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    0.5.0 • Public • Published


    IMPORTANT!: Direct support for YAML has been dropped starting with this release (v0.5.0). Using YAML with straints moving forward is documented in the What Else? section below. In addition, there are a few other breaking changes in this release. Please see the changelog for details.

    What is this?

    Straints is a javascript object validator. Validation rules are declaratively defined in a single JSON or YAML file.

    For example:

        name: [ exists ]
        email: [ exists, email ]

    With the above (YAML) configuration, executing a straints validation with a 'create_user' context will ensure that a JS object has name and email properties and that email is a valid email.

    The goal of this project is to provide easy setup and maintenance of validation rules across an entire web or node application.


    • Manage site-wide front and back end validations from a single file
    • Reusable validation contexts and constraint configurations
    • Validate object properties against each other
    • Validate nested objects or nested arrays of objects
    • Conditional context and constraint validation
    • Function and/or Promise based async validation methods supported

    Awesome. How do I get it?


    npm install straints --save


    bower install straints --save

    What's the Setup?

    For Node/Express

    var express = require('express');
    var straints = require('straints');
    straints.configure({ ... });
    var app = express();

    The app.use() call simply adds the straints instance to the request object. So, in your controllers you can do

    req.straints.validate(req.body, "create_user").then(
            if (results.valid())
                // all validations passed
                // one or more validations failed
            // errors occurred during validation

    Note that .validate() will only return a Promise instance if an implementation has been provided in the configuration, otherwise a 'then' function is returned (e.g, function(success, failure)).

    To get at the 'default' instance (provided through straints.usage) use

    var instance = straints.getInstance();

    To create a new instance (and pass config options as necessary).

    var instance = straints.newInstance({ ... });

    The config options passed will be merged atop those originally passed to .configure() for the new instance.

    As a convenience you can

    var validate = instance.validator;

    to get the configured validator implementation for the instance.

    For the Browser

    The js for the browser can be found in /dist.

    <script src=".../dist/straints.min.js"></script>

    Or, if you wish to use the default validator:

    <script src=".../dist/straints-alyze.min.js"></script>

    Then you can do

        var instance = straints.configure({ ... }).getInstance();    

    How does all this work?

    Configuration Options

    The config options and their defaults.

    • load (null)
      Specify the validation schema data here. Here are the options:

      1. (function)
        Must be of the form function(callback) { callback(data); }, where 'data' is the VSD object.

      2. (object)
        The VSD object itself.

    • validator (alyze instance)
      Specify the test method implementation(s) to use here (See Validators section).

    • promise (null)
      Specify the promise implementation class to use here. If none specified a 'then' style method is returned from .validate().

    • levels ('constrain')
      Array or comma-delimited string of named validation levels that can be used in the validation schema. Note that 'constrain' is always required and will be prepended to whatever is specified here.

    • log (null)
      There is some logging available. Specify the log function here (.configure() only).

    Validation Schema Definition (VSD)

    Let's start with an example:

        name: [ is.notNull ]
        email: [ is.notNull, email ]
        include: [ person ]
          position: [ is.basketballPosition ]
            include: [ person ]
                include: [ basketball.player ]
          name: [ is.notNull ]
          coach: [ is.notNull ]
          players: [ is.notNull ]
      - { name: notNull, test: 'null', flip: true }
      - { name: basketballPosition, test: itemIn, params: [ [ point, guard, forward, water ] ] }


    A 'context' defines how an object will be validated. If a path level in the VSD has constrain, include, or nested child directives, or a validation level specified in levels in the configuration, then it can be used as a context.

    In the above our contexts are:

    • person
    • basketball.player

    The constrain directive applies rules by property name. These rules are required if the context is included in the validation session.

    For instance, the 'person' context requires that:

    • 'name' is not null
    • 'email' is not null
    • 'email' is a valid email address

    It is also possible to specify property validations by rule.

    For example, to get the same effect for the 'person' context, you could do

        ~is.NotNull: [ name, email ]
        ~email: [ email ]

    Simply prefix the constraint with a tilde (~) so the straints parser knows to treat it as a rule rather than a property.

    Additional Validation Levels (VLs)

    It is possible to specify additional VLs in the VSD. First you have to register the level name(s) you wish to use in configuration (levels) so it will get picked up. Then you can simply use the name as a 'context directive' and specify validations in the same way you can with constrain.

    Note that you cannot use include, nested, or constrain as VL names since they are already reserved as context directives.


    A context can include other contexts by listing their names in an array.

    In the above example, the 'basketball.player' context includes the 'person' context. This means that all of the configuration for the 'person' context now also applies to 'basketball.player' as well.

    Conditional inclusion at the context level is also available.

    For instance:

      - { if: [ greatShooter, greatPasser ], then: starter, else: benchwarmer }

    If you had team tryouts and a person object validated as 'greatShooter' and 'greatPasser', then you might want to try to validate as 'starter' as well. Otherwise, 'benchwarmer' may be more appropriate.

    Here are the properties for the include condition object:

    • name (string)
      If you wish to reference the condition elsewhere in the VSD this makes it much easier.

    • if (array,string)
      The contexts that will be executed to resolve the condition. On success, then contexts will be included if provided. On failure, else contexts will be included if provided.

    • then (array,string)
      The contexts that will be included when the if condition succeeds. If there is no if then inclusion is automatic.

    • else (array,string)
      The contexts that will be included when the if condition fails. If there is no if then this is ignored.


    If you need to validate an object that includes another object, you can use the nested context directive. This works similarly to constrain except that you are specifying new contexts for each property rather than constraints.

    In the above example, a '' object must have a 'coach', as specified by the constrain directive of that context. But, we also have

        include: person

    This means that we want to validate the 'coach' property as an object, and so here we specify a context for that property that, in this case, includes the context we wish to validate against ('person').

    The Quadruple Underscore

    While nested alone works well for single objects, what if you want to validate a list of objects?

    Here's the nested 'players' part of the '' context.

            include: basketball.player

    We nest again inside of 'players' to get at the objects in the list. Then you can use the ____ value to apply the context to every element of the array.

    This also works on an object of objects as well. Just remember that ____ will apply its contextual validation rules to EVERY object found as a property value of the parent object.


    Constraints can be defined anywhere in the VSD, but wherever they appear they should be in an array. There are 3 ways to specify a constraint in the VSD.

    1. Validator Methods

    The simplest way is to specify the name of a test method in the validator implementation.

    email: [ email ]
    # or
    ~email: [ email ]

    Either of the above ensures that the 'email' property has a valid email address.

    You may also prefix a test method with the name of another object property by separating them with a colon (:).

    email: [ property:testMethod ]
    # or
    ~property:testMethod: [ email ]

    Here 'property' is validated against testMethod with the result being applied to 'email'.

    2. Constraint References

    You can include an individual constraint by specifying the path to it in the VSD.

        size: [ is.notNull ]
      - { name: notNull, test: 'null', flip: true }

    You can also reference an array of constraints.

        size: [ sizes ]
      - { name: isNotNull, test: 'null', flip: true }
      - { test: itemIn, params: [ [ small, medium, large ] ] }

    Like validator methods you can prefix a constraint reference with a property name and also specify a rule by constraint reference.

    3. Constraint Objects

    Finally, you can create a constraint definition. Its unique identifier is the path to it in the VSD.

      - { name: notNull, test: 'null', flip: true }

    Define constraint objects using these attributes:

    • name (string)
      If you wish to reference the constraint elsewhere in the VSD this makes it much easier.

    • test (string)
      A rule or rule expression that defines the constraint (See Constraint Rule Expressions).

    • if (string)
      A rule or rule expression that will determine if test is executed.

    • param (array)
      Arguments passed as a single parameter to ALL validator methods appearing in test.

    • params (array)
      Arguments passed to ALL validator methods appearing in test. Ignored if param is given

    • flip (boolean)
      Set to true to reverse the boolean value ultimately returned by test.

    • payload (any)
      Arbitrary data to pass along with the constraint into the results object.

    Constraint Rule Expressions

    A rule expression is made up of validator methods or constraint references separated by logic gates.

    - { name: color, test: hexadecimal or (number and not negative) }

    The above defines a couple of ways a constraint can validate a color value using a rule expression.

    Most logic gates sit between two boolean values or 'sides' of an expression. There is no "operator precedence" here, the logic is simply evaluated from left to right. Use parentheses to get the results desired.

    The following logic gates are available.

    • and
      Both sides must be true.

    • or
      At least one side must be true.

    • nor
      Both sides must be false.

    • nand
      At least one side must be false.

    • xnor
      Both sides must be the same boolean value.

    • xor
      Both sides must not be the same boolean value.

    • not
      Negates the boolean value after it.

    Let's revisit property name prefixes now.

        - { test: itemIn, param: [ hex, rgb, named ] }
        - { test: (color_type:is.hex and hexadecimal) or (color_type:is.named and in.colors) }
      - { name: hex, test: equal, params: 'hex' }
      - { name: named, test: equal, params: 'named' }
      - { name: colors, test: itemIn, param: [ yellow, red, gold ] }

    Here we validate 'color' to be hexadecimal when 'color_type' is 'hex' or as a named color value when 'color_type' is 'named'.

    Here's an alternative way to validate 'color'.

      - { if: color_type:is.hex, test: hexadecimal }
      - { if: color_type:is.named, test: in.colors }

    One difference here is that 'color' won't fail if 'color_type' is 'rgb' because using an if in the constraint means that test only runs if the condition is met.

    Note that the expression syntax available to test can also be used in the if condition.

    Using a constraint array reference in a rule expression is illegal and will cause an error. It is also not advisable to reference an if constraint in a rule expression because if its test is not run the expression will fail.

    Parameter Property References

    Suppose you want to use data from the target object as a parameter to a test method.

    For example, an email and its confirmation:

        - email
        - { test: equal, params: }

    Nifty, eh?

    The target objects are exposed like so:

    • s - session target
      This is the full object being validated, the one passed to instance.validate().

    • t - current target
      This is the current object being validated. It will be the same as s, unless the current object is nested.

    Only dot-notation and alphanumeric (plus underscores) property names are supported when specifying a parameter dependency in this manner.

    To do something more advanced you can use %{ ... } interpolation syntax.

    - { test: equal, params: "%{t['email']}" }
    The Quadruple Underscore (again)

    The ____ can also be used in the realm of constraints.

        - is.notNumeric
        - email
        - is.address

    Here, the ____ applies its constraints to all properties of the object. This means that the 'email' and 'address' (and ALL other) properties on the validation target must not be numeric.

    Consequently, this also works for the same

        - ____

    Running a Validation

    instance.validate(target, contexts, validate): Promise|Function
    • target (object)
      The object to be validated

    • contexts (string or array)
      One or more contexts to be validated against

    • validate (boolean function(result, info))
      Called for every constraint test during the validation session

      • result (boolean)
        validation result

      • info (object)
        additional information pertaining to the validation test.

        • target (object)
          target object (same as starget unless nested)

        • starget (object)
          session target object (the one originally passed to .validate())

        • name (string)
          target object property name

        • sname (string)
          session target object property name

        • rule (object)
          validating constraint object

        • level (string)
          the validation level

    Validation Results

    The validation results object will be passed to either of the callbacks provided to the Promise or 'then' function returned by .validate(). Here's a rundown on the information available in this object.

    • target
      The JS or JSON object that validation was run against.

    • contexts
      The contexts involved in the validation session.

      The identifiers (by property) of the constraints whose tests were required at validation level 'xxx'.

    • constraints
      The constraints involved in the validation session indexed by their identifiers.

    • isComplete
      A boolean value that will be true if validation completed successfully.

    • error
      This will be null far successful validation or contain some information on failure.

    • findConstraints(property, level, value) & findProperties(constraint, level, value)
      Functions that return an array of constraint/property names by property/constraint, validation level, and value.

      • property (string)
        Object property name to get constraints for. If not given then all properties are checked.

      • constraint (string)
        Constraint identifier to get properties for. If not given then all constraints are checked.

      • level (string)
        The validation level to check. If not given then constrain is assumed.

      • value (boolean|null)
        Value to check for. This can only be true, false, or null. If not given then false is assumed.

    • validFor(level)
      For the given validation level, returns true if all tests passed, false if any failed, and null if none were run or if the level does not exist.

    • valid()
      Returns true if validation completed successfully and no constrain tests failed.

    • payload(cname)
      Returns payload data for the named constraint.


    As far as straints is concerned, a validator is simply a JS object with test methods and/or child objects with test methods.

    straints is bundled and configured with alyze as its validator.

    straints.configure({ validator: alyze });

    But you can use something else or configure this however you like.

    straints.configure({ validator: myTestMethods });
    // OR
    straints.configure({ validator: { alyze: alyze, async: someAsyncMethods, other: myTestMethods } });

    If your validator is setup like the second example above then in the VSD you would reference methods like this:

    - { name: required, test: not alyze.missing }

    A test method is called with the owning object as its this reference. So here, missing is called with the alyze object as this.

    Validation Methods

    Validation methods (or test methods) should be of the following form:

    testMethod(value: any, ...args: any[]): boolean|Function|Promise

    The test method should accept the test value as the first argument and may also accept or require additional arguments.

    If the return value is

    • a Promise its .then() is called with success and error callbacks
    • a function it is called with success and error callbacks
    • a boolean then it is handled as a successful validation

    If the value returned is none of the above then it is an error.

    What Else?

    Using YAML VSDs

    Starting with v0.5.0, direct support for loading YAML files has been dropped. This was done due to the wealth of code bundlers and build processors in the JS ecosystem that can easily include a YAML parser alongside straints in your app.

    The YAML VSD examples in the documentation remain as it is easier to read and digest than the equivalent JSON. As such, it is fitting that using YAML with straints be documented here as well.

    In previous straints versions, the 'js-yaml' module was used.

    npm install js-yaml --save

    Once installed, you can configure straints with a load function.

    var fs = require('fs');
    var jsyaml = require('js-yaml');
    var straints = require('straints');
    var vsdFile = 'validation.yaml';
        load: function(callback)
            fs.readFile(vsdFile, 'utf8', function(error, data)
                if (!error) callback(jsyaml.safeLoad(data));

    This is almost exactly how straints did it before. Be sure to handle any filesystem errors.

    You can use this pattern to load just about any type of file that can be converted to JSON.


    { dist files } { updates } { feedback } { license } { versioning }


    The goal is to eventually have a small website for straints complete with examples, better documentation, and possibly some other doodads. Until then, have a look at the /test folder for some decent examples.


    npm test


    Codacy Badge


    Happy Validating!


    npm i straints@0.5.0





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