a prof of concept for "Transfer Objects" or "Parameter Objects"

validator.js

function field() {
    var type = 'string',
        optional = false,
        regexp = null,
        custom = null;

    return {
        'string': function(){ type = 'string'; return this },
        'int': function(){ type = 'number'; return this },
        'optional': function(){ optional = true; return this },
        'required': function(){ optional = false; return this },
        'match': function(exp){ regexp = exp; return this },
        'custom': function(fn){ custom = fn; return this },

        isValid: function(v){
            if (typeof v === 'undefined') return optional;
            if (typeof v !== type) return false;
            if (regexp && !regexp.test(v)) return false;
            if (custom && !custom(v)) return false;
            return true;
        },
        getErrorMessage: function(v){
            if (typeof v === 'undefined' && !optional) 
                return 'Required but missing';
            if (typeof v !== type) 
                return 'Expecting type ' + (type) + ' but got ' + (typeof v);
            if (regexp && !regexp.test(v)) 
                return 'Did not match ' + regexp.toString();
            if (custom && !custom(v)) 
                return 'Did not pass custom validation';

            return this.isValid(v) ? 'Ok' : 'Unknown error';
        }
    };
}

function validate(rules, hashmap){
    var errors = [],
        hashmap = hashmap || {};

    // Augment the hashmap with validation methods
    hashmap.checkRules = function(){
        errors = [];
        for (var name in rules) if (rules.hasOwnProperty(name)) {
            if (!rules[name].isValid(this[name])) {
                errors.push(
                    'Field "' + name + '" is not valid: ' + 
                    rules[name].getErrorMessage(this[name])
                );
            }
        }
        return errors;
    };
    hashmap.isValid = function(){
        errors = this.checkRules();
        return errors.length === 0;
    };
    hashmap.getErrors = function(){
        return errors;
    };

    return hashmap;
}


// Define the validation rules
var rules = {
    name: field().match(/^[A-Za-z]+$/),
    age: field().int().optional().custom(function(v){ return v > 18 && v < 67 })
};

// Assign values by "hashmap"
var req = {
    name: 'Ivan'
};

var obj = validate(rules, req);
// Also assign values 'interactively'
obj.age = 23;

// Validation is OK
if (!obj.isValid()) {
    alert(obj.getErrors());
} else {
    alert('Ok');
}

// Make it fail
obj.name = 'Iván';

if (!obj.isValid()) {
    alert(obj.getErrors());
} else {
    alert('Ok');
}

// Check a single field validation
alert(rules.name.getErrorMessage('Iván'));