https://github.com/C2FO/c2fo/blob/develop/api/lib/registration/routes.js#L50-L62

api_handler_using_null.js

// promise handles user not found by returning `null`
// advantage - don't have to write a reject handler
// disadvantage - requires developer to know the implementation detail of the facade function
//                that it returns `null` or not. Higher possibility of forgeting to write `null`
//                check and cause null pointer exception
function checkEmail(emailAddress) {
    return facade.user.getUserByEmailAddress(emailAddress).chain(function (user) {
        if (!user) {
            return {available: true};
        }

        return facade.user.isUserAnInvitedTaker(user).chain(function (isInvited) {
            return isInvited ? {invited: true} : {available: false}
        });
    }).chain(res.handler.successHandler(), res.handler.errorHandler(errors.UNKNOWN_ERROR));
}

api_handler_using_reject.js

// promise handles user not found by rejecting the promise and returning an error
// advantage - success handler handles the common case and never causes a null pointer exception
// disadvantage - recovering from `not found` is explicit and has to occur in reject handler
//                requires better understanding of errors in the case we need to recover from the errors
function checkEmail(emailAddress) {
    return facade.user.getUserByEmailAddress(emailAddress).chain(function (user) {
        return facade.user.isUserAnInvitedTaker(user).chain(function (isInvited) {
            return isInvited ? {invited: true} : {available: false};
        });
    }, function(err) {
        if (err.objectNoFound()) {
            return {available: true};
        } 
        
        return comb.rejected(err);
    }).chain(res.handler.successHandler(), res.handler.errorHandler(errors.UNKNOWN_ERROR));
}

facade.js

function getUserByEmailAddress(email) {
    return User.first({emailAddress: email}).chain(function(user) {
        if (!user) {
            return ErrorPromises.NotFound(`No user found for emailAddress = ${email}`)
        }

        return user;
    });
}

original_file.js

// https://github.com/C2FO/c2fo/blob/develop/api/lib/registration/routes.js#L50-L62

function checkEmail(emailAddress) {
    return facade.user.getUserByEmailAddress(emailAddress).chain(function (user) {
        if (user) {
            return facade.user.isUserAnInvitedTaker(user).chain(function (isInvited) {
                if (isInvited) {
                    res.handler.success({invited: true});
                }else {
                    res.handler.success({available: false});
                }
            });
        } else  {
            res.handler.success({available: true});
        }
    }, res.handler.errorHandler(errors.UNKNOWN_ERROR));
}

Regex to find .chain with if condition as the first line => chain\s*\(\s*function\s*\(\s*([^)]+?)\s*\)\s*{\n*\s*if