JSON – Introduction & Syntax

What is JSON?

JSON (JavaScript Object Notation) is a lightweight text format used for storing and exchanging data between a client and a server.

It is easy to read and write, and its syntax is similar to JavaScript objects.

• JSON is plain text and language independent.
• JSON is used to send, receive, and store data.
• It can be easily converted to JavaScript objects and vice versa.
• Originally specified by Douglas Crockford.

Example

{
  "name": "John",
  "age": 30,
  "car": null}

Defines an object with three properties:

• "name" → "John"
• "age" → 30
• "car" → null

Why Use JSON?

• It’s lightweight and easy to transmit across networks.
• Text-only format, compatible with any programming language.
• Ideal for APIs and server communication.
• Stores complex data structures (objects and arrays).

JSON and JavaScript

JSON is syntactically identical to JavaScript object notation.

This means:

• JavaScript can easily parse JSON strings into objects.
• JavaScript can convert objects into JSON strings.

Built-in JavaScript JSON Methods

Method	Description	Example
JSON.parse()	Converts JSON string → JavaScript object	const obj = JSON.parse(text);
JSON.stringify()	Converts JavaScript object → JSON string	const text = JSON.stringify(obj);

Example:

let text = '{"name":"John", "age":30}';
const obj = JSON.parse(text);
console.log(obj.name); // John

const jsonString = JSON.stringify(obj);
console.log(jsonString); // {"name":"John","age":30}

JSON Example with Array

{
  "employees": [
    { "firstName": "John", "lastName": "Doe" },
    { "firstName": "Anna", "lastName": "Smith" },
    { "firstName": "Peter", "lastName": "Jones" }
  ]
}

Accessing data in JavaScript:

const obj = JSON.parse(text);
console.log(obj.employees[1].firstName); // Anna

JSON Syntax Rules

JSON syntax is derived from JavaScript object notation:

• Data is written as name/value pairs
• Pairs are separated by commas
• Curly braces {} hold objects
• Square brackets [] hold arrays
• Keys must be in double quotes
• No comments allowed

JSON Name/Value Pair Example

"name": "John"

• "name" → key
• "John" → value

JSON Data Types

In JSON, values must be one of the following:

• String
• Number
• Object
• Array
• Boolean
• Null

Example:

{
  "string": "Hello",
  "number": 25,
  "boolean": true,
  "array": [1, 2, 3],
  "object": { "key": "value" },
  "nullValue": null}

Difference Between JSON and JavaScript Object

Feature	JavaScript Object	JSON
Keys	Can be unquoted	Must be in double quotes
Values	Can include functions, dates, undefined	Only strings, numbers, objects, arrays, booleans, or null
Comments	Allowed	Not allowed

Example:

// JavaScript Object
let obj = { name: "John", age: 30 };

// JSON String
let json = '{"name":"John", "age":30}';

JSON Files

• File extension: .json
• MIME type: application/json