node package manager


A validation framework based on inline comments


Build Status

Leaf is a simple validation framework for validating models. It parses doc comments out of a constructor function to identify how to validate each property of an object.


Install via NPM: npm install leaf-validation


This library is meant to be used to validate domain objects (i.e. models). To ease the pain of configuration, it uses doc comments in the constructor of your model to determine how to validate its fields.


Say you wanted a user model with the following requirements:

  1. email - must be present, and a valid email address
  2. username - must be present, between 3 and 30 characters and consist of only letters, numbers and the underscore
  3. role - must be exactly one of "admin", "mod" or "user", or not given

You could represent all of these requirements with the following constructor:

function User() {
     * @validator required
     * @validator email
     */ = '';
     * @validator required
     * @validator length(3, 30)
     * @validator regex(/^\w+$/)
    this.username = '';
     * @validator optional
     * @validator values([ 'admin', 'mod', 'user' ])
    this.role = null;

And you could perform validation on it simply like this:

var leaf = require('leaf-validation'),
    userValidator = new leaf.Validator(User),
    user = new User();
user.role = 'foo';
userValidator.validate(, function(err) {
    if (err) {
// the error would be something like: 
    email: [ 'This field is required' ],
    username: [ 'This field is required' ],
    role: [ 'Must be one of "admin", "user" or "mod"' ]

required and optional

All of a property's validators are run during validation unless explicitly stated as optional. In effect, the required validator is implicit, but it's use is recommended so that the correct error message is delivered to the user.

So, if you want to set up validation on a property, but you only want it the validators to be run if the value is actually given, you would use the optional validator.


There are a bunch of built-in validators that will cover most simple cases. You can of course build your own.

Bundled validators:

  • length: validates string length
    • length(3): string length must be greater than or equal to 3
    • length(null, 3): string length must be less than or equal to 3
    • length(3, 5): string length must be between 3 and 5 (inclusive)
  • required: validates that a value must exist. A value exists if it is truthy: the result of !!value is true
    • required(true): does not trim whitespace
  • optional: stops validation if the value is not present; same signature as required
  • regex: validates a string against a regular expression
    • regex(/^\w{3,}$/) - must consist of letters, numbers and underscore, and be at least 3 characters long
  • range: validates a number is within a given range; same signature as length
  • number: validates that a value is a number (using parseFloat())
  • boolean: validates that a value is a boolean
    • boolean: 0, 1, 'true', 'false', true and false are all considered booleans (case insensitive)
    • boolean(true): only allows actual booleans: true and false
  • email: validates that a string is an email address; this is merely a shorthand for regex(/^.+@.+\..+$/)
  • values: validates that a value is one of many values
    • values([ 'foo', 'bar', 'baz' ]): validates if the value is 'foo', 'bar' or 'baz'

Validator factory and custom validators

You can specify a factory object to the Validator constructor, which instructs Leaf on how to create the validators. By default, uses a simple map consisting of the bundled validators (specifically, the map is require('./src/validators')).

If you want to change the map, to include your own validators for example, you could do the following:

var map = leaf.validators; //the default validators 
map.evenInteger = function() {
    return {
        validate: function(value, context, callback) {
            value = parseInt(value);
            var isValid = !isNaN(value) && value % 2 === 0;
            callback(isValid ? null : true);
        getErrorMessage: function() {
            return 'Must be an even integer';
function MyObject() {
    /** @validator evenInteger */
    this.integer = 0;
//create a custom validator factory, and inject it into 
//the Validator constructor 
var factory = new leaf.Factory(map);
var myObjectValidator = new leaf.Validator(MyObject, factory);

Other options

By default, the validator will stop validating a property when it reaches the first error. You can force it to continue validation by passing true as the second argument to validate():

//stop on first error 
userValidator.validate(new User(), function(err) {...});
//don't stop 
userValidator.validate(new User(), true, function(err) {...});


git clone
cd leaf
npm install
npm test