★ JSON Schema · from JSON

JSON to JSON Schema Converter

A draft 2020-12 schema inferred from your sample, with required lists, nullable types, string formats and nested objects factored into $defs. Useful for API docs, OpenAPI components and validating payloads in any language.

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Convert to

Root type name

Strictness

Your JSON

JSON Schema

3 types · 16 fields

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Root",
  "type": "object",
  "properties": {
    "id": {
      "type": "integer"
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "display-name": {
      "type": "string"
    },
    "createdAt": {
      "type": "string",
      "format": "date-time"
    },
    "plan": {
      "type": "null"
    },
    "tags": {
      "type": "array",
      "items": {
        "type": "string"
      }
    },
    "team": {
      "$ref": "#/$defs/Team"
    },
    "invoices": {
      "type": "array",
      "items": {
        "$ref": "#/$defs/Invoice"
      }
    }
  },
  "required": [
    "id",
    "email",
    "display-name",
    "createdAt",
    "plan",
    "tags",
    "team",
    "invoices"
  ],
  "$defs": {
    "Team": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string",
          "format": "uuid"
        },
        "name": {
          "type": "string"
        },
        "website": {
          "type": "string",
          "format": "uri"
        }
      },
      "required": [
        "id",
        "name",
        "website"
      ]
    },
    "Invoice": {
      "type": "object",
      "properties": {
        "number": {
          "type": "string"
        },
        "amount": {
          "type": "number"
        },
        "paid": {
          "type": "boolean"
        },
        "paidAt": {
          "type": [
            "string",
            "null"
          ],
          "format": "date-time"
        },
        "note": {
          "type": "string"
        }
      },
      "required": [
        "number",
        "amount",
        "paid",
        "paidAt"
      ]
    }
  }
}

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How the JSON Schema mapping works

Each JSON value is read, merged with every other sample of the same key, and then mapped to the closest JSON Schema type:

JSON to JSON Schema mapping
In your JSON	JSON Schema
string	`{ "type": "string" }`
timestamp / email / uuid / URL	`"format": date-time / email / uuid / uri`
integer / decimal	`"integer" / "number"`
null alongside a value	`"type": ["string", "null"]`
key present in every sample	`listed in "required"`
nested object	`"$ref": "#/$defs/Name"`
mixed values	`anyOf`
empty array	`"items": {}`

Worked example

A user record with a nested team, a list of invoices (one missing a note, one with a null payment date) and an awkward display-name key:

{
  "id": 1042,
  "email": "ada@example.com",
  "display-name": "Ada",
  "createdAt": "2026-11-07T09:30:00Z",
  "plan": null,
  "tags": ["beta", "founder"],
  "team": {
    "id": "7f3c9a2e-1b4d-4c8e-9f2a-6d5e4c3b2a19",
    "name": "Analytical Engines",
    "website": "https://example.com"
  },
  "invoices": [
    { "number": "INV-001", "amount": 49.5, "paid": true, "paidAt": "2026-10-01T00:00:00Z" },
    { "number": "INV-002", "amount": 49, "paid": false, "paidAt": null, "note": "Card declined" }
  ]
}

Becomes this JSON Schema, generated by the same engine that runs the converter above (3 types, 16 fields):

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Root",
  "type": "object",
  "properties": {
    "id": {
      "type": "integer"
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "display-name": {
      "type": "string"
    },
    "createdAt": {
      "type": "string",
      "format": "date-time"
    },
    "plan": {
      "type": "null"
    },
    "tags": {
      "type": "array",
      "items": {
        "type": "string"
      }
    },
    "team": {
      "$ref": "#/$defs/Team"
    },
    "invoices": {
      "type": "array",
      "items": {
        "$ref": "#/$defs/Invoice"
      }
    }
  },
  "required": [
    "id",
    "email",
    "display-name",
    "createdAt",
    "plan",
    "tags",
    "team",
    "invoices"
  ],
  "$defs": {
    "Team": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string",
          "format": "uuid"
        },
        "name": {
          "type": "string"
        },
        "website": {
          "type": "string",
          "format": "uri"
        }
      },
      "required": [
        "id",
        "name",
        "website"
      ]
    },
    "Invoice": {
      "type": "object",
      "properties": {
        "number": {
          "type": "string"
        },
        "amount": {
          "type": "number"
        },
        "paid": {
          "type": "boolean"
        },
        "paidAt": {
          "type": [
            "string",
            "null"
          ],
          "format": "date-time"
        },
        "note": {
          "type": "string"
        }
      },
      "required": [
        "number",
        "amount",
        "paid",
        "paidAt"
      ]
    }
  }
}

JSON Schema gotchas

format is advisory

Many validators only check format when you turn it on (Ajv needs ajv-formats). Without it, a bad email still passes.

Extra keys are allowed

The schema doesn't set additionalProperties: false, so unknown keys pass. Add it if you want a strict contract.

Draft versions differ

2020-12 uses $defs; older tools may expect definitions (draft-07). Rename the key if your validator is old.

Questions developers actually ask

Can I use this in OpenAPI?

OpenAPI 3.1 uses JSON Schema 2020-12 directly. Move the $defs into components/schemas and update the $ref paths.

How do I validate data against it in JavaScript?

Use Ajv 2020 (import Ajv from "ajv/dist/2020") with ajv-formats, compile the schema once, then call the validator on each payload.

Why is a field required?

It appeared in every sample. Paste more samples (as an array) and keys that are sometimes missing drop out of required.

Is my JSON sent to a server?

No. Parsing and type generation run in your browser, so you can paste real API responses with tokens or customer data. Nothing leaves the tab.

How does it know a field is optional?

Paste an array of objects, or a response that contains one. Every element is read and merged: keys missing from some elements become optional, and keys that are sometimes null become nullable.

Other languages

Or go back to the JSON to Types hub.

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Fine print

Tools: on the house.
Engineer: by the hour.

A codebase that doesn't need this tool is the upgrade. I set up end-to-end type safety so your API, database and UI agree by construction.

I'm Himanshu, an AI and full-stack engineer. I build MVPs, AI agents and the growth plumbing around them for early-stage startups.

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