# The JSON Grammar: Six Types and a Few Strict Rules

> JSON is smaller than it looks. The whole format is six value types and a handful of structural characters, governed by rules that are stricter than most people remember: no comments, no trailing commas, and keys that must be quoted strings.

Source: https://ronutz.com/en/learn/json-grammar  
Updated: 2026-06-29  
Related tools: https://ronutz.com/en/tools/json-formatter

---

## A deliberately small format

JSON was designed to be the smallest data format that is still useful, and that smallness is the whole point: it is easy to write a parser, easy to read, and hard to disagree about. The grammar is defined identically by RFC 8259 and ECMA (European Computer Manufacturers Association)-404, and it fits on a postcard. A JSON document is exactly one value, and a value is one of six things: an object, an array, a string, a number, a boolean, or null. Everything else is built from those.

## The six value types

An object is an unordered set of members wrapped in curly braces, where each member is a string key, a colon, and a value: `{"name": "Ada", "active": true}`. An array is an ordered list of values in square brackets: `[1, 2, 3]`. A string is text in double quotes, with a fixed set of backslash escapes for special characters. A number is a decimal literal, which has subtleties worth [its own article](https://ronutz.com/en/learn/json-numbers-and-precision). A boolean is `true` or `false`, lowercase. And null is the literal `null`. Objects and arrays nest freely, and that nesting is what lets a flat grammar describe arbitrarily complex data.

## The rules people forget

JSON is stricter than the JavaScript object literals it resembles, and the differences are where validation fails. Keys must be double-quoted strings: `{name: 1}` is invalid, and so is `{'name': 1}`, because single quotes are not allowed anywhere. There are no comments in JSON, despite how often people want them. There are no trailing commas, so `[1, 2,]` is an error. Numbers cannot have a leading zero or a leading plus, and they cannot be `NaN` or `Infinity`, because those are not representable in the grammar. A document must contain a single value and nothing after it, so two values in a row, or trailing text, is invalid.

## Why a validator points to a location

Because the grammar is so rigid, every error has a precise location: the parser knows exactly which character violated which rule. A good validator reports not just that the document is invalid but where, by line and column, and which structural path it was reading at the time. That path, written as a JSON Pointer, turns a vague "syntax error" into "the value at /users/2/email is malformed," which is the difference between minutes and seconds of debugging. The formatter parses against this grammar and reports failures exactly that way.