What Is JSON?

What does JSON stand for?

JSON stands for JavaScript Object Notation. Despite the name, it is completely language-independent — every major programming language can read and write JSON natively or through a well-maintained library.

JSON was specified by Douglas Crockford in the early 2000s as a lightweight alternative to XML for transmitting data between a server and a browser. It succeeded wildly: today JSON is the standard format for web APIs, configuration files, and structured data interchange.

Basic structure

A JSON document is text — a sequence of characters that follows a strict grammar. At the top level it must be either:

A JSON object — an unordered set of key/value pairs enclosed in { }.
A JSON array — an ordered list of values enclosed in [ ].

Everything else in JSON is a value, and values can be nested arbitrarily deep.

Data types

JSON has exactly six value types.

Objects

An object is a collection of key/value pairs. Keys must be double-quoted strings. Values can be any JSON type.

{
  "username": "alice",
  "age": 30,
  "active": true
}

Arrays

An array is an ordered list. Elements can be any value type and do not need to share a type.

["red", "green", "blue"]

[1, "two", true, null, { "nested": "object" }]

Strings

Strings must be wrapped in double quotes — single quotes are not valid JSON. A handful of characters must be backslash-escaped:

Character	Escape sequence
Double quote	`\"`
Backslash	`\\`
Newline	`\n`
Tab	`\t`

Numbers

JSON numbers can be integers or floating-point. There is no separate int / float distinction.

JSON does not support Infinity, -Infinity, or NaN.

Booleans

Exactly two values: true and false. Both are lowercase.

null

The single value null represents the intentional absence of a value. It is not the same as an empty string "" or the number 0.

A realistic example

Here is a JSON payload a server might return for a user profile:

{
  "id": 1042,
  "username": "alice_dev",
  "email": "[email protected]",
  "isPremium": false,
  "bio": null,
  "tags": ["javascript", "open-source"],
  "address": {
    "city": "London",
    "country": "GB"
  },
  "createdAt": "2024-03-15T09:41:00Z"
}

JSON has no native date type — dates are represented as strings. The ISO 8601 format ("2024-03-15T09:41:00Z") is the widely adopted convention.

Common JSON mistakes

Trailing commas — the last item in an object or array must not have a trailing comma:

// ✗ Invalid
{
  "name": "Alice",
  "age": 30,
}

// ✓ Valid
{
  "name": "Alice",
  "age": 30
}

Single-quoted strings — valid JavaScript, but not valid JSON:

// ✗ Invalid
{ 'name': 'Alice' }

// ✓ Valid
{ "name": "Alice" }

Unquoted keys — another habit from JavaScript object literals that breaks JSON:

// ✗ Invalid
{ name: "Alice" }

// ✓ Valid
{ "name": "Alice" }

Comments — JSON does not support comments. If you need them in config files, look at JSONC or JSON5, which are separate (non-standard) formats.

Formatted vs minified JSON

Production JSON is often transmitted without whitespace to reduce payload size:

{"id":1042,"username":"alice_dev","isPremium":false,"tags":["javascript","open-source"]}

This is valid but nearly impossible to read. A JSON formatter re-adds indentation and line breaks. A JSON validator checks the grammar and reports errors precisely. Both run entirely in your browser.

Tips for working with JSON

Validate before you debug — a missing comma or mismatched bracket is the most common culprit.
Use 2-space indentation for a readable balance between depth and line length.
Minify before sending over the network to reduce bandwidth.
Pick a consistent date format (ISO 8601) in your APIs and document it.
When reading JSON from an untrusted source, validate its structure before use — valid JSON is not necessarily the shape your code expects.