JSON.stringify()

Beginner
5 min read
Updated: Nov 2026
3 examples
JSON.stringify tutorial — turn JavaScript values into JSON text
Definition

Definition and Usage

JSON.stringify() turns a JavaScript value into a JSON text string—ready for APIs, files, or localStorage.

It is the reverse of JSON.parse(). Optional replacer and space arguments let you filter properties and pretty-print the output.

Beginner path.

Stringify a plain object → try space for readable text → use a replacer when you need to drop fields. Pair with JSON.parse().

Reference

Syntax

JSON
JSON.stringify(value)
JSON.stringify(value, replacer)
JSON.stringify(value, replacer, space)
  • value The JavaScript value to convert (object, array, string, number, boolean, or null).
  • replacer Optional function (key, value) => …, or an array of property names to keep.
  • space Optional number or string for indentation (pretty-print). Often 2 or "\\t".
Watch outs

Special Cases

  • undefined Dropped on objects; becomes null in arrays.
  • Date Becomes an ISO-8601 string (via toJSON).
  • functions / Symbol Omitted on objects; null in arrays.
  • NaN / Infinity Serialized as null.
  • circular refs Throw TypeError — JSON has no cycles.
  • BigInt Throws unless you convert it first (for example to a string).
JSON Schema

Output Shape

What you stringify should match the JSON shape your API or JSON Schema expects. Stringify only writes text; schema tools validate after someone parses it again.

JSON
const user = { name: "Ada", age: 36 };
const text = JSON.stringify(user);
const again = JSON.parse(text);
// again matches { "type": "object", "properties": { "name": …, "age": … } }
Code

Examples

Use View Output for a quick result, or Try It Yourself to edit and run in the browser.

Example 1 — Basic Stringify

Turn an object into compact JSON text.

JSON
const user = {
  name: "Ada",
  age: 36,
  active: true
};

const text = JSON.stringify(user);
console.log(text);
console.log(typeof text);
Try It Yourself

How It Works

  1. Pass a plain object to JSON.stringify.
  2. You get a single-line JSON string.
  3. typeof of the result is "string".

Example 2 — Pretty-Print with space

Pass null for replacer and 2 for indentation.

JSON
const user = {
  name: "Ada",
  skills: ["math", "code"]
};

console.log(JSON.stringify(user, null, 2));
Try It Yourself

How It Works

  1. Second argument null means “no replacer”.
  2. 2 indents each level with two spaces.
  3. Use pretty text for logs and config; keep compact for APIs.

Example 3 — Replacer

Drop secrets (or any field) by returning undefined from a replacer.

JSON
const user = {
  name: "Ada",
  password: "secret",
  age: 36
};

const text = JSON.stringify(user, function (key, value) {
  if (key === "password") {
    return undefined;
  }
  return value;
});

console.log(text);
Try It Yourself

How It Works

  1. The replacer runs for each key/value pair.
  2. Returning undefined omits that property.
  3. Return every other value unchanged so it stays in the JSON.

Frequently Asked Questions

A string of JSON text for the input value. Without space it is usually one compact line. JSON.stringify(undefined) returns undefined, not a string.
Object properties with undefined are omitted. Array slots that are undefined become null. Functions and Symbols are treated the same way.
Pass a replacer: an array of property names to keep, or a function (key, value) that returns undefined to drop a property.
JSON cannot represent cycles. JSON.stringify throws TypeError if an object refers back to itself. Flatten the data or remove the cycle first.

Next: JSONP

Learn the legacy callback pattern around JSON.

JSONP tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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