JSON

What Is JSON?

JSON (JavaScript Object Notation) is a lightweight, text-based data-interchange format that is easy for humans to read and write and easy for machines to parse and generate. Originally derived from JavaScript, JSON is language-independent and is now the de facto standard for data exchange on the web.

JSON represents data as key-value pairs organized into objects and arrays. Its simplicity made it the dominant format for REST APIs, configuration files, and data storage — replacing the more verbose XML in most modern applications.

A Brief History

JSON was formalized by Douglas Crockford in the early 2000s, though the format itself was already implicit in JavaScript syntax. The specification was published at json.org in 2001. RFC 4627 followed in 2006, and the current ECMA-404 standard was published in 2013.

Before JSON, XML was the dominant data exchange format on the web. JSON's minimalism — no attributes, processing instructions, or namespaces — made it immediately popular with developers building AJAX applications. By 2010, JSON had largely displaced XML for web APIs.

JSON Data Types

JSON supports exactly six data types. Every value in a valid JSON document must be one of these:

{
  "string":  "hello world",
  "number":  42,
  "float":   3.14,
  "boolean": true,
  "null":    null,
  "array":   [1, 2, 3],
  "object":  { "nested": "value" }
}

JSON Syntax Rules

JSON has strict syntax rules. A small deviation causes a parse error. Here are the six most important rules to remember:

Invalid vs. Valid JSON

{
  'name': 'Alice',       // single quotes — invalid
  age: 30,              // unquoted key — invalid
  "scores": [1, 2, 3,], // trailing comma — invalid
  "note": undefined     // undefined — invalid
}

{
  "name": "Alice",
  "age": 30,
  "scores": [1, 2, 3],
  "note": null
}

JSON in the Modern Web

REST APIs

JSON is the standard body format for REST API requests and responses. Content-Type: application/json is the most common MIME type on the modern web. Virtually every programming language has a built-in JSON parser, making interoperability seamless.

Configuration Files

package.json, tsconfig.json, .eslintrc, and many other developer tools use JSON for configuration. The structured format and universal parser support make it practical, though the lack of comments is a persistent criticism.

NoSQL Databases

Document databases like MongoDB, CouchDB, and Amazon DynamoDB store data as JSON or JSON-like documents. This allows flexible, schema-less storage that maps naturally to application objects.

Frontend State and Data

localStorage and sessionStorage store strings, so JSON.stringify/JSON.parse are used constantly to serialize JavaScript objects. API responses from fetch() arrive as JSON text and are parsed before use.

JSON vs. Other Formats

JSON is not always the best choice. Here is how it compares to other common data formats:

JSON Schema

JSON Schema is a vocabulary for validating the structure of JSON documents. A schema describes the expected types, required fields, and constraints (minimum, maximum, pattern) of a JSON value. The current version is JSON Schema 2020-12.

Tools like ajv (JavaScript), jsonschema (Python), and built-in validators in many IDEs support JSON Schema. OpenAPI (formerly Swagger) uses JSON Schema to describe API request and response bodies, making it the backbone of modern API documentation.

Common JSON Pitfalls

Frequently Asked Questions