What is JSON?

JavaScriptJunior~2 min read

JSON is one of the most important concepts in frontend and web development in general. Simply put:

JSON (JavaScript Object Notation) is a text format for storing and exchanging data, understandable to both humans and computers.

Example of JSON

javascript

{
  "name": "Oleh",
  "age": 25,
  "isAdmin": false,
  "skills": ["JavaScript", "React", "Node.js"],
  "address": {
    "city": "Kyiv",
    "zip": 123456
  }
}

This is a regular JSON document. Essentially it is a string that holds structured data. Such data is easy to read and transmit over the network.

Where JSON is used

Exchanging data between the client and the server The server sends JSON, the browser reads it:

javascript

fetch('/api/user')
  .then(res => res.json())
  .then(data => console.log(data.name));

Storing data in localStorage

javascript

const user = { name: 'Oleh', age: 25 };
localStorage.setItem('user', JSON.stringify(user));

Settings and configuration (for example, package.json in Node.js projects):

javascript

{
  "name": "my-project",
  "version": "1.0.0",
  "scripts": {
    "start": "node index.js"
  }
}

JSON syntax rules

Rule	Description
Keys are always in double quotes	`"name": "Oleh"`
Strings are also in double quotes	`"city": "Kyiv"`
Only simple types are supported	`string`, `number`, `boolean`, `null`, `object`, `array`
No functions	`function() {}` → error
No `undefined` and `Symbol`	they are ignored
Comma between elements	but not after the last element

Basic JSON data types

Type	Example
string	`"Oleh"`
number	`25`
boolean	`true`, `false`
null	`null`
object	`{ "a": 1, "b": 2 }`
array	`[1, 2, 3]`

Difference between JSON and JavaScript objects

Feature	JSON	JavaScript object
Keys	Only in `"double quotes"`	Can be without quotes
Strings	Only `"double"`	`'single'` or `"double"`
Types	Only simple ones (`string`, `number`, etc.)	Any (`function`, `undefined`, `symbol`)
Purpose	Storing/exchanging data	Working with data in JS code

Converting between an object and JSON

javascript

const obj = { name: 'Oleh', age: 25 };

// object -> JSON
const json = JSON.stringify(obj);
console.log(json);
// '{"name":"Oleh","age":25}'

// JSON -> object
const parsed = JSON.parse(json);
console.log(parsed.name); // 'Oleh'

Summary

Parameter	JSON
Full name	JavaScript Object Notation
Format	Text
Used for	Storing and exchanging data
Typical operations	`JSON.stringify()` and `JSON.parse()`
Based on	JavaScript syntax
Supported	In all languages (Python, Java, C#, Go, etc.)

In short

JSON is a universal data format that lets you transmit complex structures (objects, arrays and so on) as a plain text string.

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