What Is JSON? An Introduction to JavaScript Object Notation

Introduction to JSON

JSON, which stands for JavaScript Object Notation, is a lightweight data-interchange format. It is easy for humans to read and write, and it is easy for machines to parse and generate. Originally popularized by Douglas Crockford in the early 2000s, JSON has become the de facto standard for transmitting data in web APIs, replacing XML in almost all modern applications.

Despite having “JavaScript” in its name, JSON is completely language-independent. Most modern programming languages have robust built-in support for parsing and writing JSON data.

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JSON Syntax Rules

JSON is built on two primary structures:

1. A collection of name/value pairs (commonly realized as an object, dictionary, or hash table).
2. An ordered list of values (realized as an array, list, or vector).

Core Syntax Constraints

• Data is represented in key/value pairs.
• Keys must always be wrapped in double quotes (e.g., "name": "Alice").
• Values must be one of the six valid data types (see below).
• Key/value pairs are separated by commas.
• No trailing commas are allowed at the end of objects or arrays.

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Valid JSON Data Types

A JSON value can only be one of the following:

Type	Description	Example
String	Wrapped in double quotes, supports unicode characters.	"Hello World"
Number	Double-precision floating-point format (no octal or hexadecimal).	42 or -3.14
Object	An unordered collection of key/value pairs enclosed in {}.	{"id": 1}
Array	An ordered collection of values enclosed in [].	[1, 2, "three"]
Boolean	Lowercase logical values.	true or false
Null	Represents an empty or non-existent value.	null

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A Complete JSON Example

Here is a standard JSON payload describing a user profile:

{
  "userId": 101,
  "username": "coder_jane",
  "verified": true,
  "roles": ["admin", "developer"],
  "profile": {
    "firstName": "Jane",
    "lastName": "Smith",
    "avatarUrl": null
  }
}

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Frequently Asked Questions

Is JSON a programming language?

No. JSON is strictly a static text-based data representation format. It cannot contain executable functions, logic, or variables.

Can JSON files contain comments?

No, the standard JSON specification (RFC 8259) does not support comments (// or /* */). If you need configuration files with comments, alternative formats like JSONC or YAML are preferred.