JSON

JSON is a text format for exchanging data. Servers send their responses in it, settings are stored in it, and browsers save data with it. JSON looks a lot like JavaScript objects and arrays, but it is always a string.

What JSON looks like

JSON writes objects in curly braces and arrays in square brackets. The rules are stricter than in JavaScript: keys and strings use double quotes only, and a trailing comma is not allowed.

{
  "name": "Anna",
  "age": 25,
  "tags": ["admin", "editor"]
}

JSON can hold strings, numbers, true, false, null, arrays and objects. It has no functions and no undefined.

JSON.stringify — object to string

JSON.stringify() turns a JavaScript value into a JSON string. This is called serialization:

const user = { name: 'Anna', age: 25 };
 
console.log(JSON.stringify(user)); // {"name":"Anna","age":25}

Such a string can be sent to a server or saved, and restored later.

JSON.parse — string to object

JSON.parse() does the opposite: it reads a JSON string and returns an ordinary object you can work with:

const text = '{"name":"Oleg","age":30}';
 
const user = JSON.parse(text);
 
console.log(user.age + 1); // 31

Readable output

The third argument of JSON.stringify() sets the indentation. With it the JSON is split into lines and is easier for a person to read:

const product = { title: 'Tea', price: 5 };
 
console.log(JSON.stringify(product, null, 2));
// {
//   "title": "Tea",
//   "price": 5
// }

The second argument, null, is for selecting properties — it is rarely used.

What gets lost

Not everything survives the trip to JSON. Functions and properties set to undefined disappear, and a date becomes a string:

const data = {
  name: 'Anna',
  greet() {},
  phone: undefined,
  created: new Date(Date.UTC(2026, 0, 15)),
};
 
console.log(JSON.stringify(data)); // {"name":"Anna","created":"2026-01-15T00:00:00.000Z"}

After JSON.parse() the created field will be a string, not a date.

Common mistake

JSON.parse() throws a SyntaxError when the string is not valid JSON. Single quotes or a trailing comma are the usual culprits:

try {
  JSON.parse("{'name': 'Anna'}");
} catch (error) {
  console.log(error.name); // SyntaxError
}

Always parse data that comes from outside inside try...catch.