A simple JSON validation package (subset of JSON Schema)






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json-validate is a small library that performs, shockingly enough, validation of JSON documents. It supports a limited subset of JSON Schema.


The usual simple:

npm install json-validate

In order to run in a browser, you simply need to include the json-validate.js file, after having included underscore.js on which it depends.

Why not JSON Schema, one of the existing implementations?

The reason this library exists is because I initially needed JSON validation in a project. I tried to use some of the JSON Schema implementations that exist, but most were out of date. Those that weren't tended to break in the contexts in which I was using them, and were generally rather larger and more complex than my needs. It turned out to be simpler to just write a validator that matches my needs.

I release this library on the assumption that if I needed it, others might too. I may add support for more of JSON Schema, and I will certainly take pull requests (so long as they don't cause excessive bloat).

One notable difference is that JSON Schema support schema referencing one another. Doing that is an explicit non-goal of this library. But it ought to be easy to implement JSON referencing separately (in another small library) and feed schemata with references resolved into json-validate.


The API is very simple, the following example probably tells you all you need to know:

var jv = new JSONValidate();
var result = jv.validate(object, schema);
if (result.ok) {
    // victory \o/
else {
    console.log("JSON has the following errors: " + result.errors.join(", ") + " at path " + result.path);

var jv = new JSONValidate()

A simple constructor that takes no arguments.

var result = jv.validate(object, schema);

This takes an object that is parsed JSON (or any in-memory equivalent) and a schema that corresponds to the subset of JSON Schema described in the following section. Note that validate() will throw if you schema is invalid.

The return value is an object with the following fields:

  • ok: true if the JSON is valid, false otherwise.
  • errors: a list of human-readable strings describing the errors that were encountered. In most cases json-validate will only return one single error as it does not currently try to keep processing the JSON when it finds a problem, but in some cases it can return several errors at once, and this is likely to increase going forward (though likely limited to finding several problems with just one item). If there were no errors this array is empty.
  • path: a string indicating the path in the object at which the errors were found. It looks like $root.field1.field2[5]. The idea is that if you replace $root with the object you passed you will get to the problematic value (this is intended for human debugging though).

Supported schema constructs

A schema is basically a nested structure of objects that describe the constraints on a JSON instance at that nesting level. Each of these objects is keyed off a specific fundamental type, and the constraints that can be applied to the value depend on that fundamental type.


The object is null.

Example schema:

{ "type": "null" }

No additional constraints.


Absolutely anything goes here.

Example schema:

{ "type": "any" }

No additional constraints.


The object is a boolean value (true or false). Note that this is strictly for booleans, and not for values that are truthy or falsy.

Example schema:

{ "type": "boolean" }

Other example:

    "type": "boolean"
,   "enum": [true]

Additional constraints:

  • enum: The object must match one of the values in the enum array. Since the boolean type only takes two values this constraint is only ever useful if you wish to restrict a value to either always being true or always being false (as in the second example above).


The object is a string. A number of constraints on length, patterns, and enumerations apply. Note that these are strictly strings and not objects that stringify.

Example schemata:

{ "type": "string" }

    "type": "string"
,   "enum": ["carrot", "banana", "donkey"]

    "type": "string"
,   "pattern": "a{3}\\d\\d"

    "type": "string"
,   "minLength": 2
,   "maxLength": 17

Additional constraints:

  • enum: The object must match one of the values in the enum array. All of these must be strings.
  • pattern: A regular expression that the string must match. Note that since it is conveyed as a string you will need to escape backslashes. Additional constraints:
  • minLength and maxLength: The minimal and maximal length of the string (inclusive).


The object is a number. It can be further constrained on enumeration, minimum, and maximum values.

Example schemata:

{ "type": "number" }

    "type": "number"
,   "enum": [5, 17, 23, 42]

    "type": "number"
,   "minimum": 23
,   "exclusiveMaximum": 57.2

Additional constraints:

  • enum: The object must match one of the values in the enum array. All of these must be numbers.
  • minimum, maximum, exclusiveMinimum, and exclusiveMaximum: minimal and maximal boundaries on the number's value, either inclusive or exclusive.


The object is, well, an object. Its properties can be enumerated and themselves recursively defined and constrained.

Example schemata:

{ "type": "object" }

    "type": "object"
,   "properties": {
        "fullName":  { "type": "string", "required": true }
    ,   "age":       { "type": "number", "minimum": 0 }
    ,   "superHeroIdentity": {
            "type": "object"
        ,   "coolName": { "type": "string" }
        ,   "superPower": {
                "type": "string"
            ,   "enum": ["flying", "telekinesis", "parsing MIME"]

Additional constraint:

  • properties: This is a simply an object the keys of which are those that are being constrained on the object (object keys not listed here are not only allowed but also unconstrained). The values for those keys are the types of the values for the matching fields in JSON instances.

In addition to its regular type information, each property value can also take a boolean constraint called required. If set to true, then this field must be present in the instance (the default is for it to be optional).


The object is an array. This can be further constrained with minimal and maximal lengths, as well as with constraints on the types of the objects contained in the array.

Example schemata:

{ "type": "array" }

    "type":         "array"
,   "minItems":     3
,   "maxItems":     200
,   "items":        { "type": "number" }
,   "uniqueItems":  true

    "type":             "array"
,   "items":            [ { "type": "number" }, { "type": "string" }, { "type": "string" } ]
,   "additionalItems":  true

Additional constraints:

  • minItems and maxItems: The minimal and maximal length of the array, inclusive.
  • items (with a type): When items is a type definition, then all array members will be validated against that type.
  • items (with an array): When items is an array of type definitions, then the array members at a given offset will be validated against the type definition at the same offset. The array instance is implicitly constrained to be of the same length as the items constraint, unless additionalItems is specified.
  • additionalItems: In the case of an array-based items, a boolean that allows for additional array members after those constrained by the provided types. Those additional members are not validated.
  • uniqueItems: If true, checks that the values in the array are unique (using ===, recursively).

Union types

It is also possible to define union types, which is to say, cases in which validation will be tried against multiple type definitions until one matches or the list of options in the union is exhausted.

Example schemata:

{ "type": ["number", "string"] }

    "type": [
        { "type": "string", "pattern": "^\\w+$" }
    ,   { "type": "array", "minItems": 3 }
    ,   "boolean"

A union is defined by providing an array of options for the type. Each item in that array may be either just the string name of a fundamental type, or a full-fledged type definition. If none of the types match, then the validation fails.

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