How to Fix JSON Syntax Errors (Common Mistakes & Solutions)
JSON parsing errors are among the most common issues developers face. A single misplaced comma, missing quote, or extra bracket can break your entire configuration file, API response, or data pipeline. Here are the most common JSON errors and how to fix them.
Error 1: Trailing Commas
The most common JSON error. JavaScript allows trailing commas, but JSON does not. Wrong: {"name": "John", "age": 30,} — The comma after 30 is invalid. Fix: {"name": "John", "age": 30} — Remove the trailing comma. This is especially common when you delete the last property but forget to remove the preceding comma.
Error 2: Single Quotes Instead of Double Quotes
JSON requires double quotes for strings. Wrong: {'name': 'John'} — Single quotes are not valid JSON. Fix: {"name": "John"} — Always use double quotes for both keys and string values.
Error 3: Unquoted Keys
JavaScript objects allow unquoted keys, JSON does not. Wrong: {name: "John"} — Keys must be in double quotes. Fix: {"name": "John"} — Wrap all keys in double quotes.
Error 4: Comments in JSON
JSON does not support comments. Wrong: {"name": "John" // user name} — Comments are not allowed. Fix: Remove the comment or use a separate documentation file. If you need comments in configuration, consider using JSONC or YAML instead.
Error 5: Missing Brackets or Braces
Unmatched brackets cause cryptic errors. Use a JSON validator to highlight exactly where the mismatch occurs. Common causes: copy-pasting partial JSON, manual editing mistakes, or truncated API responses.
Error 6: Invalid Values
JSON supports: strings (double-quoted), numbers, booleans (true/false), null, objects, and arrays. It does NOT support: undefined, functions, NaN, Infinity, Date objects, or single-quoted strings. Convert invalid values before serializing to JSON.
Error 7: Duplicate Keys
While not always an error, duplicate keys cause unpredictable behavior. Wrong: {"name": "John", "name": "Jane"} — Which name wins? The JSON spec says behavior is undefined. Most parsers use the last value, but this is a common source of bugs.
How to Find and Fix JSON Errors
Use NowTo Tools' JSON Validator to instantly find syntax errors with exact line numbers and descriptive error messages. Paste your JSON, click Validate, and the tool highlights exactly what is wrong and where.
Prevention Tips
Use a code editor with JSON syntax highlighting. Enable format-on-save for JSON files. Use a linter in your development workflow. When building JSON programmatically, always use JSON.stringify() instead of manual string concatenation.
Validate your JSON now: JSON Validator — instant error detection, completely private.