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Quicktype infers JSON Schema from sample JSON data. It works well when you have example outputs but no model or schema definition yet. Quicktype currently emits draft-06 JSON Schema. That reflects the tool’s output format, not a recommendation to target draft-06 in new hand-written schemas.

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Install

npm install -g quicktype

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Basic usage

Given a sample JSON file: 
{
  "name": "Alice Johnson",
  "email": "alice@acme.com",
  "role": "Product Manager"
}
Quicktype infers types from the values it sees and wraps everything in a $ref + definitions structure. The type name (Sample) is derived from the filename. Unlike Genson, Quicktype sets additionalProperties: false by default.

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Multiple samples

Pass multiple samples to improve inference. Quicktype merges them and detects optional fields: a field missing from some samples becomes non-required: 
{"name": "Alice", "email": "alice@acme.com", "phone": "+1-555-0100"}
phone is correctly detected as optional since it only appears in one sample. When multiple source files are provided, the type is named TopLevel instead of being derived from a filename.

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Nested objects

Quicktype extracts nested objects into separate definitions and references them via $ref: 
{
  "name": "Alice",
  "address": {
    "street": "123 Main St",
    "city": "Springfield",
    "country": "US"
  }
}
The key name address is PascalCased to Address for the definition.

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Arrays

Quicktype infers item types from array contents: 
{
  "question": "What is your favorite color?",
  "options": ["red", "green", "blue"],
  "tags": ["survey", "color"]
}
Quicktype does not add minItems or maxItems; add those manually. See Bounded Arrays.

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Arrays of objects

When arrays contain objects, Quicktype extracts the item schema into a separate definition, singularizing the field name: 
{
  "contacts": [
    {"name": "Alice", "email": "alice@acme.com"},
    {"name": "Bob", "email": "bob@acme.com"}
  ]
}
The field contacts produces a definition named Contact (singular).

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Type inference across samples

When multiple samples expose different field sets, Quicktype correctly distinguishes required from optional and infers integer vs number: 
{"id": 1, "label": "first"}
id (whole numbers) is typed as integer while score (decimal) is typed as number. score is optional since it only appears in one sample.

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Limitations

Quicktype infers structure and types but does not add semantic constraints. The generated schema will not include:
  • enum values
  • minLength, maxLength, or pattern for strings
  • minItems, maxItems for arrays
  • description on fields or the schema
It also uses draft-06 and wraps every type in $ref + definitions rather than the more modern $defs. The output is a starting point. Tighten it by adding constraints manually or follow Improve your schema.