TypeError: Converting circular structure to JSON — cause and fix

When you call JSON.stringify() on an object that contains a reference back to itself (directly or through a chain), JavaScript throws:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'child' -> object with constructor 'Object'
    --- property 'parent' closes the circle

What causes circular references

• Parent-child back-references: a child node that holds a reference to its parent, forming a cycle.
• DOM nodes: HTML elements have circular references through parentNode, childNodes, etc. Never stringify a DOM element.
• Logging frameworks / error objects: request and response objects in Express, Fastify, and others have circular references by design.
• ORM entities: Sequelize, TypeORM, and Mongoose models with bidirectional associations are often circular.

Four ways to fix it

1. Use a replacer function (standard JSON.stringify)

function safeStringify(obj) {
  const seen = new WeakSet();
  return JSON.stringify(obj, (key, value) => {
    if (typeof value === 'object' && value !== null) {
      if (seen.has(value)) return '[Circular]';
      seen.add(value);
    }
    return value;
  });
}

2. Use the json-stringify-safe package

import stringify from 'json-stringify-safe';
const json = stringify(circularObj, null, 2);  // replaces cycles with '[Circular ~]'

3. Use structuredClone + delete the cycle

// Remove the back-reference before stringifying
const copy = structuredClone(obj);
delete copy.child.parent;  // break the cycle
JSON.stringify(copy);

4. Map to a plain DTO before serializing

// Best for ORMs / API responses: explicitly shape the output
const dto = {
  id: user.id,
  name: user.name,
  posts: user.posts.map(p => ({ id: p.id, title: p.title })),
  // omit user.posts[n].author (which points back to user)
};
JSON.stringify(dto);

Detecting cycles without stringifying

function hasCycle(obj, seen = new WeakSet()) {
  if (typeof obj !== 'object' || obj === null) return false;
  if (seen.has(obj)) return true;
  seen.add(obj);
  return Object.values(obj).some(v => hasCycle(v, seen));
}