JSONパーサー

reviverでは元の桁を取り戻せません。reviverが動く時点で、数値はすでに浮動小数点数に変換されているからです。

const value = JSON.parse(text);

// The reviver runs on every key and value, which is how you
// intercept large integers before precision is lost.
const value2 = JSON.parse(text, function (key, val) {
  if (typeof val === 'number' && !Number.isSafeInteger(val)) {
    // this[key] is the raw value; the original TEXT is not
    // available here, which is the limitation.
    console.warn('unsafe integer at', key);
  }
  return val;
});

Pythonで重複キーを検出する移植性のある方法は、object_pairs_hookだけです。

import json
from decimal import Decimal

# parse_float and parse_int receive the raw TEXT of the number,
# so unlike JavaScript you can keep full precision.
data = json.loads(text, parse_float=Decimal, parse_int=int)

# Detect duplicate keys instead of silently keeping the last
def no_dupes(pairs):
    seen = {}
    for k, v in pairs:
        if k in seen:
            raise ValueError(f'duplicate key: {k}')
        seen[k] = v
    return seen

data = json.loads(text, object_pairs_hook=no_dupes)

// UseNumber keeps the original text instead of converting
// to float64, so large integers survive.
dec := json.NewDecoder(bytes.NewReader(data))
dec.UseNumber()

var v any
if err := dec.Decode(&v); err != nil {
    return err
}

ObjectMapper mapper = new ObjectMapper();

// Fail loudly on duplicates rather than keeping the last one
mapper.enable(DeserializationFeature.FAIL_ON_READING_DUP_TREE_KEY);

// Keep big integers exact
mapper.enable(DeserializationFeature.USE_BIG_INTEGER_FOR_INTS);