The fastest diagnostic routine
Copy the smallest complete payload that reproduces the error. Run it through a JSON validator and look at the reported parsing failure. Work from the position it identifies, but also inspect the character immediately before it: a missing quote or comma can make the parser complain later than the actual mistake.
When the syntax is valid, pretty-print the JSON to inspect nesting. Validation proves that text follows the JSON grammar; it does not prove that the server accepts the schema, field names or values.
1–3: single quotes, unquoted keys and trailing commas
1. Invalid: {'name':'Maya'}. Valid: {"name":"Maya"}. JSON uses double quotes for both object keys and strings. 2. Invalid: {name:"Maya"}. Valid: {"name":"Maya"}. JavaScript object literal shorthand is not standard JSON.
3. Invalid: {"items":[1,2,]}. Valid: {"items":[1,2]}. JSON does not allow a trailing comma after the final item of an array or the last property of an object. A permissive code editor may accept it as JavaScript, while a JSON API will not.
4–5: unsupported values and comments
4. Invalid: {"result":undefined}. Valid alternatives include {"result":null} or omitting the key entirely, depending on the contract. JSON supports null, true, false, numbers, strings, objects and arrays; undefined is not a JSON value.
5. Invalid: {"active":true // enabled}. Valid: {"active":true}. Standard JSON has no comment syntax. If you need comments for a configuration file, choose a format that explicitly permits them or keep documentation outside the JSON payload.
6–7: backslashes and broken brackets
6. Backslashes inside a JSON string must be escaped. A Windows path such as C:\temp\new can accidentally contain escape sequences, including tab and newline. The correct JSON representation of the literal path is {"path":"C:\\temp\\new"}. Use a serializer when possible instead of manually building strings.
7. Arrays and objects must close in the reverse order they open. Invalid: {"items":[1,2}. Valid: {"items":[1,2]}. Indentation often makes a missing ] or } immediately visible. Do not blindly append a bracket without checking what structure you meant to close.
Valid JSON can still be wrong for your API
The payload {"quantity":"12"} is valid JSON, but the quantity is a string. Some APIs require the number 12 instead. Similarly, the string "0012" may be an important identifier, not an integer you should coerce. Syntax validation will not choose the correct meaning for you.
Minification removes irrelevant whitespace; it does not fix an invalid schema or remove private data. For sensitive customer or production information, use a trusted local workflow. RezSat's JSON Formatter & Validator performs parsing in your browser and does not intentionally send the content to the site's backend.
Useful links and sources
These links point to the original tool or official documentation. Features and free-tier limits may change.
- RFC 8259 — The JavaScript Object Notation (JSON) Data Interchange Format — The formal JSON syntax specification
- MDN — JSON.parse() — Parsing behavior and syntax errors in JavaScript
- MDN — JSON data format — Practical introduction to JSON values and structures