parse.js

// Works the same as JSON.parse, but also handles BigInt type,
// by returning BigInt for each string that uses 123n format:
function parse(text) {
    return JSON.parse(text, (_, value) => {
        if (typeof value === 'string') {
            const m = value.match(/(-?\d+)n/);
            if (m && m[0] === value) {
                value = BigInt(m[1]);
            }
        }
        return value;
    });
}