The fastest JSON Schema Validator

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Defining custom keywords

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Define keyword with validation function

Validation function will be called during data validation and it will be passed:

The access to the parent data object and the current property name allow to create keywords that modify the validated data (modifying option MUST be used in keyword definition in this case).

The function should return validation result as boolean. It can return an array of validation errors via .errors property of itself (otherwise a standard error will be used).

This way to define keywords is useful for:

Please note: In cases when validation flow is different depending on the schema and you have to use ifs, this way to define keywords will have worse performance than compiled keyword returning different validation functions depending on the schema.

Example. constant keyword (a synonym for draft6 keyword const, it is equivalent to enum keyword with one item):

ajv.addKeyword('constant', { validate: function (schema, data) {
  return typeof schema == 'object' && schema !== null
          ? deepEqual(schema, data)
          : schema === data;
}, errors: false });

var schema = { "constant": 2 };
var validate = ajv.compile(schema);
console.log(validate(2)); // true
console.log(validate(3)); // false

var schema = { "constant": { "foo": "bar" } };
var validate = ajv.compile(schema);
console.log(validate({foo: 'bar'})); // true
console.log(validate({foo: 'baz'})); // false

const keyword is already available in Ajv.

Please note: If the keyword does not define custom errors (see Reporting errors in custom keywords) pass errors: false in its definition; it will make generated code more efficient.

To add asynchronous keyword pass async: true in its definition.

Define keyword with “compilation” function

Compilation function will be called during schema compilation. It will be passed schema, parent schema and schema compilation context and it should return a validation function. This validation function will be passed during validation:

The access to the parent data object and the current property name allow to create keywords that modify the validated data (modifying option MUST be used).

The function should return validation result as boolean. It can return an array of validation errors via .errors property of itself (otherwise a standard error will be used).

In some cases it is the best approach to define keywords, but it has the performance cost of an extra function call during validation. If keyword logic can be expressed via some other JSON-schema then macro keyword definition is more efficient (see below).

All custom keywords types can have an optional metaSchema property in their definitions. It is a schema against which the value of keyword will be validated during schema compilation.

Example. range and exclusiveRange keywords using compiled schema:

ajv.addKeyword('range', { type: 'number', compile: function (sch, parentSchema) {
  var min = sch[0];
  var max = sch[1];

  return parentSchema.exclusiveRange === true
          ? function (data) { return data > min && data < max; }
          : function (data) { return data >= min && data <= max; }
}, errors: false, metaSchema: {
  type: 'array',
  items: [ { type: 'number' }, { type: 'number' } ],
  additionalItems: false
} });

var schema = { "range": [2, 4], "exclusiveRange": true };
var validate = ajv.compile(schema);
console.log(validate(2.01)); // true
console.log(validate(3.99)); // true
console.log(validate(2)); // false
console.log(validate(4)); // false

See note on custom errors and asynchronous keywords in the previous section.

Define keyword with “macro” function

“Macro” function is called during schema compilation. It is passed schema, parent schema and schema compilation context and it should return another schema that will be applied to the data in addition to the original schema.

It is the most efficient approach (in cases when the keyword logic can be expressed with another JSON-schema) because it is usually easy to implement and there is no extra function call during validation.

In addition to the errors from the expanded schema macro keyword will add its own error in case validation fails.

Example. range and exclusiveRange keywords from the previous example defined with macro:

ajv.addKeyword('range', { type: 'number', macro: function (schema, parentSchema) {
  return {
    minimum: schema[0],
    maximum: schema[1],
    exclusiveMinimum: !!parentSchema.exclusiveRange,
    exclusiveMaximum: !!parentSchema.exclusiveRange
  };
}, metaSchema: {
  type: 'array',
  items: [ { type: 'number' }, { type: 'number' } ],
  additionalItems: false
} });

Example. contains keyword from version 5 proposals that requires that the array has at least one item matching schema (see https://github.com/json-schema/json-schema/wiki/contains-(v5-proposal)):

ajv.addKeyword('contains', { type: 'array', macro: function (schema) {
  return { "not": { "items": { "not": schema } } };
} });

var schema = {
  "contains": {
    "type": "number",
    "minimum": 4,
    "exclusiveMinimum": true
  }
};

var validate = ajv.compile(schema);
console.log(validate([1,2,3])); // false
console.log(validate([2,3,4])); // false
console.log(validate([3,4,5])); // true, number 5 matches schema inside "contains"

contains keyword is already available in Ajv with option v5: true.

See the example of defining recursive macro keyword deepProperties in the test.

Define keyword with “inline” compilation function

Inline compilation function is called during schema compilation. It is passed four parameters: it (the current schema compilation context), keyword (added in v3.0 to allow defining multiple keywords with a single function), schema and parentSchema and it should return the code (as a string) that will be inlined in the code of compiled schema. This code can be either an expression that evaluates to the validation result (boolean) or a set of statements that assigns the validation result to a variable.

While it can be more challenging to define keywords with “inline” functions, it has several advantages:

Example even keyword:

ajv.addKeyword('even', { type: 'number', inline: function (it, keyword, schema) {
  var op = schema ? '===' : '!==';
  return 'data' + (it.dataLevel || '') + ' % 2 ' + op + ' 0';
}, metaSchema: { type: 'boolean' } });

var schema = { "even": true };

var validate = ajv.compile(schema);
console.log(validate(2)); // true
console.log(validate(3)); // false

'data' + (it.dataLevel || '') in the example above is the reference to the currently validated data. Also note that schema (keyword schema) is the same as it.schema.even, so schema is not strictly necessary here - it is passed for convenience.

Example range keyword defined using doT template:

// 
var doT = require('dot');
var inlineRangeTemplate = doT.compile("\
{{ \
  var $data = 'data' + (it.dataLevel || '') \
    , $min = it.schema.range[0] \
    , $max = it.schema.range[1] \
    , $gt = it.schema.exclusiveRange ? '>' : '>=' \
    , $lt = it.schema.exclusiveRange ? '<' : '<='; \
}} \
var valid{{=it.level}} = {{=$data}} {{=$gt}} {{=$min}} && {{=$data}} {{=$lt}} {{=$max}}; \
");

ajv.addKeyword('range', {
  type: 'number',
  inline: inlineRangeTemplate,
  statements: true,
  metaSchema: {
    type: 'array',
    items: [ { type: 'number' }, { type: 'number' } ],
    additionalItems: false
  }
});
// 

'valid' + it.level in the example above is the expected name of the variable that should be set to the validation result.

Property statements in the keyword definition should be set to true if the validation code sets the variable instead of evaluating to the validation result.

The main challenge of defining inline keywords is that you have to write both the code that will execute during schema compilation (compile-time) and the code that will execute during data validation (validation-time - this code can be generated either using strings concatenation or using templates, see the examples below).

Ajv uses doT templates to generate the code of validation functions that makes it easier to separate compile-time and validation-time code because of the different syntax used in templates and in the code. Ajv also uses different variable names for compile-time and validation-time variables to make it easier to differentiate - compile-time variable names start with $ character.

Also you have to bear in mind that while compile-time variables exist in the scope of the function you wrote to compile the keyword, so they are isolated, validation-time variables share the scope with all the variables in the scope of a single validation function. So if your keyword has subschemas you have to append the schema level (it.level) to the variable names.

See schema compilation context for more information on which properties and utilities from the schema compilation context you can use.

Schema compilation context

The first parameter passed to inline keyword compilation function (and the 3rd parameter passed to compile and macro keyword functions) is it, the schema compilation context. All the properties and functions documented here are safe to use in your keywords, they won’t be renamed or change their meaning without major version change.

it object has the following properties:

Validation time variables

There is a number of variables and expressions you can use in the generated (validation-time) code of your keywords.

Ajv utilities

There are sevral useful functions you can use in your inline keywords. These functions are available as properties of it.util object:

.copy(Object obj[, Object target]) -> Object

Clone or extend the object. If one object is passed, it is cloned. If two objects are passed, the second object is extended with the properties of the first.

.toHash(Array arr) -> Object

Converts the array of strings to the object where each string becomes the key with the value of true.

it.util.toHash(['a', 'b', 'c']) // { a: true, b: true, c: true }
.equal(value1, value2) -> Boolean

Performs deep equality comparison. This function is used in keywords enum, constant, uniqueItems and can be used in custom keywords.

.getProperty(String key) -> String

Converts the string that is the key/index to access the property/item to the JavaScript syntax to access the property (either “.” notation or “[…]” notation).

it.util.getProperty('a')   // ".a"
it.util.getProperty('1')   // "['1']"
it.util.getProperty("a'b") // "['a\\'b']"
it.util.getProperty(1)     // "[1]"
.schemaHasRules(Object schema, Object rules) -> String

Determines whether the passed schema has rules that should be validated. This function should be used before calling it.validate to compile subschemas.

it.util.schemaHasRules(schema, it.RULES.all) // true or false
.escapeQuotes(String str) -> String

Escapes single quotes in the string, so it can be inserted in the generated code inside the string constant with the single quotes.

.toQuotedString(String str) -> String

Converts the string to the JavaScript string constant in single quotes (using the escaped string).

it.util.toQuotedString("a'b") // "'a\\'b'"
.getData(String jsonPointer, Number dataLevel, Array paths) -> String

Returns the validation-time expression to safely access data based on the passed relative json pointer (See examples).

it.util.getData('2/test/1', it.dataLevel, it.dataPathArr)
// The result depends on the current level
// if it.dataLevel is 3 the result is "data1 && data1.test && data1.test[1]"
.escapeJsonPointer(String str) -> String

Converts the property name to the JSON-Pointer fragment.

.unescapeJsonPointer (String str) -> String

Converts JSON-Pointer fragment to the property name.

.unescapeFragment(String str) -> String

Converts the property name to the JSON-Pointer fragment that can be used in URI.

.escapeFragment(String str) -> String

Converts the JSON-Pointer fragment from URI to the property name.

Reporting errors in custom keywords

All custom keywords but macro keywords can optionally create custom error messages.

Synchronous validating and compiled keywords should define errors by assigning them to .errors property of the validation function. Asynchronous keywords can return promise that rejects with new Ajv.ValidationError(errors), where errors is an array of custom validation errors (if you don’t want to define custom errors in asynchronous keyword, its validation function can return the promise that resolves with false).

Inline custom keyword should increase error counter errors and add error to vErrors array (it can be null). This can be done for both synchronous and asynchronous keywords. See example range keyword.

When inline keyword performs validation Ajv checks whether it created errors by comparing errors count before and after validation. To skip this check add option errors (can be "full", true or false) to keyword definition:

ajv.addKeyword('range', {
  type: 'number',
  inline: inlineRangeTemplate,
  statements: true,
  errors: true // keyword should create custom errors when validation fails
  // errors: 'full' // created errors should have dataPath already set
  // errors: false // keyword never creates errors, Ajv will add a default error
});

Each error object should at least have properties keyword, message and params, other properties will be added.

Inlined keywords can optionally define dataPath and schemaPath properties in error objects, that will be assigned by Ajv unless errors option of the keyword is "full".

If custom keyword doesn’t create errors, the default error will be created in case the keyword fails validation (see Validation errors).

Short-circuit validation

In some cases inline keyword can terminate validation and return the result as soon as it encounters the error. It is only practical if the keyword you define has many criteria to validate and you want it to be able to fail fast. You only need to do it if your keyword defines errors itself, otherwise Ajv will return when it creates the default error (if the conditions below are met).

Two conditions should be checked before keyword can return the result:

If these conditions are met your keyword can immediately return result. In case the current schema is synchronous (it.async is not true) you can add this to keyword’s generated code when it encounters error err:

if (vErrors === null) vErrors = [err];
else vErrors.push(err);
validate.errors = vErrors;
return false;

In case the current schema is asynchronous (it.async is truthy) to return result you need:

if (vErrors === null) vErrors = [err];
else vErrors.push(err);
throw new ValidationError(vErrors); // ValidationError is in the scope

In case allErrors option is used the keyword should continue validation after it encounters an error trying to find as many errors as possible.

If allErrors option is not used but it.compositeRule is truthy the keyword may short-circuit its own validation but it should not return the final validation result.