The six JSON data types
Object — an unordered set of key-value pairs in curly braces: {"key": "value"}. Array — an ordered list of values in square brackets: [1, 2, 3]. String — text in double quotes: "hello". Number — an integer or decimal, no quotes: 42 or 3.14. Boolean — exactly true or false, unquoted. Null — the literal null, representing "no value."
Object structure rules
Keys must always be double-quoted strings — never single-quoted, never unquoted. Each key-value pair is separated by a colon, and pairs are separated by commas, with no comma after the last pair: {"a": 1, "b": 2} is valid; {"a": 1, "b": 2,} is not.
Array structure rules
Arrays hold an ordered sequence of any JSON values — including mixed types or nested objects/arrays — separated by commas, again with no trailing comma: [1, "two", true, null, {"nested": true}].
String rules
Strings must use double quotes, not single quotes. Special characters need escaping with a backslash: \" for a quote, \\ for a backslash, \n for a newline, and \uXXXX for arbitrary Unicode characters. Our JSON Escape tool handles this automatically.
What makes JSON invalid
The most common mistakes: trailing commas after the last item, single quotes instead of double quotes, unquoted object keys, and using undefined (which doesn't exist in JSON — use null instead). Any of these will cause a strict parser to reject the document; run it through our JSON Validator to find the exact line and column.