Summarising the other answers, here are the JSON schema generators proposed so far:

Online:

• https://www.liquid-technologies.com/online-json-to-schema-converter (1 input)
• http://www.jsonschema.net (1 input)
• https://easy-json-schema.github.io (1 input)
• https://json.ophir.dev/ (1 input, nice UI)

Python:

• https://github.com/gonvaled/jskemator (1 input but allows iteration)
• https://github.com/perenecabuto/json_schema_generator (1 input)
• https://github.com/rnd0101/json_schema_inferencer (1 input I think)
• https://pypi.python.org/pypi/genson/ (multiple inputs)
• https://pypi.python.org/pypi/skinfer (multiple inputs)

NodeJS:

• https://github.com/Nijikokun/generate-schema (multiple inputs (pass object array))
• https://github.com/easy-json-schema/easy-json-schema (1 input)
• https://github.com/aspecto-io/genson-js (multiple inputs)

Ruby:

• https://github.com/maxlinc/json-schema-generator (1 input)

A library that does exactly this is hypothesis-jsonschema

Hypothesis is a library that can generate arbitrary data that conforms to a given specification.

hypothesis-jsonschema makes it possible to convert JSON Schema into specifications that can be used by Hypothesis.

Here is an example showing a unit test written using Hypothesis and hypothesis-jsonschema:

from hypothesis import given

from hypothesis_jsonschema import from_schema


@given(from_schema({"type": "integer", "minimum": 1, "exclusiveMaximum": 10}))
def test_integers(value):
    assert isinstance(value, int)
    assert 1 <= value < 10