how can we pass parameter to custom express-validator? - node.js

Question

I would like to prevent a registration with an email address which already exists. Is it possible to use express-validator's new syntax for this? For example:
router.post('/register', [
check('email').custom((value, {req}) => {
return new Promise((resolve, reject) => {
Users.findOne({email:req.body.email}, function(err, user){
if(err) {
reject(new Error('Server Error'))
}
if(Boolean(user)) {
reject(new Error('E-mail already in use'))
}
resolve(true)
});
});
})
]
....
How would i pass Users?

Answer 1

express-validator is only aware of the request object itself, what keeps its complexity pretty low for the end-user.
More importantly, it only truly knows about the request's input locations -- body, cookies, headers, query and params.
Your custom validator is completely correct. That being said, it might not be testable, as you seem to be depending on global context.
In order to make it testable, the 2 options that I see are:
1. Inject req.Users:
This one would involve using some middleware that sets your store objects onto req:
// Validator definition
const emailValidator = (value, { req }) => {
return req.Users.findOne({ email: value }).then(...);
}
// In production code
// Sets req.Users, req.ToDo, req.YourOtherBusinessNeed
app.use(myObjectsStore.middleware);
app.post('/users', check('email').custom(emailValidator));
// In tests
req = { Users: MockedUsersObject };
expect(emailValidator('foo#bar.com', { req })).rejects.toThrow('email exists');
2. Write a factory function that returns an instance of your validator:
This is my preferred solution, as it doesn't involve using the request object anymore.
// Validator definition
const createEmailValidator = Users => value => {
return Users.findOne({ email: value }).then(...);
};
// In production code
app.post('/users', [
check('email').custom(createEmailValidator(myObjectsStore.Users)),
]);
// Or in tests
expect(createEmailValidator(MockedUsersObject)('foo#bar.com')).rejects.toThrow('email exists');
Hope this helps!

Answer 2

Converting my comments into a final, conclusive answer here :
A validator is simply supposed to validate the fields of request entities against the given criteria of data type / length / pattern.
You would need to write the method yourself, to determine if the user pre-exists or not. An express-validator ( or rather any validator ) would not do the task of cherry picking if the item exists in your list of items ( or your data-source), neither should it interact with the data-source concerned.