ajv-errors
Custom error messages in JSON-Schema for Ajv validator
Last updated a month ago by esp .
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ajv-errors

Custom error messages in JSON-Schema for Ajv validator

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Please note

ajv-errors v2 supports ajv v7.

If you are using ajv v6, you should use ajv-errors v1

Contents

Install

npm install ajv-errors

Usage

Add the keyword errorMessages to Ajv instance:

const Ajv = require("ajv").default
const ajv = new Ajv({allErrors: true})
// Ajv option allErrors is 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:

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

const 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:

const 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",
  },
}

const 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:

const 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:

const 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",
    },
  },
}

const 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 using _ property:

const 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',
  },
}

const 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:

const schema = {
  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}",
    },
  },
}

Referring to Field Names

Field names can be referred to by using the relative 0# JSON-pointer.

Example which prints additional properties that cause validation error:

const schema = {
  type: "object",
  properties: {
    size: {
      type: "number",
    },
  },
  additionalProperties: {
    not: true,
    errorMessage: “extra property is ${0#}”
  }
}

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

Supporters

Roger Kepler

Enterprise support

ajv-errors package is a part of Tidelift enterprise subscription - it provides a centralised commercial support to open-source software users, in addition to the support provided by software maintainers.

Security contact

To report a security vulnerability, please use the Tidelift security contact. Tidelift will coordinate the fix and disclosure. Please do NOT report security vulnerability via GitHub issues.

License

MIT

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