JSON Syntax Rules — The Golden Rules

JSON has a small, precise grammar. Break any of these rules and every parser will reject the document. The rules are simple enough to memorize in five minutes.

1. Keys must be double-quoted strings — "name", not name or 'name'
2. Strings must use double quotes — single quotes are never valid
3. No trailing commas — no comma after the last item in an object or array
4. No comments — JSON has no // or /* */ syntax
5. true, false, and null are lowercase — True, False, Null are all invalid
6. Numbers have no quotes — write 42, not "42" (unless you want a string)
7. The root must be a valid JSON value — object, array, string, number, boolean, or null

A valid object:

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "score": null
}

A valid array:

["apple", "banana", "cherry"]

A valid nested structure:

{
  "user": {
    "id": 1,
    "roles": ["admin", "editor"]
  }
}

Single quotes — invalid:

{'name': 'Alice'}

Trailing comma — invalid:

{"name": "Alice", "age": 30,}

Unquoted key — invalid:

{name: "Alice"}

Comment — invalid:

{
  "port": 3000
  // This is the default port
}

Uppercase boolean — invalid:

{"active": True}

Douglas Crockford intentionally removed comments from JSON because he observed that developers were using them to hold parsing directives — turning JSON into a configuration language with side effects. JSON is a pure data format. If you need comments, consider JSONC (used by VS Code) or YAML, or store comments as a _comment string field.

JSON5 relaxes several of these rules — it allows comments, trailing commas, single quotes, and unquoted keys. But JSON5 is not JSON. Standard parsers like JSON.parse() will reject JSON5 syntax. Use JSON5 only if your toolchain explicitly supports it.

Run your JSON through the JSON Validator to catch any rule violations instantly. For a reference on which specific mistakes are most common and how to fix them, see Common JSON Mistakes.