JSON Schema generator and validator

Each value in the sample maps to a schema keyword. With several samples, the generator merges them and only keeps what holds for all of them.

No extra properties adds "additionalProperties": false to every object, so unknown keys fail validation. Add examples copies up to three sample values into examples, which documentation tools show. The schema is a starting point: add enum, minimum, maxLength or pattern where you know the rules, because a sample cannot tell the generator what values are allowed.

Most simple schemas work the same in both. These are the changes you are likely to meet:

The validator is written for this page and runs in your browser. It reports every failing rule, not only the first, each with the path of the value ($.users[0].email), its JSON Pointer and the schema path of the rule. It evaluates these keywords:

type · enum · const · minimum · maximum · exclusiveMinimum · exclusiveMaximum · multipleOf · minLength · maxLength · pattern · format · items · prefixItems · additionalItems · contains · minContains · maxContains · minItems · maxItems · uniqueItems · properties · patternProperties · additionalProperties · required · propertyNames · minProperties · maxProperties · dependentRequired · dependentSchemas · dependencies · allOf · anyOf · oneOf · not · if · then · else · $ref · $defs · definitions · $anchor

$ref works for local references (#/$defs/address, # and $anchor names), including recursive schemas. Remote references, unevaluatedProperties, unevaluatedItems and $dynamicRef are not evaluated; when a schema uses them, the result says so instead of passing silently. Unknown keywords (like OpenAPI's nullable) are listed too, since validators ignore them.

JavaScript (Ajv). The default Ajv class is for draft-07; import ajv/dist/2020 for 2020-12. Format checks need ajv-formats, and allErrors collects every error instead of stopping at the first.

import Ajv2020 from 'ajv/dist/2020';
import addFormats from 'ajv-formats';

const ajv = new Ajv2020({ allErrors: true });
addFormats(ajv);
const validate = ajv.compile(schema);
if (!validate(data)) console.log(validate.errors);

Python (jsonschema).

from jsonschema import Draft202012Validator

validator = Draft202012Validator(schema, format_checker=Draft202012Validator.FORMAT_CHECKER)
for error in validator.iter_errors(data):
    print(list(error.absolute_path), error.message)

Prefer TypeScript types to a schema? The JSON to TypeScript converter writes interfaces or Zod schemas from the same sample.

• Objects accept unknown keys by default. Without additionalProperties: false, a typo like "emial" passes. Add it where you want strict input.
• Use the right $schema. A 2020-12 schema with prefixItems run through a draft-07 validator is not checked as a tuple, because draft-07 does not know that keyword.
• Integer is a value check. In JSON Schema, 1.0 is a valid integer, since the number has no fractional part.
• pattern is not anchored. "pattern": "\\d+" matches "abc1". Write ^\\d+$ to match the whole string.

Fix broken JSON first with the JSON formatter, or query parts of it with the JSONPath tester. More tools for building and testing APIs are in API tools on DevHunt.