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What Is JSON and Why Do Developers Use It?
JSON is the most widely used data format on the web. If you work with APIs, web development, or configuration files you encounter it constantly. Here is a clear explanation of what it is and how it works.
ToolSpot AI Team
Editorial
June 29, 20265 min read
Use our free JSON Formatter and Validator to format, validate, and beautify JSON - no signup needed.
What Is JSON and Why Do Developers Use It?
JSON shows up everywhere in modern software development. API responses, configuration files, database records, web application data, package manifests - JSON is the default format for structured data exchange across the web. Yet many developers who use it daily have never taken ten minutes to understand it systematically.
This guide explains what JSON is, why it became dominant, how to read and write it correctly, and the common mistakes to avoid.
What does JSON stand for?
JSON stands for JavaScript Object Notation. It was originally derived from JavaScript object syntax but is now a language-independent format used across virtually every programming language and platform.
It was formalised by Douglas Crockford in the early 2000s as a lightweight alternative to XML for data exchange. Its simplicity and human readability drove rapid adoption and it is now the de facto standard for web APIs.
What JSON looks like
JSON represents data as structured text. Here is a simple example:
{
"name": "Alice Johnson",
"age": 32,
"email": "alice@example.com",
"isActive": true,
"scores": [95, 87, 92],
"address": {
"city": "Austin",
"state": "TX",
"zip": "78701"
}
}
Everything between the outer curly braces is a JSON object. It contains key-value pairs where keys are always strings in double quotes and values can be any of the six valid JSON data types.
The six JSON data types
String - text wrapped in double quotes
"name": "Alice Johnson"
Number - integer or decimal, no quotes
"age": 32
"price": 9.99
Boolean - true or false (lowercase, no quotes)
"isActive": true
Null - represents an empty or absent value
"middleName": null
Array - an ordered list of values in square brackets
"scores": [95, 87, 92]
Object - a collection of key-value pairs in curly braces
"address": {"city": "Austin", "state": "TX"}
Arrays and objects can be nested inside each other to any depth, which is what makes JSON flexible enough to represent complex data structures.
JSON syntax rules
These rules are strictly enforced. A single violation makes the entire JSON document invalid.
Keys must be strings in double quotes. Single quotes are not valid JSON.
Valid: "name": "Alice"
Invalid: name: "Alice" or 'name': 'Alice'
String values must use double quotes. Single quotes are not valid for values either.
No trailing commas. The last item in an object or array must not have a comma after it.
Valid: {"a": 1, "b": 2}
Invalid: {"a": 1, "b": 2,}
No comments. JSON does not support comments of any kind. This is one of the most common frustrations for developers coming from languages that allow inline comments.
Numbers cannot have leading zeros. 007 is invalid. 7 is valid.
No undefined or functions. Unlike JavaScript objects, JSON cannot contain functions, undefined values, or special objects like Date. Dates are typically represented as ISO 8601 strings: "2024-03-15T14:30:00Z"
Why JSON became the standard
Before JSON, XML was the dominant format for data exchange. XML is verbose and requires closing tags that repeat the element name, making it bulky to write and expensive to transmit over networks.
Compare the same data in XML and JSON:
XML:
<person>
<name>Alice Johnson</name>
<age>32</age>
<email>alice@example.com</email>
</person>
JSON:
{"name": "Alice Johnson", "age": 32, "email": "alice@example.com"}
JSON is more compact, easier to read, and maps naturally to data structures in most programming languages (dictionaries in Python, objects in JavaScript, maps in Java and Go). Parsing JSON is also computationally cheaper than parsing XML.
The rise of REST APIs and JavaScript-heavy web applications in the mid-2000s drove JSON to near-universal adoption. Today virtually every web API returns JSON by default.
Common uses of JSON
REST API responses - when you make an API request to any modern web service, the response almost always comes back as JSON. Weather APIs, payment APIs, social media APIs, mapping APIs - all JSON.
Configuration files - package.json in Node.js projects, tsconfig.json for TypeScript, .eslintrc for ESLint, launch.json in VS Code - JSON configuration files are everywhere in development tooling.
NoSQL databases - MongoDB stores documents in BSON (Binary JSON). Firestore, DynamoDB, and other document databases all use JSON-like structures natively.
Data interchange between services - microservices communicating over HTTP typically exchange JSON payloads.
Local storage in browsers - web applications store user preferences and session data as JSON strings in browser localStorage.
How to read and write JSON in practice
Most modern programming languages have built-in JSON support.
In JavaScript:
const jsonString = '{"name": "Alice", "age": 32}'
const obj = JSON.parse(jsonString) // string to object
const backToString = JSON.stringify(obj) // object to string
In Python:
import json
json_string = '{"name": "Alice", "age": 32}'
data = json.loads(json_string) # string to dict
back_to_string = json.dumps(data) # dict to string
In any language the two core operations are parsing (string to data structure) and serialising (data structure to string).
Common JSON mistakes
Using single quotes instead of double quotes - the most frequent error, especially for developers who write JavaScript where single quotes are common.
Trailing commas - forgetting that the last element in an array or object cannot have a trailing comma. Many code editors and linters catch this automatically.
Using undefined or NaN - these are JavaScript-specific values that are not valid JSON. If you need to represent the absence of a value use null.
Deeply nested structures - technically valid but practically difficult to work with. If your JSON is nested five or more levels deep, consider restructuring the data or breaking it into multiple smaller documents.
Inconsistent key naming - JSON is case sensitive. Name, name, and NAME are three different keys. Agree on a naming convention (camelCase is most common in JSON from JavaScript APIs, snake_case in Python APIs) and be consistent.
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Frequently asked questions
They look similar but they are different things. A JavaScript object is a runtime data structure in memory. JSON is a text format for representing data. JavaScript objects can contain functions, undefined values, and circular references - none of which are valid in JSON. JSON is stricter and is language-independent. You convert between them using JSON.parse() and JSON.stringify() in JavaScript.
Douglas Crockford, who formalised JSON, has explained that he removed comment support intentionally to prevent people from using JSON as a configuration format with parsing directives embedded in comments. He wanted JSON to be a pure data interchange format. In practice this decision frustrates many developers - workarounds include using a key like underscore comment for notes within the data, or using JSONC supported by some tools.
Both are human-readable data formats used for configuration and data exchange. YAML uses indentation instead of brackets and supports comments, making it arguably more readable for configuration files. JSON is more strictly defined and faster to parse, making it better for data interchange over networks. Kubernetes configs and GitHub Actions use YAML. Web APIs almost universally use JSON.
JSON cannot natively represent binary data - it only supports text. Binary data (images, files, certificates) is typically Base64 encoded and stored as a JSON string. This increases the size by approximately 33% but allows binary content to be included in a JSON payload. For large files it is usually more efficient to reference a URL to the binary rather than embedding it.
There is no defined maximum size in the JSON specification. Practical limits come from the systems processing the JSON - memory available for parsing, API payload limits (many REST APIs cap requests at 1MB to 10MB), and database document size limits (MongoDB has a 16MB document limit). For large datasets JSON is often split into multiple smaller documents or replaced with more efficient formats like Protocol Buffers or MessagePack.
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