Simplifying JSON Validation with Ajv (Another JSON Validator)

Simplifying JSON Validation with Ajv (Another JSON Validator)

Introduction

For the full implementation, you can reference the GitHub repository: https://github.com/jdevto/ajv-demo

Ajv (Another JSON Validator) is a popular tool for validating JSON data against schemas. By default, Ajv uses the older Draft-07 of JSON Schema, but with some additional effort, it can be configured to work seamlessly with the newer Draft 2019-09. In this article, we will demonstrate how to use Ajv with the newer Draft 2019-09, ensuring your JSON data validation leverages the latest schema features.

• meta-data.json
• format.json
• content.json
• json-schema-draft-2019-09.json

You can download these meta-schemas from the official JSON Schema repository:

curl -L -o schemas/core.json https://json-schema.org/draft/2019-09/meta/core
curl -L -o schemas/applicator.json https://json-schema.org/draft/2019-09/meta/applicator
curl -L -o schemas/validation.json https://json-schema.org/draft/2019-09/meta/validation
curl -L -o schemas/meta-data.json https://json-schema.org/draft/2019-09/meta/meta-data
curl -L -o schemas/format.json https://json-schema.org/draft/2019-09/meta/format
curl -L -o schemas/content.json https://json-schema.org/draft/2019-09/meta/content
curl -L -o schemas/json-schema-draft-2019-09.json https://json-schema.org/draft/2019-09/schema

---

Code Example

Below is the code for validating JSON data against a schema using Ajv and Draft 2019-09:

const Ajv2019 = require("ajv/dist/2019");
const addFormats = require("ajv-formats");
const fs = require("fs");
const path = require("path");

// Initialize Ajv2019 instance
const ajv = new Ajv2019({ strict: false });

// List of meta-schemas to load
const metaSchemas = [
  "core.json",
  "applicator.json",
  "validation.json",
  "meta-data.json",
  "format.json",
  "content.json",
  "json-schema-draft-2019-09.json",
];

// Load and add meta-schemas if not already registered
metaSchemas.forEach((file) => {
  const schema = JSON.parse(fs.readFileSync(path.join(__dirname, "schemas", file), "utf8"));
  if (!ajv.getSchema(schema.$id)) {
    ajv.addMetaSchema(schema);
  } else {
    console.log(`Meta-schema "${schema.$id}" already added. Skipping.`);
  }
});

// Add format validation support
addFormats(ajv);

// Define a schema using Draft 2019-09
const schema = {
  $schema: "https://json-schema.org/draft/2019-09/schema",
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "integer", minimum: 0 },
  },
  required: ["name", "age"],
};

// Sample data to validate
const validData = { name: "Alice", age: 25 };
const invalidData = { name: "Bob", age: -5 };

try {
  const validate = ajv.compile(schema);

  [validData, invalidData].forEach((data, index) => {
    if (validate(data)) {
      console.log(`✅ Validation succeeded for sample ${index + 1}.`);
    } else {
      console.error(`❌ Validation failed for sample ${index + 1}:`);
      console.error(validate.errors);
    }
  });
} catch (error) {
  console.error("❌ Error compiling schema:", error.message);
}

---

Explanation of the Code

1. Initializing Ajv2019: We use the Ajv2019 module specifically for Draft 2019-09 schemas. The strict mode is disabled for compatibility.

2. Registering Meta-Schemas: The meta-schemas are dynamically loaded from the schemas directory and registered with Ajv.

3. Adding Format Validation: The addFormats function enhances Ajv by enabling support for common JSON formats like email, uri, etc.

4. Defining a Schema: The schema specifies:

• A name property (string, required).
• An age property (integer, required, must be ≥ 0).

1. Compiling and Validating Data:

• The validData object passes validation.
• The invalidData object fails because age is negative.

---

Running the Code

1. Save the code to a file (e.g., validate.js).
2. Ensure the schemas directory contains all required meta-schemas.
3. Run the script:

   node validate.js

Expected output:

Meta-schema "https://json-schema.org/draft/2019-09/meta/core" already added. Skipping.
...
✅ Validation succeeded for sample 1.
❌ Validation failed for sample 2:
[
  {
    instancePath: "/age",
    schemaPath: "#/properties/age/minimum",
    keyword: "minimum",
    params: { comparison: ">=", limit: 0 },
    message: "must be >= 0"
  }
]

---

Key Takeaways

1. Meta-Schema Management:

• Properly register all meta-schemas to avoid unresolved references.

1. Error Reporting:

• Ajv provides detailed error messages for failed validations.

1. Extensibility:

• This setup can easily be extended for more complex schemas and data validations by adding additional meta-schemas or customizing validation rules. For instance, you could include custom keywords, validate dynamic data structures, or support nested schemas to handle advanced use cases.

---

Conclusion

For the complete code and examples, visit the GitHub repository: https://github.com/jdevto/ajv-demo

Validating JSON data using Ajv with Draft 2019-09 is straightforward if you handle meta-schemas correctly. This approach ensures that your data adheres to the latest JSON Schema standards while leveraging Ajv's powerful validation capabilities. By automating meta-schema registration, you can streamline the validation process and reduce potential errors.