express-ajv

Middleware function for using ajv to validate express requests.

Validating requests using express-ajv will not only remove validation logic from your controller, but will also let you leverage the power of json schemas to write very complex validators in very little time.

Installation

npm install express-ajv

Notice that this library relies on ajv as a peer dependency.

Usage

import express from 'express';
import { createValidator } from 'express-ajv';

const schemas = {
    getUsersSchema: { ... } // your schema here
};
const validate = createValidator(schemas);
const app = express();

app.get('/users', validate('getUsersSchema'), (req, res) => ... );

Whenever your endpoint is consumed, the validate middleware will validate the properties body, params and query from express requests against your schemas, using the schema identified by the provided string. If the validation fails, an error will be generated and passed to express next function.

API

The main function exposed by the library is createValidator which is the method that will create the middleware validator function to be used by your express app. createValidator takes two parameters:

• schemas: SchemaCollection - an object representing a collection of schemas. The keys in this object will should be used as the schema name provided to the middleware like shown in the snippet above.
• options: ValidatorOptions - optional object with configs to be used by the library. Available options are:
  • ajv: Ajv - an existing instance of ajv. If not provided, a new instance will be created. Default: undefined
  • ajvOptions: Ajv.Options - options to be provided to ajv if a new instance is to be created. Will be ignored if options.ajv is provided. Default: undefined
  • contextExtractor: (req: Request) => any - function to extract properties from the request and use as additional context to custom ajv validators.

The middleware returned by createValidator should be used as follows:

const validate = createValidator(schemas, options);

...

app.get('/', validate('YOUR_SCHEMA_KEY'), (req, res) => ... );

where 'YOUR_SCHEMA_KEY' should be one of the keys of the schemas object provided to createValidator. Finally, the middleware function returned by createValidator will have a property called ajv containing the ajv instance used for validations:

const options = {
    ajv: new Ajv(ajvOptions);
}
const validate = createValidator(schemas, options);

// validate.ajv === options.ajv

or

const validate = createValidator(schemas);

// validate.ajv will be a new Ajv instance

Using the middleware to alter the properties in the request

Using the right configuration in ajv allows the user to leverage the validation functions to change the data being validated (see the filtering data, assigning defaults and coercing types for details and examples). Note that providing those options will change the original data in the request, for example:

Doing something like curl -H "Content-Type: application/json" -X GET http://localhost:8080/users?age=10 would likely cause age to be parsed by express as a string. However if you use:

// schema
{
    "properties": {
        "query": {
            "properties": {
                "age": {
                    "minimum": 0,
                    "type": "number"
                }
            },
            "required": ["name"],
            "type": "object"
        }
    },
    "type": "object"
}

// validator
const options = {
    ajvOptions: {
        coerceTypes: true
    }
};
const validate = createValidator(schemas, options);

app.get('/', validate('YOUR_SCHEMA_KEY'), (req, res) => {
    // req.query.age will be cast to a number
});

This functionality can be used to, among other things, provide standardization and sanitization to your api requests.

Considerations about schemas

Like mentioned above the middleware will validate the properties body, params and query from express requests against your schemas. Because of that, your schemas must follow a format similar to the example below:

// my-schema.json. Example for a typical GET request
{
    "properties": {
        "query": {
            "properties": {
                "name": {
                    "minLength": 3,
                    "type": "string"
                },
                "age": {
                    "minimum": 0,
                    "type": "number"
                }
            },
            "required": ["name"],
            "type": "object"
        },
        "params": {
            "properties": {
                "userId": {
                    "type": "string",
                    "pattern": "[a-f0-9]{24}"
                }
            },
            "required": ["userId"],
            "type": "object"
        }
    },
    "required": ["query", "params"],
    "type": "object"
}

Given that can be a little tedious declaring the base schema properties

{
    "properties": {
        "query": {
            ...
        },
        "params": {
            ...
        }
    },
    "required": ["query", "params"],
    "type": "object"
}

on every schema, the library exposes two helper functions createGetRequestSchema and createPostRequestSchema. Both functions follow the signature below:

(baseSchema: object, baseParamsSchema?: object) => RequestSchema

The objects you provide as the base schemas will be used as the schemas for query (or body) and params

Examples

Examples can be found here.