🧰 JSON Schema Kit

Some very simple helper functions for writing concise JSON Schema — perfect for OpenAI Structured Outputs.

✨ Quick Taste

import { object, string, number, array, nullable } from 'json-schema-kit'

object({
  name: string(),
  price: number({ minimum: 0 }),
  description: nullable(string()),
  categories: array(string()),
})

All functions just return plain JavaScript objects. Freely modify or extend them to fit your needs.

🚀 Installation

npm install json-schema-kit

🆚 Comparison

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "price": { "type": "number", "description": "Price in dollars" },
    "discount": { "anyOf": [{ "type": "number" }, { "type": "null" }] },
    "categories": { "type": "array", "items": { "type": "string", "enum": ["electronics", "clothing", "books"] } },
    "dimensions": {
      "type": "object",
      "properties": {
        "width": { "type": "number" },
        "height": { "type": "number" }
      },
      "required": ["width", "height"],
      "additionalProperties": false
    }
  },
  "required": ["name", "price", "discount", "categories", "dimensions"],
  "additionalProperties": false
}

object({
  name: string(),
  price: number({ description: 'Price in dollars' }),
  discount: nullable(number()),
  categories: array(string({ enum: ['electronics', 'clothing', 'books'] })),
  dimensions: object({
    width: number(),
    height: number(),
  }),
})

🤖 OpenAI Structured Outputs

JSON Schema Kit is perfectly suited for OpenAI's Structured Outputs.
For example, here's how to use it with the Vercel AI SDK:

const schema = object({
  summary: string(),
  sentiment: string({ enum: ['positive', 'neutral', 'negative'] }),
})

await generateObject({
  model: openai(...),
  schema: jsonSchema(schema),
  prompt: 'Analyze this review: "Great product, works perfectly!"',
})

🔗 Using References

Use $ref to create reusable schema definitions and reference them throughout your schema:

const person = object({
  name: string(),
  age: number(),
})

const team = object({
  leader: $ref('person'),
  members: array($ref('person')),
})

team.$defs = { person }

Read more about $ref and $defs in the JSON Schema Documentation

🔀 Using Union Types

Create union types using anyOf to allow multiple possible schemas:

const contact = anyOf([
  object({ email: string() }),
  object({ phone: string() }),
])

Read more about anyOf in the JSON Schema Documentation

🤔 "But what about Zod?"

Great question! Zod is a versatile and comprehensive library, spanning thousands of lines of code. However, it's not specifically built for generating JSON Schemas, which can lead to unexpected results during conversion. In contrast, JSON Schema Kit provides full control — all in under 100 lines of code.