What is JSON?

JSON (JavaScript Object Notation) is a lightweight, text-based format for representing structured data. It's how APIs send data over a network, how config files get written, and how you persist data to disk or localStorage.

Valid JSON values

JSON supports exactly six types: string, number, boolean, null, object, and array. Notably absent: undefined, functions, and comments, none of those exist in JSON.

const config = {
  name: 'Kodstigen',
  version: 2,
  isPublic: true,
  maintainer: null,
  tags: ['learning', 'javascript'],
};
console.log(JSON.stringify(config));

JSON vs a JavaScript object literal

They look almost identical, but JSON is a text format: keys and string values must use double quotes, and it can't contain functions, undefined, Date objects, or comments. A JavaScript object literal has none of those restrictions, it's just code.

// Valid JSON text (a string):
const jsonText = '{"name":"Ada","active":true}';

// A JS object literal (code, not a string):
const obj = { name: 'Ada', active: true };