JSON.stringify() Complete Guide

JSON.stringify() is a built-in JavaScript method that converts JavaScript objects, arrays, and primitive values into JSON strings. This process is called serialization and is essential for storing data, sending data over networks, and working with APIs.

Whether you're building a web application, working with REST APIs, or storing data locally, understandingJSON.stringify() is fundamental to JavaScript development. It's the counterpart to JSON.parse(), which converts JSON strings back into JavaScript objects.

Basic Syntax

JSON.stringify(value, replacer, space)

Simple Examples

Converting an Object

const device = {
  hostname: 'router-01',
  ipAddress: '192.168.1.1',
  port: 443,
  active: true
};

const jsonString = JSON.stringify(device);
console.log(jsonString);

// Output:
// {"hostname":"router-01","ipAddress":"192.168.1.1","port":443,"active":true}

You can also use our JSON Formatter tool to format and beautify JSON strings online without writing any code.

Converting an Array

const protocols = ['SSH', 'HTTPS', 'SNMP'];
const jsonString = JSON.stringify(protocols);
console.log(jsonString);

// Output: ["SSH","HTTPS","SNMP"]

Learn more about working with JSON arrays in our comprehensive guide.

Pretty Printing (Formatting)

Use the third parameter to add indentation and make JSON readable. This is similar to what ourJSON Formatter tool does:

const device = {
  hostname: 'router-01',
  config: {
    port: 443,
    protocol: 'HTTPS'
  }
};

// Indent with 2 spaces
const formatted = JSON.stringify(device, null, 2);
console.log(formatted);

// Output:
// {
//   "hostname": "router-01",
//   "config": {
//     "port": 443,
//     "protocol": "HTTPS"
//   }
// }

You can use a number (spaces) or a string (like '\t' for tabs).

Using the Replacer Parameter

Filter Properties with Array

const device = {
  hostname: 'router-01',
  ipAddress: '192.168.1.1',
  internalId: 'secret-123',
  port: 443
};

// Only include specific properties
const jsonString = JSON.stringify(device, ['hostname', 'ipAddress', 'port']);
console.log(jsonString);

// Output: {"hostname":"router-01","ipAddress":"192.168.1.1","port":443}
// 'internalId' is excluded

Transform Values with Function

const device = {
  hostname: 'router-01',
  password: 'secret123',
  port: 443
};

// Hide sensitive data
const jsonString = JSON.stringify(device, (key, value) => {
  if (key === 'password') {
    return '***HIDDEN***';
  }
  return value;
}, 2);

console.log(jsonString);

// Output:
// {
//   "hostname": "router-01",
//   "password": "***HIDDEN***",
//   "port": 443
// }

Handling Special Cases

const obj = { a: 1, b: undefined, c: 3 };
JSON.stringify(obj);
// Output: {"a":1,"c":3}

const obj = { a: 1, fn: function() {} };
JSON.stringify(obj);
// Output: {"a":1}

const obj = { timestamp: new Date('2025-01-09') };
JSON.stringify(obj);
// Output: {"timestamp":"2025-01-09T00:00:00.000Z"}

const obj = {};
obj.self = obj;
JSON.stringify(obj); // Error: Converting circular structure to JSON

Common Use Cases

const data = { hostname: 'router-01', port: 443 };

fetch('/api/devices', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(data)
});

const settings = { theme: 'dark', notifications: true };
localStorage.setItem('settings', JSON.stringify(settings));

// Retrieve and parse later
const stored = localStorage.getItem('settings');
const settings = JSON.parse(stored);

const original = { a: 1, b: { c: 2 } };
const clone = JSON.parse(JSON.stringify(original));
// Note: This only works for simple objects without functions/dates

Best Practices

• •Use try-catch: Wrap in error handling for circular references or large objects
• •Pretty print during development: Use JSON.stringify(obj, null, 2) for debugging, or use our JSON Formatter for quick formatting
• •Filter sensitive data: Use replacer function to exclude passwords or tokens
• •Consider performance: Stringify is synchronous and can block for large objects. Use ourJSON Minifier to reduce file size for production
• •Validate before parsing: Always validate JSON strings with ourJSON Validator before calling JSON.parse()

JSON Tools

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External References