express-json-validator-middleware

An Express middleware to validate requests against JSON Schemas

Usage no npm install needed!

<script type="module">
  import expressJsonValidatorMiddleware from 'https://cdn.skypack.dev/express-json-validator-middleware';
</script>

README

Express JSON Validator Middleware

Express middleware for validating requests against JSON schemas.

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Why validate with JSON schemas?

  • Simple - JSON schemas are a simple and expressive way to describe a data structure.
  • Standard - JSON schemas are not specific to JavaScript. In fact, they are used just about everywhere.
  • Fail-Fast - Catch errors early in your logic, evading confusing errors later.
  • Separate Validation - Keep your routes clean. Validation logic doesn't need to be defined in your route handlers.
  • Error Messaging - Ajv provides you with rich error objects that can easily be transformed into human-readable format.
  • Documentation - Creating a JSON Schema is another way of documenting your application.

Install

npm install express-json-validator-middleware

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 https://github.com/ajv-validator/ajv/tree/v6#options
 */
 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`.
 */
app.post("/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 are writing JSON schemas in TypeScript, you will need to cast your schema to the const type e.g.

const addressSchema = {
  type: "object",
  required: ["street"],
  properties: {
    street: {
      type: "string",
    }
  },
} as const;

This is required so that TypeScript doesn't attempt to widen the types of values in the schema object. If you omit the as const statement TypeScript will raise a compilation error. The discussion in this issue provides further 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`.
 */
app.post("/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
    },
  },
};

app.post(
  "/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: /street/?uuid=af3996d0-0e8b-4165-ae97-fdc0823be417

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;
}

app.post(
  "/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
validator.ajv;

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

Ajv versions

The major version 1.x of this module uses ajv@5, read their changelog and migration guide here.

Major version 2.x uses ajv@6 in order to support draft-07 of JSON Schema. You have to manually configure Ajv to support draft-06 schemas (see https://github.com/ajv-validator/ajv/tree/v6#using-version-6).

Tests

Tests are written using Mocha & Chai.

npm install
npm test

More documentation on JSON Schema

Credits