If you have ever pasted a JSON response into your code only to have JSON.parse throw an error pointing at a perfectly valid-looking spot, you are not alone. A new tutorial published on DEV.to on October 17, 2026, breaks down the seven most common mistakes that cause these failures. The guide provides before-and-after examples for each error, helping developers quickly identify and resolve issues that stem from confusing JavaScript syntax with strict JSON standards.

Syntax and Formatting Traps

The first three common errors involve basic syntax differences between JavaScript and JSON. Trailing commas, which are often allowed in JavaScript object literals, will cause JSON.parse to fail immediately. Similarly, using single quotes instead of double quotes for keys and string values is a frequent mistake, as JSON strictly requires double quotes. Another classic error is omitting quotes around keys, which is valid in JavaScript but invalid in JSON. The tutorial emphasizes that every key and string value must be wrapped in double quotes to be considered valid JSON.

Unsupported Data Types and Characters

JSON does not support certain JavaScript values like undefined, NaN, and Infinity. The guide recommends replacing these with null or removing the key entirely. Another tricky issue involves characters that need escaping, such as backslashes in file paths (e.g., "C:\\Users\\ali"). A single backslash starts an escape sequence, so doubling them is necessary. Additionally, JSON numbers are stricter than JavaScript numbers: no leading zeros, plus signs, bare decimal points, or hexadecimal notation are allowed. Large numbers exceeding 9007199254740991 also lose precision and should be sent as strings.

Debugging and Prevention Strategies

When JSON.parse fails, the error message can vary by browser and engine, sometimes only providing a character position. The tutorial suggests wrapping the parse in a try/catch block and logging the error message. If the message is unclear, pasting the JSON into a formatter can instantly reveal the structure or error. For prevention, the best habit is to let a library handle JSON generation. Using JSON.stringify in JavaScript or json.dumps in Python automatically handles quotes, escaping, and commas, eliminating most manual errors.

Key Takeaways

  • JSON strictly requires double quotes for all keys and string values.
  • Trailing commas and comments are invalid in standard JSON.
  • Values like undefined, NaN, and Infinity must be replaced with null or omitted.
  • Backslashes and line breaks inside strings must be escaped correctly.
  • JSON numbers cannot have leading zeros, plus signs, or hex formats.
  • Use JSON.stringify or equivalent library functions to generate JSON safely.
  • Large numbers should be transmitted as strings to avoid precision loss.
  • JSON formatters can quickly identify syntax errors when error messages are ambiguous.

The Bottom Line

Stop hand-crafting JSON strings; let your language's serialization library handle the syntax so you can focus on logic instead of debugging quote marks.