JSON.stringify vs JSON.parse — Differences Explained

Published: July 25, 2025 · 6 min read

These two functions are the core of JSON handling in JavaScript. They work in opposite directions, and understanding them fully — including their quirks — will save you from hard-to-debug bugs.

The One-Line Summary

Function	Direction	Returns
`JSON.parse(str)`	JSON string → JS value	Object, array, string, number, boolean, or null
`JSON.stringify(val)`	JS value → JSON string	String

JSON.parse() in Depth

Signature: JSON.parse(text, reviver?)

// Basic usage const obj = JSON.parse('{"name":"Alice","age":30}'); // → { name: "Alice", age: 30 } // Throws SyntaxError on invalid input JSON.parse("{bad json}"); // SyntaxError! // With reviver — transform values as they're parsed const data = JSON.parse('{"created":"2025-01-01T00:00:00Z","value":42}', (key, val) => { if (key === "created") return new Date(val); return val; }); console.log(data.created instanceof Date); // true

JSON.stringify() in Depth

Signature: JSON.stringify(value, replacer?, space?)

// Basic usage JSON.stringify({ a: 1, b: "hello" }); // '{"a":1,"b":"hello"}' // Pretty-print with indent JSON.stringify({ a: 1 }, null, 2); // '{ // "a": 1 // }' // With tab indentation JSON.stringify({ a: 1 }, null, "\t"); // Replacer as array — whitelist specific keys JSON.stringify({ name: "Alice", password: "secret", age: 30 }, ["name", "age"]); // '{"name":"Alice","age":30}' — password is excluded // Replacer as function — transform or filter values JSON.stringify({ name: "Alice", joined: new Date("2025-01-01") }, (key, val) => { if (val instanceof Date) return val.toISOString().split("T")[0]; // date only return val; }); // '{"name":"Alice","joined":"2025-01-01"}'

What JSON.stringify Silently Drops

This is the most common source of bugs. These JavaScript values are not representable in JSON:

const tricky = { fn: () => "hello", // → dropped (key removed) undef: undefined, // → dropped (key removed) sym: Symbol("x"), // → dropped (key removed) nan: NaN, // → null inf: Infinity, // → null negInf: -Infinity, // → null date: new Date(), // → ISO string (auto-converted) regex: /abc/, // → {} (empty object!) map: new Map([[1,2]]), // → {} (empty object!) }; console.log(JSON.stringify(tricky, null, 2)); // { // "nan": null, // "inf": null, // "negInf": null, // "date": "2025-07-25T10:00:00.000Z", // "regex": {}, // "map": {} // }

Key rule: If a value would serialize as undefined, it gets dropped entirely when it's an object property. If it's in an array, it becomes null.

JSON.stringify([1, undefined, 3]); // "[1,null,3]" JSON.stringify({ a: undefined }); // "{}"

toJSON() — Custom Serialization

You can control how any object is serialized by adding a toJSON() method. JSON.stringify calls it automatically.

class Money { constructor(amount, currency) { this.amount = amount; this.currency = currency; } toJSON() { return `${this.amount} ${this.currency}`; } } const price = new Money(29.99, "USD"); JSON.stringify({ price }); // '{"price":"29.99 USD"}'

Circular Reference Error

Trying to stringify an object that references itself throws a TypeError.

const obj = { name: "Alice" }; obj.self = obj; // circular reference! JSON.stringify(obj); // TypeError: Converting circular structure to JSON

Solutions: use a replacer function to detect and skip circular refs, or use a library like flatted or json-stringify-safe for serializing circular structures.

Quick Reference

Use Case	Use This
Parse API response body	`JSON.parse(text)` or `response.json()`
Send data in fetch body	`JSON.stringify(data)`
Save to localStorage	`localStorage.setItem(key, JSON.stringify(val))`
Read from localStorage	`JSON.parse(localStorage.getItem(key))`
Deep clone (simple objects)	`JSON.parse(JSON.stringify(obj))`
Pretty-print for debugging	`JSON.stringify(obj, null, 2)`
Filter sensitive fields	`JSON.stringify(obj, ["field1","field2"])`

Try Our JSON Tools

JSON Formatter — equivalent to JSON.stringify(data, null, 2) in your browser
Fix JSON — repair malformed JSON that causes JSON.parse() to throw
JSON Tree Viewer — explore any JSON structure visually