Convert to JSON, replacing cyclic dependencies

toJSON.js

function toJSON(object, fieldsToOmit) {
    const map = new Map();
    let index = 0;
    return JSON.stringify(object, (key, value) => {
        if (~fieldsToOmit.indexOf(key)) return undefined;
        return value != null && typeof value === 'object' 
            ? map.get(value) || (map.set(value, '$' + ++index), value) 
            : value;
    }, 2);
}