What is JSON?

JSON in simple terms

Think of JSON as a way to write down data as plain text using two familiar building blocks: a list of key/value pairs (an object), and an ordered list(an array). Values can be text, numbers, true/false, nothing (null), or more objects and arrays nested inside. That is the whole idea: a simple, predictable text shape that any program can turn back into data.

A JSON example

Here is a small JSON object describing a user:

{
  "id": 42,
  "name": "Ada Lovelace",
  "active": true,
  "roles": ["admin", "editor"],
  "address": {
    "city": "London",
    "zip": "SW1A 1AA"
  },
  "lastLogin": null
}

The whole thing is one object (wrapped in { }). Inside it are keys ("id", "name") each paired with a value. "roles" holds an array ([ ]), and "address"  holds another nested object. Paste an example like this into the JSON Viewer to explore it as a collapsible tree.

JSON syntax rules

• Data is written as key/value pairs: "key": value.
• Keys must be double-quoted strings. Single quotes are not valid JSON.
• Pairs are separated by commas; there is no trailing comma after the last one.
• Objects use curly braces { }; arrays use square brackets [ ].
• Comments are not allowed in standard JSON.
• The whole document is a single value (usually an object or an array).

If your text breaks these rules (single quotes, trailing commas, comments), it is not strictly valid. The JSON Validator shows the exact error, and the JSON Formatter can auto-repair common mistakes.

JSON data types

JSON supports exactly six types of value:

• String: double-quoted text, e.g. "hello".
• Number: an integer or decimal, e.g. 42 or 9.5 (no separate integer type).
• Boolean: true or false.
• Null: null, meaning "no value".
• Array: an ordered list, e.g. [1, 2, 3].
• Object: a collection of key/value pairs, e.g. { "a": 1 }.

There is no date, currency, or "undefined" type. Dates are conventionally stored as ISO strings like "2026-07-08T10:00:00Z".

Where JSON is used

• Web APIs: most REST APIs send and receive JSON.
• Configuration: files like package.json and tsconfig.json.
• Databases: document stores (MongoDB) and JSON columns in SQL databases.
• Data exchange: exporting and importing data between systems.

JSON vs XML

JSON and XML both structure data as text, but JSON is more compact and maps directly onto the objects and arrays of modern programming languages, which is why it has largely replaced XML for web APIs. XML is more verbose and supports attributes, namespaces, and comments. You can convert between them with XML to JSON and JSON to XML.

Working with JSON

Every tool on this site runs entirely in your browser, so your data never leaves your machine. Common next steps:

• Format or minify JSON to make it readable or compact.
• Validate it against the strict JSON standard.
• Explore a large document as a collapsible tree.
• Open a .json file if someone sent you one.

Frequently asked questions