ajv-errors

Custom error messages in JSON-Schema for Ajv validator

Install

npm install ajv-errors

Usage

Add the keyword errorMessages to Ajv instance:

var Ajv = require('ajv');
var ajv = new Ajv({allErrors: true, jsonPointers: true});
// Ajv options allErrors and jsonPointers are required
require('ajv-errors')(ajv /*, {singleError: true} */);

See Options below.

Single message

Replace all errors in the current schema and subschemas with a single message:

var schema = {
  type: 'object',
  required: ['foo'],
  properties: {
    foo: { type: 'integer' }
  },
  additionalProperties: false,
  errorMessage: 'should be an object with an integer property foo only'
};

var validate = ajv.compile(schema);
console.log(validate({foo: 'a', bar: 2})); // false
console.log(validate.errors); // processed errors

Processed errors:

[
  {
    keyword: 'errorMessage',
    message: 'should be an object with an integer property foo only',
    // ...
    params: {
      errors: [
        { keyword: 'additionalProperties', dataPath: '' /* , ... */ },
        { keyword: 'type', dataPath: '.foo' /* , ... */ }
      ]
    }
  }
]

Messages for keywords

Replace errors for certain keywords in the current schema only:

var schema = {
  type: 'object',
  required: ['foo'],
  properties: {
    foo: { type: 'integer' }
  },
  additionalProperties: false,
  errorMessage: {
    type: 'should be an object', // will not replace internal "type" error for the property "foo"
    required: 'should have property foo',
    additionalProperties: 'should not have properties other than foo'
  }
};

var validate = ajv.compile(schema);
console.log(validate({foo: 'a', bar: 2})); // false
console.log(validate.errors); // processed errors

Processed errors:

[
  {
    // original error
    keyword: type,
    dataPath: '/foo',
    // ...
    message: 'should be integer'
  },
  {
    // generated error
    keyword: 'errorMessage',
    message: 'should not have properties other than foo',
    // ...
    params: {
      errors: [
        { keyword: 'additionalProperties' /* , ... */ }
      ]
    },
  }
]

For keywords "required" and "dependencies" it is possible to specify different messages for different properties:

var schema = {
  type: 'object',
  required: ['foo', 'bar'],
  properties: {
    foo: { type: 'integer' },
    bar: { type: 'string' }
  },
  errorMessage: {
    type: 'should be an object', // will not replace internal "type" error for the property "foo"
    required: {
      foo: 'should have an integer property "foo"',
      bar: 'should have a string property "bar"'
    }
  }
};

Messages for properties and items

Replace errors for properties / items (and deeper), regardless where in schema they were created:

var schema = {
  type: 'object',
  required: ['foo', 'bar'],
  allOf: [{
    properties: {
      foo: { type: 'integer', minimum: 2 },
      bar: { type: 'string', minLength: 2 }
    },
    additionalProperties: false
  }],
  errorMessage: {
    properties: {
      foo: 'data.foo should be integer >= 2',
      bar: 'data.bar should be string with length >= 2'
    }
  }
};

var validate = ajv.compile(schema);
console.log(validate({foo: 1, bar: 'a'})); // false
console.log(validate.errors); // processed errors

Processed errors:

[
  {
    keyword: 'errorMessage',
    message: 'data.foo should be integer >= 2',
    dataPath: '/foo',
    // ...
    params: {
      errors: [
        { keyword: 'minimum' /* , ... */ }
      ]
    },
  },
  {
    keyword: 'errorMessage',
    message: 'data.bar should be string with length >= 2',
    dataPath: '/bar',
    // ...
    params: {
      errors: [
        { keyword: 'minLength' /* , ... */ }
      ]
    },
  }
]

Default message

When the value of keyword errorMessage is an object you can specify a message that will be used if any error appears that is not specified by keywords/properties/items:

var schema = {
  type: 'object',
  required: ['foo', 'bar'],
  allOf: [{
    properties: {
      foo: { type: 'integer', minimum: 2 },
      bar: { type: 'string', minLength: 2 }
    },
    additionalProperties: false
  }],
  errorMessage: {
    type: 'data should be an object',
    properties: {
      foo: 'data.foo should be integer >= 2',
      bar: 'data.bar should be string with length >= 2'
    },
    _: 'data should have properties "foo" and "bar" only'
  }
};

var validate = ajv.compile(schema);
console.log(validate({})); // false
console.log(validate.errors); // processed errors

Processed errors:

[
  {
    keyword: 'errorMessage',
    message: 'data should be an object with properties "foo" and "bar" only',
    dataPath: '',
    // ...
    params: {
      errors: [
        { keyword: 'required' /* , ... */ },
        { keyword: 'required' /* , ... */ }
      ]
    },
  }
]

The message in property _ of errorMessage replaces the same errors that would have been replaced if errorMessage were a string.

Templates

Custom error messages used in errorMessage keyword can be templates using JSON-pointers or relative JSON-pointers to data being validated, in which case the value will be interpolated. Also see examples of relative JSON-pointers.

The syntax to interpolate a value is ${<pointer>}.

The values used in messages will be JSON-stringified:

to differentiate between false and "false", etc.
to support structured values.

Example:

{
  "type": "object",
  "properties": {
    "size": {
      "type": "number",
      "minimum": 4
    }
  },
  "errorMessage": {
    "properties": {
      "size": "size should be a number bigger or equal to 4, current value is ${/size}"
    }
  }
}

Options

Defaults:

{
  keepErrors: false,
  singleError: false
}

keepErrors: keep original errors. Default is to remove matched errors (they will still be available in params.errors property of generated error). If an error was matched and included in the error generated by errorMessage keyword it will have property emUsed: true.
singleError: create one error for all keywords used in errorMessage keyword (error messages defined for properties and items are not merged because they have different dataPaths). Multiple error messages are concatenated. Option values:
- false (default): create multiple errors, one for each message
- true: create single error, messages are concatenated using "; "
- non-empty string: this string is used as a separator to concatenate messages