A simple and intuitive object validation library

json, schema, JSON Schema, validation, validator
bower install js-schema



js-schema is a new way of describing object schemas in JavaScript. It has a clean and simple syntax, and it is capable of serializing to/from the popular JSON Schema format. The typical use case is declarative object validation.

Latest release: 1.0.1 (2015/05/06)


Defining a schema:

var Duck = schema({              // A duck
  swim : Function,               //  - can swim
  quack : Function,              //  - can quack
  age : Number.min(0).max(5),    //  - is 0 to 5 years old
  color : ['yellow', 'brown']    //  - has either yellow or brown color

The resulting function (Duck) can be used to check objects against the declared schema:

// Some animals
var myDuck = { swim : function() {}, quack : function() {}, age : 2, color : 'yellow' },
    myCat =  { walk : function() {}, purr  : function() {}, age : 3, color : 'black'  },
    animals = [ myDuck, myCat, {}, /*...*/ ];

// Simple checks
console.log( Duck(myDuck) ); // true
console.log( Duck(myCat)  ); // false

// Using the schema function with filter
var ducks   = animals.filter( Duck );                        // every Duck-like animal
var walking = animals.filter( schema({ walk : Function }) ); // every animal that can walk

It is also possible to define self-referencing data structures:

var Tree = schema({ left : [ Number, Tree ], right : [ Number, Tree ] });

console.log( Tree({ left : 3, right : 3 })                        ); // true
console.log( Tree({ left : 3, right : { left: 5, right: 5 } })    ); // true
console.log( Tree({ left : 3, right : { left: 5, right: 's' } })  ); // false

Error reporting:

  swim: function() {},
  quack: function() {},
  age: 6,
  color: 'green'

// {
//   age: 'number = 6 is bigger than required maximum = 5',
//   color: 'string = green is not reference to string = yellow AND
//           string = green is not reference to string = brown'
// }


Include js-schema in your project with var schema = require('js-schema'); in node.js or with <script src="js-schema.min.js"></script> in the browser. AMD module loading is also supported.

The first parameter passed to the schema function describes the schema, and the return value is a new function called validator. Then the validator can be used to check any object against the described schema as in the example above.

There are various patterns that can be used to describe a schema. For example, schema({n : Number}) returns a validation function which returns true when called with an object that has a number type property called n. This is a combination of the object pattern and the instanceof pattern. Most of the patterns are pretty intuitive, so reading a schema description is quite easy even if you are not familiar with js-schema. Most patterns accept other patterns as parameters, so composition of patterns is very easy.

Extensions are functions that return validator by themselves without using the schema function as wrapper. These extensions are usually tied to native object constructors, like Array, Number, or String, and can be used everywhere where a pattern is expected. Examples include Array.of(X), Number.min(X).

For serialization to JSON Schema use the toJSON() method of any schema (it returns an object) or call JSON.stringify(x) on the schema (to get a string). For deserialization use schema.fromJSON(json). JSON Schema support is still incomplete, but it can reliably deserialize JSON Schemas generated by js-schema itself.


Basic rules

There are 10 basic rules used for describing schemas:

  1. Class (where Class is a function, and has a function type property called schema) matches x if Class.schema(x) === true.
  2. Class (where Class is a function) matches x if x instanceof Class.
  3. /regexp/ matches x if /regexp/.test(x) === true.
  4. [object] matches x if x is deep equal to object
  5. [pattern1, pattern2, ...] matches x if any of the given patterns match x.
  6. { 'a' : pattern1, 'b' : pattern2, ... } matches x if pattern1 matches x.a, pattern2 matches x.b, etc. For details see the object pattern subsection.
  7. primitive (where primitive is boolean, number, or string) matches x if primitive === x.
  8. null matches x if x is null or undefined.
  9. undefined matches anything.
  10. schema.self references the schema returned by the last use of the schema function. For details see the self-referencing subsection.

The order is important. When calling schema(pattern), the rules are examined one by one, starting with the first. If there's a match, js-schema first resolves the sub-patterns, and then generates the appropriate validator function and returns it.


The following example contains patterns for all of the rules. The comments denote the number of the rules used and the nesting level of the subpatterns (indentation).

var Color = function() {}, x = { /* ... */ };

var validate = schema({                    // (6) 'object' pattern
  a : [ Color, 'red', 'blue', [[0,0,0]] ], //     (5) 'or' pattern
                                           //         (2) 'instanceof' pattern
                                           //         (7) 'primitive' pattern
                                           //         (4) 'deep equality' pattern
  b : Number,                              //     (1) 'class schema' pattern
  c : /The meaning of life is \d+/,        //     (3) 'regexp' pattern
  d : undefined,                           //     (9) 'anything' pattern
  e : [null, schema.self]                  //     (5) 'or' pattern
                                           //         (8) 'nothing' pattern
                                           //         (10) 'self' pattern

console.log( validate(x) );

validate(x) returns true if all of these are true:

  • x.a is either 'red', 'blue', an instance of the Color class, or an array that is exactly like [0,0,0]
  • x.b conforms to Number.schema (it return true if x.b instanceof Number)
  • x.c is a string that matches the /The meaning of life is \d+/ regexp
  • x doesn't have a property called e, or it does but it is null or undefined, or it is an object that matches this schema

The object pattern

The object pattern is more complex than the others. Using the object pattern it is possible to define optional properties, regexp properties, etc. This extra information can be encoded in the property names.

The property names in an object pattern are always regular expressions, and the given schema applies to instance properties whose name match this regexp. The number of expected matches can also be specified with ?, + or * as the first character of the property name. ? means 0 or 1, * means 0 or more, and + means 1 or more. A single * as a property name matches any instance property that is not matched by other regexps.

An example of using these:

var x = { /* ... */ };

var validate = schema({
  'name'             : String,  // x.name must be string
  'colou?r'          : String   // x must have a string type property called either
                                // 'color' or 'colour' but not both
  '?location'        : String,  // if x has a property called 'location' then it must be string
  '*identifier-.*'   : Number,  // if the name of a property of x matches /identifier-.*/ then
                                // it must be a number
  '+serialnumber-.*' : Number,  // if the name of a property of x matches /serialnumber-.*/ then
                                // it must be a number and there should be at least one such property
  '*'                : Boolean  // any other property that doesn't match any of these rules
                                // must be Boolean

assert( validate(x) === true );


The easiest way to do self-referencing is using schema.self. However, to support a more intuitive notation (as seen in the Tree example above) there is an other way to reference the schema that is being described. When executing this:

var Tree = schema({ left : [ Number, Tree ], right : [ Number, Tree ] });

js-schema sees in fact { left : [ Number, undefined ], right : [ Number, undefined ] } as first parameter, since the value of the Tree variable is undefined when the schema function is called. Consider the meaning of [ Number, undefined ] according to the rules described above: 'this property must be either Number, or anything else'. It doesn't make much sense to include 'anything else' in an 'or' relation. If js-schema sees undefined in an or relation, it assumes that this is in fact a self-reference.

Use this feature carefully, because it may easily lead to bugs. Only use it when the return value of the schema function is assigned to a newly defined variable.



There are five functions that can be used for describing number ranges: min, max, below, above and step. All of these are chainable, so for example Number.min(a).below(b) matches x if a <= x && x < b. The Number.step(a) matches x if x is a divisible by a.


The String.of method has three signatures:

  • String.of(charset) matches x if it is a string and contains characters that are included in charset
  • String.of(length, charset) additionally checks the length of the instance and returns true only if it equals to length.
  • String.of(minLength, maxLength, charset) is similar, but checks if the length is in the given interval.

charset must be given in a format that can be directly inserted in a regular expression when wrapped by []. For example, 'abc' means a character set containing the first 3 lowercase letters of the english alphabet, while 'a-zA-C' means a character set of all english lowercase letters, and the first 3 uppercase letters. If charset is undefined or null then there's no restriction on the character set.


The Array.like(array) matches x if x instanceof Array and it deep equals array.

The Array.of method has three signatures:

  • Array.of(pattern) matches x if x instanceof Array and pattern matches every element of x.
  • Array.of(length, pattern) additionally checks the length of the instance and returns true only if it equals to length.
  • Array.of(minLength, maxLength, pattern) is similar, but checks if the length is in the given interval.


Object.reference(object) matches x if x === object.

Object.like(object) matches x if x deep equals object.


Function.reference(func) matches x if x === func.

Future plans

Better JSON Schema support. js-schema should be able to parse any valid JSON schema and generate JSON Schema for most of the patterns (this is not possible in general, because of patterns that hold external references like the 'instanceof' pattern).


Feel free to open an issue or send a pull request if you would like to help improving js-schema or find a bug.

People who made significant contributions so far:


Using npm:

npm install js-schema

Using bower:

bower install js-schema


To build the browser version you will need node.js and the development dependencies that can be installed with npm (type npm install ./ in the project directory). build.sh assembles a debug version using browserify and then minifies it using uglify.


The MIT License

Copyright (C) 2012 Gábor Molnár gabor@molnar.es