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Express JSON Validator Middleware

Express middleware for validating
requests against JSON schemas with Ajv.

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Why validate with JSON schemas?

  • Expressive — JSON schemas are an expressive way to describe data structures.
  • Standard — JSON schemas are portable. There are validator implementations in many languages.
  • Separate validation — Avoid the need to handle request validation in your route handlers.
  • Error messaging — Ajv provides rich and descriptive error objects.
  • Documentation — Schemas can help document your application.



npm install express-json-validator-middleware

If you’re upgrading from v2 to v3, make sure you read the migration notes.

Getting started

import { Validator } from "express-json-validator-middleware"; /** * Define a JSON schema. */ const addressSchema = { type: "object", required: ["street"], properties: { street: { type: "string", } }, }; /** * Initialize a `Validator` instance, optionally passing in * an Ajv options object. * * @see */ const { validate } = new Validator(); /** * The `validate` method accepts an object which maps request * properties to the JSON schema you want them to be validated * against e.g. * * { requestPropertyToValidate: jsonSchemaObject } * * Validate `request.body` against `addressSchema`. */"/address", validate({ body: addressSchema }), (request, response) => { /** * Route handler logic to run when `request.body` has been validated. */ response.send({}); });

Coming from express-jsonschema? Read the migration notes.

Schemas in TypeScript

If you’re writing JSON schemas in TypeScript, you’ll need to use the
AllowedSchema type e.g.

import { AllowedSchema } from "express-json-validator-middleware";

const addressSchema: AllowedSchema  = {
  type: "object",
  required: ["street"],
  properties: {
    street: {
      type: "string",

This is required so TypeScript doesn’t attempt to widen the types of values
in the schema object. If you omit this type, TypeScript will raise an error.

See issues #39
and #102
for more background.

Error handling

On encountering invalid data, the validator will call next() with a
ValidationError object. It is recommended to setup a general error handler
for your app where you handle ValidationError errors.

Example – error thrown for the body request property:

ValidationError {
    name: "JsonSchemaValidationError",
    validationErrors: {
        body: [AjvError]

More information on Ajv errors.

Example Express application

import express from "express"; import { Validator, ValidationError } from "express-json-validator-middleware"; const app = express(); app.use(express.json()); const addressSchema = { type: "object", required: ["number", "street", "type"], properties: { number: { type: "number", }, street: { type: "string", }, type: { type: "string", enum: ["Street", "Avenue", "Boulevard"], }, }, }; const { validate } = new Validator(); /** * Validate `request.body` against `addressSchema`. */"/address", validate({ body: addressSchema }), (request, response) => { /** * Route handler logic to run when `request.body` has been validated. */ response.send({}); }); /** * Error handler middleware for validation errors. */ app.use((error, request, response, next) => { // Check the error is a validation error if (error instanceof ValidationError) { // Handle the error response.status(400).send(error.validationErrors); next(); } else { // Pass error on if not a validation error next(error); } }); app.listen(3000);

Validating multiple request properties

Sometimes your route may depend on the body and query both having a specific
format. In this example we use body and query but you can choose to validate
any request properties you like. This example builds on the
Example Express application.

const tokenSchema = { type: "object", required: ["token"], properties: { token: { type: "string", minLength: 36, maxLength: 36 }, }, }; "/address", validate({ body: addressSchema, query: tokenSchema }), (request, response) => { /** * Route handler logic to run when `request.body` and * `request.query` have both been validated. */ response.send({}); } );

A valid request must now include a token URL query. Example valid URL:

Using dynamic schema

Instead of passing in a schema object you can also pass in a function that will
return a schema. It is useful if you need to generate or alter the schema based
on the request object.

Example: Loading schema from a database (this example builds on the
Example Express application):

function getSchemaFromDb() { /** * In a real application this would be making a database query. */ return Promise.resolve(addressSchema); } /** * Middleware to set schema on the `request` object. */ async function loadSchema(request, response, next) { try { request.schema = await getSchemaFromDb(); next(); } catch (error) { next(error); } } /** * Get schema set by the `loadSchema` middleware. */ function getSchema(request) { return request.schema; } "/address", loadSchema, validate({ body: getSchema }), (request, response) => { /** * Route handler logic to run when `request.body` has been validated. */ response.send({}); } );

Ajv instance

The Ajv instance can be accessed via validator.ajv.

import { Validator, ValidationError } from "express-json-validator-middleware";

const validator = new Validator();

// Ajv instance

Ajv must be configured before you call Validator.validate() to add middleware
(e.g. if you need to define custom keywords.

Upgrading from v2 to v3

v2.x releases of this library use Ajv v6.
v3.x of this library uses Ajv v8.

Notable changes between Ajv v6 and v8:

  • All formats have been moved to ajv-formats.
    If you’re using formats in your schemas, you must install this package to continue
    using them.
  • The structure of validation errors has changed.
  • Support has been dropped for JSON Schema draft-04.

For full details, read the Ajv migration guide: Changes from Ajv v6.12.6 to v8.0.0.

If you have any Ajv plugins as dependencies, update them to their newest versions.
Older versions of Ajv plugins are less likely to be compatible with Ajv v8.


Tests are written using node-tap.

npm install

npm test

More documentation on JSON Schema


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