Skip to content


Subversion checkout URL

You can clone with
Download ZIP
node module: an object structure predicate builder (make functions to test an object's structure)
Fetching latest commit...
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


an object structure predicate builder (make functions to test an object's structure)

Easily build functions to test properties of objects - useful for validation, verifying data integrity, and application-level schema checking.

Fun fact: tracery "is the stonework elements that support the glass in a Gothic window."


Define a structure with an object mapping between property names and types. Types can be either JavaScript builtins or predicate (boolean-returning) functions - even another tracery function, for easy composition. This is very powerful and lets your describe very complex document structures easily.

    var tracery = require('tracery')
    var Tags = tracery([String])

    var Movie = tracery({
      title: String,
      director: String,
      year: Number,
      genre: String,
      tags: Tags

    var Flavor = tracery({
      sour: Boolean,
      bitter: Boolean,
      sweet: Boolean,
      spicy: Boolean

    var Document = tracery({
      id: String,
      movies: [Movie],
      flavors: tracery.Collection(Flavor)

    var goodDoc = {
      id: 'doc123',
      movies: [{title: 'Born to be born', director: 'Ken Bobson', year: 1982, genre: 'Action', tags: ['cheesy']}],
      flavors: {
        'butter popcorn': {sour: false, bitter: false, sweet: false, spicy: false},
        'wasabi': {sour: false, bitter: false, sweet: false, spicy: true},
        'sweet and sour shrimp': {sour: true, bitter: false, sweet: true, spicy: false}

    // => true

combined types

Using higher order functions (for example, with connective), you can combine predicates to effectively "mix in" multiple document types:

    var and = require('connective').and

    var Person = tracery({
      name: String

    var Employee = and(Person, tracery({
      salary: Number,
      reportsTo: Person

optional and nullable types

Let's say not every employee has a supervisor. tracery has builtin helpers tracery.Optional (which can be the value or undefined) and tracery.Nullable (which can be the value or null):

    var Employee = and(Person, tracery({
      salary: Number,
      reportsTo: tracery.Nullable(Person)

      name: 'bob',
      salary: 10,
      reportsTo: null
    // -> true


Sometimes documents have variable property names, but you'd still like to check that the property values have a specific structure (for example, when using an object as a dictionary or hash table). We can use tracery.Collection:

    var State = tracery({
      capital: String,
      counties: tracery.Collection({seat: String, population: Number})

    var California = {
      capital: 'Sacramento',
      counties: {
        // thanks wikipedia
        'Alameda': {seat: 'Oakland', population: 1510271},
        'Alpine': {seat: 'Markleeville', population: 1175}

    // => true

typed arrays

We can specify that a property should be an array with any number of elements, all of a given type like so:

    var Likes = tracery({
      movies: [Movie],
      songs: [Song]

Empty arrays will match, but sparse arrays will not. In the case that you really need them:

    var SparseLikes = tracery({
      movies: [tracery.Optional(Movie)]

no no, I meant builtin type arrays

oh! Well that's great: we support the following builtin types:

      a: ArrayBuffer,
      b: DataView,
      c: Float32Array,
      d: Float64Array,
      e: Int8Array,
      f: Int16Array,
      g: Int32Array,
      h: Uint8Array,
      i: Uint16Array,
      j: Uint32Array,
      k: Date,
      l: RegExp

typed vectors (tuples)

Arrays with an expected structure are sometimes used for memory or performance reasons to reprent vectors or tuples. For example, a Cartesian coordinate (10, 20) could be represented as an object as {x: 10, y: 20} or as [10, 20]. We can specify vectors, which must match in terms of number of elements and type of element at each position, using tracery.Vector:

    var Point = tracery.Vector([Number, Number])

    var Square = tracery.Vector([Point, Point, Point, Point])

    var Circle = tracery({
      origin: Point,
      radius: Number

See test/example.js for more.

bonus: type diffing

You can require('tracery/diff') to generate objects diffing betweed expected and actual object structures. If there is no difference, it returns false, otherwise it returns an object structure similar to the object under test, with leaves of {actual: type, expected: type, actualValue: value}.

This module is included in the package, but is not loaded by default. It is useful for debugging and for unit tests.

See test/test.diff for a readable example.


$ npm install tracery

running the tests

$ npm install
$ npm test


0.5.0 - add support for builtin typed arrays, Date, and RegExp objects in type signatures 0.4.0 - intial public release



please submit pull requests or issues


MIT (c) 2013 Agile Diagnosis. See

Something went wrong with that request. Please try again.