JSON Schema Generator

About JSON Schema Generator

This JSON Schema generator creates a schema draft by analyzing sample JSON data. It infers types (string, number, boolean, array, object, null), detects object properties, and marks all fields as required by default.

The generated schema uses JSON Schema Draft 2020-12 format and serves as a starting point for API contracts, validation rules, and documentation. You may need to refine the output to add constraints like patterns, ranges, or optional fields.

JSON Schema Type Reference

Common String Formats

Schema Constraints Reference

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "minLength": 1,
      "maxLength": 100,
      "pattern": "^[a-zA-Z ]+$"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 150
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "website": {
      "type": "string",
      "format": "uri"
    },
    "tags": {
      "type": "array",
      "items": {"type": "string"},
      "minItems": 1,
      "maxItems": 10,
      "uniqueItems": true
    },
    "metadata": {
      "type": "object",
      "minProperties": 1,
      "additionalProperties": true
    }
  },
  "required": ["name", "email"]
}

Inference Algorithm

How this generator works:

1. Parse JSON input
2. For each value:
   - null → {"type": "null"}
   - boolean → {"type": "boolean"}
   - number → {"type": "number"} (or "integer" if whole)
   - string → {"type": "string"}
   - array → {"type": "array", "items": schemaOf(firstElement)}
   - object → {"type": "object", "properties": {...}, "required": [...]}
3. Wrap in draft-2020-12 schema with $schema declaration

Limitations:
- Arrays: Only analyzes first item for items schema
- Required: Marks ALL object fields as required
- Unions: Doesn't detect multiple possible types
- Enums: Doesn't infer allowed value sets
- Formats: Doesn't detect email, uri, date formats
- Constraints: Doesn't infer min/max lengths or values

Refine the generated schema to add these details.

Validation Example with Ajv

// Install: npm install ajv

const Ajv = require('ajv');
const ajv = new Ajv();

const schema = {
  "type": "object",
  "properties": {
    "name": {"type": "string"},
    "age": {"type": "integer", "minimum": 0}
  },
  "required": ["name"]
};

const validate = ajv.compile(schema);

const data1 = {name: "John", age: 30};
const data2 = {age: 30}; // missing required name
const data3 = {name: "John", age: -5}; // invalid minimum

console.log(validate(data1)); // true
console.log(validate.errors); // null

console.log(validate(data2)); // false
console.log(validate.errors); // [{keyword: "required", ...}]

console.log(validate(data3)); // false
console.log(validate.errors); // [{keyword: "minimum", ...}]