Validating req.params with express validator

Params are parsed as string by express middleware. Say I make a req to path defined below like /some/1000

app.get('/some/:path', (req, res, next) => {  console.log(typeof req.param.path)  // outputs string})

So you need to parse the incoming parameter to integer (Number) since you've stored accountNumber as integer. So adding toInt to chain like below should solve it:

const validateReq: [  param('accountNumber').exists().toInt().custom(acctNo => accountNumberExist(acctNo)),]

accountNumber inside accounts array is a number whereas req.params.accountNumber is a string. You need to convert the data type. You can do it as

    accountNumberExist(inputAcct) {        const isfound = accounts.find(account => account.accountNumber.toString() === inputAcct);        if (isfound === undefined) throw new Error('Account Number not found');  }

I think that the problem is your query. find method runs in an asynchronous way, that's why isfound property does not contain the data you expect. Here is a simple approach using promises which works pretty well for me.

// Here is your function.accountNumberExist(inputAcct) {    return accounts.find({accountNumber: inputAcct})    .then(result => {        if (result.length == 0) {            console.log("Account Number not found");            return Promise.reject('Account Number not found');        }        return Promise.resolve();    });}