1.0.3 • Public • Published


    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, you want to assert that an object property is exactly the value null. The null builtin type matches exactly the value null:

    var Empty = tracery({
      value: null
      value: 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

    A Collection assumes keys are strings (like JavaScript objects).

    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

    Vector 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


    There's a shortcut for making a predicate to assert instanceof:

        var Foo = function () {}
        var foo = new Foo()
        var isFoo = tracery.InstanceOf(Foo)
        // => true

    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


    jden Zalastax

    please submit pull requests or issues


    MIT (c) 2015 Jason Denizac. See


    npm i tracery

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