Introduction

The Danger of Untyped JSON

APIs communicate using JSON. However, JSON itself has no built-in schema or strict typing.

If your backend expects a user_id as an integer, but the frontend sends "user_id": "123" as a string, standard JSON parsing will succeed, but your database query will immediately crash.

JSON Schema solves this problem by defining a strict "contract" for what the JSON payload must look like.

Writing a JSON Schema

A JSON Schema is simply a JSON object that describes another JSON object.

Here is a simple JSON Schema for a User Profile:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "minLength": 2
    },
    "age": {
      "type": "integer",
      "minimum": 18
    },
    "email": {
      "type": "string",
      "format": "email"
    }
  },
  "required": ["name", "email"],
  "additionalProperties": false
}

What this Schema enforces:

1. The payload must be an Object.
2. It must have a name (string, at least 2 characters long).
3. It must have an email (string, properly formatted as an email address).
4. age is optional, but if provided, it must be an integer > 18.
5. additionalProperties: false means that if any extra fields (like admin: true) are sent, the validation will fail immediately.

Testing with Our Validator

Our visual validator allows you to:

1. Paste your JSON Schema on the left.
2. Paste your JSON Payload on the right.
3. Instantly see a live, reactive breakdown of any errors.

It uses the industry-standard AJV engine, ensuring that if it passes here, it will pass in your production Node.js servers.