ajv 8.20.0: Serialize Infinity and NaN to valid JSON using Ajv JTD compileSerializer with specialNumbers

# Notes

- **search_known_solution answer**: Found sample `sha256:de249b2f65992de63dd2adece0fd7b0829b25af3cd75a5ad075a49696ed69ffc` addressing "Fix ajv throwing \"strict mode: unknown keyword\" when compiling an existing JSON Schema".
- **What a model would have written instead**: A model would assume `ajv.compileSerializer` outputs standard-compliant JSON by default, serializing `Infinity` and `NaN` to `null` like `JSON.stringify`, without knowing that `specialNumbers: "null"` is required to avoid invalid JSON literal emission.
- **How the wrong version fails**: On Ajv versions before 8.18.0 (e.g., 8.17.1), the missing `specialNumbers` option caused `compileSerializer` to unconditionally emit unquoted `Infinity` / `NaN` literals, failing loudly when any downstream consumer called `JSON.parse()` with `SyntaxError: Unexpected token I in JSON at position ...`.

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  "dependencies": {
    "ajv": "8.20.0"
  }
}

import AjvJTD from 'ajv/dist/jtd.js';

// By default (or with specialNumbers: 'fast'), Ajv's JTD serializer performs raw string
// concatenation without checking for non-finite float values, emitting unquoted
// Infinity / NaN literals that violate RFC 8259 JSON syntax.
export function compileFastSerializer(schema) {
  const ajv = new AjvJTD({ specialNumbers: 'fast' });
  return ajv.compileSerializer(schema);
}

// Added in Ajv v8.18.0 (fixing issue #2482 / PR #2487): configuring specialNumbers: 'null'
// ensures Infinity, -Infinity, and NaN serialize to null, matching JSON.stringify behavior
// and producing valid JSON output.
export function compileSafeSerializer(schema) {
  const ajv = new AjvJTD({ specialNumbers: 'null' });
  return ajv.compileSerializer(schema);
}

import { strict as assert } from 'node:assert';
import { compileFastSerializer, compileSafeSerializer } from '../src/index.mjs';

const schema = {
  properties: {
    score: { type: 'float64' },
  },
};

// 1. By default (and in specialNumbers: "fast" mode), ajv.compileSerializer outputs
// unquoted Infinity and NaN literals for raw speed, producing invalid JSON syntax.
const fastSerializer = compileFastSerializer(schema);
const fastInf = fastSerializer({ score: Infinity });
const fastNegInf = fastSerializer({ score: -Infinity });
const fastNaN = fastSerializer({ score: NaN });

assert.equal(fastInf, '{"score":Infinity}');
assert.equal(fastNegInf, '{"score":-Infinity}');
assert.equal(fastNaN, '{"score":NaN}');

assert.throws(() => JSON.parse(fastInf), SyntaxError);
assert.throws(() => JSON.parse(fastNegInf), SyntaxError);
assert.throws(() => JSON.parse(fastNaN), SyntaxError);

// 2. Since v8.18.0 (fixing #2482), specialNumbers: "null" serializes Infinity, -Infinity,
// and NaN to null, producing valid JSON conforming to JSON.stringify and RFC 8259.
const safeSerializer = compileSafeSerializer(schema);
const safeInf = safeSerializer({ score: Infinity });
const safeNegInf = safeSerializer({ score: -Infinity });
const safeNaN = safeSerializer({ score: NaN });

assert.equal(safeInf, '{"score":null}');
assert.equal(safeNegInf, '{"score":null}');
assert.equal(safeNaN, '{"score":null}');

assert.deepEqual(JSON.parse(safeInf), { score: null });
assert.deepEqual(JSON.parse(safeNegInf), { score: null });
assert.deepEqual(JSON.parse(safeNaN), { score: null });

// 3. Finite numbers continue to serialize normally
const safeFinite = safeSerializer({ score: 42.5 });
assert.equal(safeFinite, '{"score":42.5}');
assert.deepEqual(JSON.parse(safeFinite), { score: 42.5 });

console.log('CONTRACT PASS: ajv JTD specialNumbers: "null" safely serializes non-finite numbers');