JSON (JavaScript Object Notation) is the lingua franca of APIs, config files, and data exchange. It is simple — but its strictness trips up developers daily. A single stray comma can break an entire payload. This guide covers the syntax rules, the errors that cause most parse failures, and the habits that keep your JSON clean and valid.
Key takeaways
- JSON values are: string, number, boolean, null, object, or array.
- Keys and strings must use double quotes — never single.
- No trailing commas and no comments are allowed.
- Validate and pretty-print before shipping to catch errors early.
The core syntax rules
Valid JSON follows a small, strict grammar:
- Data is written as key/value pairs inside objects:
{ "key": "value" } - Keys are always double-quoted strings.
- Values may be a string, number,
true,false,null, an array[ ], or a nested object{ }. - Items are separated by commas — but never after the last item.
- The whole document must have exactly one root value.
A well-formed example:
{
"name": "WorkIQ",
"tools": 59,
"free": true,
"categories": ["pdf", "seo", "developer"],
"owner": null
}The 6 errors that break JSON most often
| # | Mistake | Fix |
|---|---|---|
| 1 | Trailing comma after the last item | Remove the final comma |
| 2 | Single quotes 'value' | Use double quotes "value" |
| 3 | Unquoted keys {name: "x"} | Quote every key {"name":"x"} |
| 4 | Comments // note | Remove them; JSON has no comments |
| 5 | Missing/closing bracket mismatch | Balance every { } and [ ] |
| 6 | Unescaped characters in strings | Escape \", \\, and control chars |
Paste, validate, and pretty-print
Spot the exact line of a syntax error and format messy JSON in one click — entirely in your browser.
Open the JSON Formatter →Best practices for clean JSON
- Pick one indentation and stick to it. Two spaces is the common default; minify only for transport, not for reading.
- Use consistent key casing.
camelCaseorsnake_case— choose one across your API. - Keep it UTF-8. Save and serve JSON as UTF-8 to avoid mojibake with accented or non-Latin characters.
- Validate in CI. Lint JSON config files automatically so a bad comma never reaches production.
- Prefer arrays for lists, objects for records. Don't fake a list with numbered keys like
"0", "1", "2".
JSON vs JavaScript objects
They look alike, but JSON is a data format, not code. JavaScript object literals allow single quotes, unquoted keys, comments, trailing commas, and functions — JSON allows none of these. When you JSON.parse() a string, it must obey JSON's stricter rules, which is why copy-pasting a JS object into a JSON file often fails.
Frequently asked questions
Can JSON have comments?
No. The standard forbids them. Use a dedicated field like _comment, or strip comments before parsing.
Are trailing commas allowed?
No. A comma after the final element is invalid and will throw a parse error in strict parsers.
Double or single quotes?
Always double quotes for keys and string values. Single quotes are not valid JSON.
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