Cannot set headers after they are sent to the client backend error

Question

I created a simple backend function which should run through different checks for dates which are sent from the frontend.

 async checkVacation(req: Request, res: Response) {
        try {
            let currentUser = req.user as User
            let dates = this.getBodyDates(req.body)
            if (dates.start == null || req.body.end == null ||this.beforeTodayCheck(dates.start.toString()) ==true ) {
                res.status(400).send("Start and/or End date is invalid")
            }
            let vacationDays = await this.getDBVacationDay(dates.start, dates.end, currentUser.Id)
            res.send(vacationDays)
        } catch (err) {
            console.error(err)
            res.status(400).send(err)
        }
    }

So basically this function should check if the given dates are null and if the given start date is before today. As soon as I provide some invalid dates, the backend gives me those errors

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

I dont know how to handle this error. Greetings, Leo

Answer 1

Putting this into an "answer" rather than a comment. As Ricky Mo stated, this occurs because of the multiple uses of res.send() .

This can be avoided by changing it to return res.send() which will ensure only one will ever get ran.

Cheers.