Convert a JSON object with circular references to a JavaScript string

A JSON object is said to have a circular reference when it contains a reference to itself. This can happen when an object is nested within itself, or when two or more objects reference each other. When you try to serialize a JSON object with circular references using JSON.stringify(), you'll get a TypeError because the method can't handle circular references.

In order to handle this, you can traverse the object and use a WeakSet to store and check seen values, and a custom replacer function to omit values already in the WeakSet. This will allow you to serialize the JSON object without running into a TypeError.

const stringifyCircularJSON = obj => {
  const seen = new WeakSet();
  return JSON.stringify(obj, (k, v) => {
    if (v !== null && typeof v === 'object') {
      if (seen.has(v)) return;
      seen.add(v);
    }
    return v;
  });
};

const obj = { n: 42 };
obj.obj = obj;
stringifyCircularJSON(obj); // '{"n": 42}'