JSON Basics: Syntax Rules and the Errors That Break It

JSON (JavaScript Object Notation) is the most common format for sending data between websites, apps and APIs. It is easy to read, but it is strict: a single misplaced comma or quote makes the whole document invalid. This guide covers the rules and the mistakes behind most parse errors.

What JSON looks like

{
  "name": "Asha",
  "age": 29,
  "active": true,
  "skills": ["design", "writing"],
  "address": { "city": "Pune", "pin": "411001" },
  "manager": null
}

A JSON document is built from objects (curly braces with key-value pairs), arrays (square brackets with a list of values) and simple values.

The six value types

• String: text in double quotes, such as "hello".
• Number: 42, 3.14 or -7. No quotes, no leading zeros.
• Boolean: true or false, in lowercase.
• Null: null, in lowercase.
• Object: unordered key-value pairs in curly braces.
• Array: an ordered list in square brackets.

The mistakes that cause most errors

Trailing commas

A comma after the last item, such as {"a": 1,}, is allowed in JavaScript but not in JSON. Remove it.

Single quotes

JSON requires double quotes. {'name': 'Asha'} is invalid; it must be {"name": "Asha"}.

Unquoted keys

In JSON, every key must be a quoted string. {name: "Asha"} is a valid JavaScript object but not valid JSON.

Comments

JSON has no comment syntax. Lines starting with // or blocks in /* */ will break parsing. Remove them or keep notes in a separate field.

Special values

NaN, Infinity and undefined are not valid JSON. Use null or a string instead.

Unescaped characters inside strings

A double quote inside a string must be written as \", and a line break as \n. Raw tabs and newlines inside a string are not allowed.

Mismatched brackets

Every { needs a }, and every [ needs a ]. In long documents, formatting with indentation makes missing brackets obvious.

Format, minify and validate

Formatting (pretty-printing) adds indentation and line breaks so people can read the structure. Minifying removes all unnecessary whitespace to make the data smaller for transfer. Validating checks that the text follows the rules. Our JSON Formatter does all three in your browser and points to the position of an error, so you can fix it without scanning the whole file.

Tips for working with JSON

• Use clear, consistent key names, such as camelCase or snake_case, but not both.
• Store dates as ISO 8601 strings, like "2026-11-08", to avoid ambiguity.
• Keep numbers as numbers. Wrapping them in quotes turns them into strings and can break calculations.
• Do not paste confidential data into online tools you do not trust. Ours runs locally in your browser.

When data is binary

JSON cannot hold raw binary data such as an image. Such data is usually converted to text first with Base64. See our guide to Base64 encoding to learn when that is a good idea and when it is not.

Frequently asked questions