router.post('/myRoute', (req, res, next) => {
    checkParams(req.body).then(paramsResult => {
        if(paramsResult.status === 'failed') return res.send(paramsResult);
        return checkUserEmail(req.body.email);
    }).then(emailResult => {
        if(emailResult.status === 'failed') return res.send(emailResult);
        return checkPasswordLength(req.body.password);
    }).then(passResult => {
        if(passResult.status === 'failed') return res.send(passResult);
        return ...
    }).then(...).catch(err => {
        return res.send({ status:'failed', message:'An error occurred.' });
    });
});

router.post('/myRoute', [checkParams, checkUserEmail, checkPasswordLength, ...], (req, res, next) => {
   // I'd set req.user = {...} in the last middleware before this route.
   createUser(req.user).then(createResult => {
        if(createResult.status === 'failed') return res.send(createResult);
        return res.send({ status:'success', message:'Your account has been created.' });
   }).catch(err => {
        return res.send({ status:'failed', message:'An error occurred.' });
   });
});