Meta-Schema Basics

The meta-schema system in @jetio/validator validates that your JSON Schemas are correctly structured according to JSON Schema specifications. This ensures your schemas are valid before you use them for data validation.

What Are Meta-Schemas?

A meta-schema is a JSON Schema that validates other JSON Schemas. It defines:

What keywords are allowed
The structure of keyword values
Relationships between keywords
Draft-specific rules and constraints

Think of it as “schema validation for schemas.”


// Your data schema
const userSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
  },
};
 
// A meta-schema validates that userSchema is correct:
// - Is 'type' a valid keyword?
// - Is 'object' a valid type value?
// - Is 'properties' structured correctly?

Why Validate Schemas?

1. Catch typos and errors early


const badSchema = {
  type: "objct", // Typo: should be 'object'
  properties: {
    name: { type: "string" },
  },
};
 
jetValidator.validateSchema(badSchema); // false

2. Prevent invalid configurations


const invalidSchema = {
  type: "number",
  minimum: "10", // String instead of number
};
 
jetValidator.validateSchema(invalidSchema); // false

3. Enforce draft compatibility


// Using a Draft 2020-12 keyword with Draft-07
const incompatibleSchema = {
  $schema: "https://json-schema.org/draft-07/schema",
  type: "object",
  unevaluatedProperties: false, // Only in 2019-09+
};
 
jetValidator.validateSchema(incompatibleSchema); // false

Supported JSON Schema Drafts

Draft	URI	Year	Key Features
Draft-06	`https://json-schema.org/draft-06/schema`	2017	`const`, `contains`, `propertyNames`
Draft-07	`https://json-schema.org/draft-07/schema`	2018	`if`/`then`/`else`, `readOnly`, `writeOnly`
Draft 2019-09	`https://json-schema.org/draft/2019-09/schema`	2019	`unevaluatedProperties`, `unevaluatedItems`, vocabulary system
Draft 2020-12	`https://json-schema.org/draft/2020-12/schema`	2020	`prefixItems`, `$dynamicRef`, improved `$ref`

Recommendation: Use Draft-07 for maximum compatibility, or Draft 2020-12 for the newest features.