consulting the items in that user's session object. If they then ask to "check out", you purchase the items in the cart for them. You don't register and deregister routes for "view cart" and "checkout". Those routes are always registered. Instead, you use the data in the session object to help you process those requests.
If you want to run the same code from several different routes, then you just move that code into a shared function and call that shared function from more than one route handler.