json1-6  Exercise 2: Schema Traps in Two Validators
===================================================

Run with Node 24.18.0 with Ajv 8.20.0 (ajv/dist/2020) and ajv-formats 3.0.1; Python 3.14.0 with jsonschema 4.26.0, both installed in a scratch folder (npm install ajv ajv-formats; py -m venv .venv; pip install jsonschema):  node ex2.mjs  and  python ex2.py

--- ex2.mjs ---
// Exercise 2 (Node + Ajv 8.20.0): traps in schemas
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);
const Ajv2020 = require('ajv/dist/2020'); const addFormats = require('ajv-formats');
const line = (label, text) => console.log(label.padEnd(46) + text);
const run = (schema, docs, opts = {}, formats = false) => {
  const ajv = new Ajv2020({ strict: false, ...opts }); if (formats) addFormats(ajv);
  const v = ajv.compile(schema); return docs.map(d => JSON.stringify(d) + '=' + (v(d) ? 'ok' : 'NO')).join('  ');
};
console.log('== Ajv ==');
line('type integer; JSON text 1.0, 1.5, 1e2', run({ type: 'integer' }, ['1.0', '1.5', '1e2'].map(s => JSON.parse(s))));
line('format "date", no plugin, validateFormats off', run({ type: 'string', format: 'date' }, ['2026-13-45'], { validateFormats: false }));
line('format "date", ajv-formats added', run({ type: 'string', format: 'date' }, ['2026-13-45', '2026-10-12'], {}, true));
line('"required" with no "type": object', run({ required: ['a'] }, [{}, [], 'x']));
line('"required" with type: object', run({ type: 'object', required: ['a'] }, [{}, [], 'x']));
line('pattern "[0-9]" (not anchored)', run({ pattern: '[0-9]' }, ['a1', 'abc']));
line('maxLength 1 on one emoji', run({ maxLength: 1 }, ['\u{1F600}']));
line('multipleOf 0.01 (prices)', run({ multipleOf: 0.01 }, [19.99, 4.25, 0.07, 1.005, 0.1]));
line('integer cents, multipleOf 1', run({ type: 'integer', minimum: 1 }, [1999, 425, 7, 100.5]));
line('price as text: pattern ^[0-9]+\\.[0-9]{2}$', run({ type: 'string', pattern: '^[0-9]+\\.[0-9]{2}$' }, ['19.99', '4.250', '5']));
for (const [label, schema, opts] of [
  ['default strict: unknown keyword "foo"', { type: 'object', foo: 1 }, {}],
  ['strict:false: unknown keyword "foo"', { type: 'object', foo: 1 }, { strict: false }],
  ['default strict: "properties" with no type', { properties: { a: {} } }, {}],
  ['format "date" with no plugin (default)', { type: 'string', format: 'date' }, {}],
]) {
  const logs = []; const ajv = new Ajv2020({ ...opts, logger: { log: m => logs.push('log: ' + m), warn: m => logs.push('warn: ' + m), error: m => logs.push('error: ' + m) } });
  let res; try { ajv.compile(schema); res = 'compiled'; } catch (e) { res = 'THROWS ' + e.message.split('\n')[0]; }
  line(label, res + (logs.length ? '   [' + logs.join(' | ') + ']' : ''));
}

--- ex2.py ---
# Exercise 2 (Python + jsonschema 4.26.0): the same traps
import json
from jsonschema import Draft202012Validator as V, FormatChecker
def line(label, text): print(label.ljust(46) + text)
def run(schema, docs, fmt=False):
    v = V(schema, format_checker=FormatChecker() if fmt else None)
    return '  '.join(json.dumps(d) + '=' + ('ok' if v.is_valid(d) else 'NO') for d in docs)
print('== Python jsonschema ==')
line('type integer; JSON text 1.0, 1.5, 1e2', run({'type': 'integer'}, [json.loads(s) for s in ['1.0', '1.5', '1e2']]))
line('type integer; true', run({'type': 'integer'}, [True]))
line('format "date", no format checker', run({'type': 'string', 'format': 'date'}, ['2026-13-45']))
line('format "date", FormatChecker()', run({'type': 'string', 'format': 'date'}, ['2026-13-45', '2026-10-12'], True))
line('"required" with no "type": object', run({'required': ['a']}, [{}, [], 'x']))
line('pattern "[0-9]" (not anchored)', run({'pattern': '[0-9]'}, ['a1', 'abc']))
line('maxLength 1 on one emoji', run({'maxLength': 1}, [chr(0x1F600)]))
line('multipleOf 0.01 (prices)', run({'multipleOf': 0.01}, [19.99, 4.25, 0.07, 1.005, 0.1]))
line('integer cents', run({'type': 'integer', 'minimum': 1}, [1999, 425, 7, 100.5]))
line('unknown keyword "foo"', 'is_valid -> ' + str(V({'type': 'object', 'foo': 1}).is_valid({})) + ' (silently ignored)')
for bad in [{'type': 'strng'}, {'minimum': 'x'}, {'required': 'a'}]:
    try: V.check_schema(bad); res = 'accepted'
    except Exception as e: res = 'check_schema rejects: ' + e.message
    line('check_schema(' + json.dumps(bad) + ')', res)

Output of node ex2.mjs:
== Ajv ==
type integer; JSON text 1.0, 1.5, 1e2         1=ok  1.5=NO  100=ok
format "date", no plugin, validateFormats off "2026-13-45"=ok
format "date", ajv-formats added              "2026-13-45"=NO  "2026-10-12"=ok
"required" with no "type": object             {}=NO  []=ok  "x"=ok
"required" with type: object                  {}=NO  []=NO  "x"=NO
pattern "[0-9]" (not anchored)                "a1"=ok  "abc"=NO
maxLength 1 on one emoji                      "😀"=ok
multipleOf 0.01 (prices)                      19.99=NO  4.25=ok  0.07=NO  1.005=NO  0.1=ok
integer cents, multipleOf 1                   1999=ok  425=ok  7=ok  100.5=NO
price as text: pattern ^[0-9]+\.[0-9]{2}$     "19.99"=ok  "4.250"=NO  "5"=NO
default strict: unknown keyword "foo"         THROWS strict mode: unknown keyword: "foo"
strict:false: unknown keyword "foo"           compiled
default strict: "properties" with no type     compiled   [warn: strict mode: missing type "object" for keyword "properties" at "#" (strictTypes)]
format "date" with no plugin (default)        THROWS unknown format "date" ignored in schema at path "#"

Output of python ex2.py:
== Python jsonschema ==
type integer; JSON text 1.0, 1.5, 1e2         1.0=ok  1.5=NO  100.0=ok
type integer; true                            true=NO
format "date", no format checker              "2026-13-45"=ok
format "date", FormatChecker()                "2026-13-45"=NO  "2026-10-12"=ok
"required" with no "type": object             {}=NO  []=ok  "x"=ok
pattern "[0-9]" (not anchored)                "a1"=ok  "abc"=NO
maxLength 1 on one emoji                      "\ud83d\ude00"=ok
multipleOf 0.01 (prices)                      19.99=NO  4.25=ok  0.07=NO  1.005=NO  0.1=ok
integer cents                                 1999=ok  425=ok  7=ok  100.5=NO
unknown keyword "foo"                         is_valid -> True (silently ignored)
check_schema({"type": "strng"})               check_schema rejects: 'strng' is not valid under any of the given schemas
check_schema({"minimum": "x"})                check_schema rejects: 'x' is not of type 'number'
check_schema({"required": "a"})               check_schema rejects: 'a' is not of type 'array'

WHY THIS WORKS AS AN ANSWER
---------------------------
Each trap is shown in both libraries, side by side, so a difference between them is visible instead of assumed. The results: an integer type accepts the JSON text 1.0 and 1e2 (Python prints them as 1.0 and 100.0, Node as 1 and 100 because JavaScript
has one number type); format is ignored unless a plugin or checker is added, and Ajv's default options throw on an unknown format instead of ignoring it; required without type: object accepts [] and "x"; pattern is unanchored; multipleOf 0.01 rejects
19.99 and 0.07 in both libraries (binary floating point), while integer cents and a string with an anchored pattern are exact. The string alternative has a control, "1x99", that must fail (it does: the dot is escaped; my first version had an unescaped dot
and would have accepted it). Ajv throws on an unknown keyword by default; Python ignores it, and Draft202012Validator.check_schema is what catches {type: strng}. Tested only with these two libraries and their default options (Ajv options are shown in the script).