JavaScript Object to JSON: Complete Conversion Guide

Convert an object to a JSON string

Pass the object to JSON.stringify. The result is a string that can be sent over HTTP, saved to localStorage, written to a file, or pasted into another tool.

const user = {
  id: 42,
  name: 'Alice',
  active: true,
  roles: ['admin', 'editor']
}

const json = JSON.stringify(user)

console.log(typeof json)
// string

console.log(json)
// {"id":42,"name":"Alice","active":true,"roles":["admin","editor"]}

Pretty print the JSON output

Add a third argument to create readable JSON. This is the most common form for debugging, config files, and examples in docs.

const pretty = JSON.stringify(user, null, 2)

// {
//   "id": 42,
//   "name": "Alice",
//   "active": true,
//   "roles": [
//     "admin",
//     "editor"
//   ]
// }

Convert only selected properties

Use a replacer array when you want an allowlist of keys. This is useful for small exports, examples, and log output.

const account = {
  id: 42,
  name: 'Alice',
  email: 'alice@example.com',
  passwordHash: 'do-not-export'
}

JSON.stringify(account, ['id', 'name'], 2)

// {
//   "id": 42,
//   "name": "Alice"
// }

For security-sensitive fields, filter data before serialization too. A replacer is useful, but it should not be the only guard that protects secrets.

Transform values while converting

A replacer function can omit, normalize, or convert fields as JSON is created.

const json = JSON.stringify(account, (key, value) => {
  if (key === 'passwordHash') return undefined
  if (value instanceof Date) return value.toISOString()
  if (typeof value === 'bigint') return value.toString()
  return value
}, 2)

This pattern is useful for API payloads, logs, and exports where you want a stable wire format.

Values that do not survive object to JSON conversion

• undefined, functions, and symbols are omitted from objects.
• NaN, Infinity, and -Infinity become null.
• Date objects become ISO strings.
• Map and Set do not serialize as useful JSON by default.
• BigInt throws unless converted first.
• Circular references throw TypeError.

Prepare objects before stringifying

If the object contains Maps, Sets, class instances, or database records, normalize it to plain JSON-compatible data first.

const settings = new Map([
  ['theme', 'dark'],
  ['density', 'compact']
])

const payload = {
  settings: Object.fromEntries(settings),
  exportedAt: new Date().toISOString()
}

JSON.stringify(payload, null, 2)