JavaScript JSON

Text that looks like data

JSON stands for JavaScript Object Notation. It is a string: quotes, braces, brackets, and commas. Servers send it. localStorage stores it. Your script does not send a live object over the network — it sends text.

Two functions sit on the JSON object. stringify goes object to string. parse goes string to object. That is the whole round trip.

JSON.stringify

Pass a value. You get a string. Objects and arrays become JSON text. Numbers, booleans, andnull become their JSON forms. Strings get quotes and escaped characters.

const task = {
  id: 1,
  title: "Read chapter",
  done: false,
};

const text = JSON.stringify(task);
console.log(text);
console.log(typeof text);

The log looks like an object, but typeof text is "string". You cannot write text.title. Parse first, or keep the original object.

JSON.parse

Pass a JSON string. You get a JavaScript value. If the text is not valid JSON, parse throws — wrap it in try / catch when the string came from a user, storage, or a network.

const text = '{"id":1,"title":"Read chapter","done":false}';
const task = JSON.parse(text);

console.log(task.title);
console.log(task.done);
console.log(typeof task);

JSON is stricter than JavaScript

Object keys must be in double quotes. Strings must use double quotes, not single quotes. Trailing commas are illegal. undefined, functions, and Date objects are not JSON values — stringify drops undefined fields and turns dates into strings.

try {
  JSON.parse("{title: 'Ada'}");
} catch (err) {
  console.log(err.message);
}

try {
  JSON.parse('{"title": "Ada",}');
} catch (err) {
  console.log(err.message);
}

console.log(JSON.parse('{"title":"Ada"}').title);

The first two strings would be fine as JavaScript object literals. They are not JSON. The third string parses.

Pretty print

A second argument to stringify is a replacer (use null to skip it). A third argument is spaces for indent. That is for logs and files people will read, not for compact storage.

const user = { name: "Lin", role: "editor" };
console.log(JSON.stringify(user, null, 2));

Where you will use it

response.json() on a fetch is parse under another name. Storage only keeps strings, so you stringify objects before setItem. Both chapters come later in this track.

What to remember

• JSON is text, not a live object.
• JSON.stringify object to string. JSON.parse string to object.
• Keys and strings use double quotes. No trailing commas.
• Parse can throw. Catch it when the text is not under your control.

Next: debugging — reading the console, the error line, and fixing one problem at a time.