What is JSON?

Introduction to JSON

JSON (JavaScript Object Notation) is a lightweight, text-based data interchange format. Despite its name containing "JavaScript," JSON is language-independent and can be used with virtually any programming language.

JSON was derived from JavaScript object literal syntax but has become the de facto standard for data exchange on the web, replacing XML in many applications due to its simplicity and smaller size.

JSON Syntax

JSON is built on two structures:

• Objects: A collection of key/value pairs enclosed in curly braces {}
• Arrays: An ordered list of values enclosed in square brackets []

Example JSON Object

{
  "name": "John Doe",
  "age": 30,
  "email": "[email protected]",
  "isActive": true,
  "address": {
    "street": "123 Main St",
    "city": "New York"
  },
  "hobbies": ["reading", "gaming", "cooking"]
}

JSON Data Types

JSON supports six data types:

1. String

A sequence of characters enclosed in double quotes. Must use double quotes, not single quotes.

"Hello, World!"

2. Number

Integer or floating-point numbers. No quotes around numbers.

42
3.14159
-17
1.5e10

3. Boolean

Either true or false (lowercase, no quotes).

true
false

4. Null

Represents an empty or non-existent value.

null

5. Object

A collection of key/value pairs. Keys must be strings in double quotes.

{
  "key1": "value1",
  "key2": "value2"
}

6. Array

An ordered list of values of any type.

[1, 2, 3, "four", true, null]

JSON Syntax Rules

1. Data is in key/value pairs
2. Keys must be strings in double quotes
3. Values must be a valid JSON data type
4. Data is separated by commas
5. Curly braces hold objects
6. Square brackets hold arrays
7. No trailing commas allowed
8. No comments allowed in standard JSON

Common Mistakes

• Using single quotes: JSON requires double quotes for strings
• Trailing commas: The last item should not have a comma after it
• Unquoted keys: Object keys must be in double quotes
• Comments: JSON does not support comments
• Undefined: JSON does not have an undefined type, use null instead

JSON vs XML

JSON has largely replaced XML for data interchange due to several advantages:

• More compact and readable
• Faster to parse
• Maps directly to data structures in most languages
• Native support in JavaScript

Where JSON is Used

• Web APIs: REST APIs typically return JSON
• Configuration files: package.json, tsconfig.json
• Data storage: NoSQL databases like MongoDB
• Local storage: Browser localStorage and sessionStorage

Try It Yourself

Now that you understand JSON, try our tools to work with JSON data: