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).
- defining keywords that do not depend on the schema value (e.g., when the value is always `true`). In this case you can add option `schema: false` to the keyword definition and the schemas won't be passed to the validation function, it will only receive the same 4 parameters as compiled validation function (see the next section).
- defining keywords that support [$data reference](https://github.com/epoberezkin/ajv#data-reference) - in this case validation function is required, either as the only option or in addition to compile, macro or inline function (see below).
__Please note__: In cases when validation flow is different depending on the schema and you have to use `if`s, this way to define keywords will have worse performance than compiled keyword returning different validation functions depending on the schema.
__Please note:__ If the keyword does not define custom errors (see [Reporting errors in custom keywords](#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.
Compilation function will be called during schema compilation. It will be passed schema, parent schema and [schema compilation context](#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.
Custom keyword can also have an optional `dependencies` property in their definitions - it is a list of required keywords in a containing (parent) schema.
"Macro" function is called during schema compilation. It is passed schema, parent schema and [schema compilation context](#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.
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)):
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](https://github.com/epoberezkin/ajv/blob/master/spec/custom.spec.js#L151).
### 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.
`'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](https://github.com/olado/doT):
`'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](https://github.com/olado/doT) 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](#schema-compilation-context) for more information on which properties and utilities from the schema compilation context you can use.
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.
- _level_ - the level of the current schema, `0` on the top level, `1` in subschemas (e.g. schemas in `properties` or `anyOf` keyword). The value of this property should be appended to the validation-time variables you use in the generated code.
- _dataLevel_ - the level of the currently validated data. It can be used to access both the property names and the data on all levels from the top. See [Validation time variables](#validation-time-variables).
- _schema_ - current level schema. The value of your keyword is `it.schema[keyword]`. This value is also passed as the 3rd parameter to the inline compilation function and the current level schema as the 4th parameter.
- _schemaPath_ - the validation time expression that evaluates to the property name of the current schema.
- _compositeRule_ - boolean indicating that the current schema is inside the compound keyword where failing some rule doesn't mean validation failure (`anyOf`, `oneOf`, `not`, `if` in `switch`). This flag is used to determine whether you can return validation result immediately after any error in case the option `allErrors` is not `true. You only need to do it if you have many steps in your keywords and potentially can define multiple errors.
- _validate_ - the function you need to use to compile subschemas in your keywords (see the [implementation](https://github.com/epoberezkin/ajv/blob/master/lib/dot/v5/switch.jst) of `switch` keyword for example).
- _util_ - [Ajv utilities](#ajv-utilities) you can use in your inline compilation functions.
- _self_ - Ajv instance.
## Validation time variables
There is a number of variables and expressions you can use in the generated (validation-time) code of your keywords.
-`'data' + (it.dataLevel || '')` - the variable name for the data at the current level.
-`'data' + ((it.dataLevel-1)||'')` - parent data if `it.dataLevel > 0`.
-`it.dataPathArr[it.dataLevel]` - the name of the property in the parent object that points to the current data if `it.dataLevel > 0`.
-`'validate.schema'` - top level schema of the current validation function at validation-time.
-`'validate.schema' + it.schemaPath` - current level schema available at validation time (the same schema at compile time is `it.schema`).
-`'validate.schema' + it.schemaPath + '.' + keyword` - the value of your custom keyword at validation-time. Keyword is passed as the second parameter to the inline compilation function to allow using the same function to compile multiple keywords.
-`'valid' + it.level` - the variable that you have to declare and to assign the validation result to if your keyword returns statements rather than expression (`statements: true`).
-`'errors'` - the number of encountered errors. See [Reporting errors in custom keywords](https://github.com/epoberezkin/ajv/blob/master/CUSTOM.md#reporting-errors-in-custom-keywords).
-`'vErrors'` - the array with errors collected so far. See [Reporting errors in custom keywords](https://github.com/epoberezkin/ajv/blob/master/CUSTOM.md#reporting-errors-in-custom-keywords).
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`.
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).
Determines whether the passed schema has rules that should be validated. This function should be used before calling `it.validate` to compile subschemas.
##### .getData(String jsonPointer, Number dataLevel, Array paths) -> String
Returns the validation-time expression to safely access data based on the passed [relative json pointer](https://tools.ietf.org/html/draft-luff-relative-json-pointer-00) (See [examples](https://gist.github.com/geraintluff/5911303)).
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](https://github.com/epoberezkin/ajv/blob/master/spec/custom_rules/range_with_errors.jst).
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:
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](https://github.com/epoberezkin/ajv#validation-errors)).
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:
- option `allErrors` should not be used (`!it.opts.allErrors` should be true).
- the current schema should not be inside composite rule (e.g. `not` or `anyOf`), when failing some keyword does not mean failing the validation (`!it.compositeRule` should be true).
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`:
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.
