You can define a toJSON property on the BigInt prototype that gets used for serialization:

    Object.defineProperty(BigInt.prototype, "toJSON", {
        get() {
            "use strict";
            return () => String(this);
        }
    });

Of course you could also serialize to a Number, but that risks losing precision. For example, the Twitter API often returns IDs as both a Number ("id") and a String ("id_str") to safely handle cases where the value might fall outside of a normal double-precision float: https://developer.twitter.com/en/docs/tweets/data-dictionary...