Common JSON Errors
JSON.parse fails when text violates the JSON specification. Browsers and validators report position or line/column to help you locate mistakes. Most errors come from treating JSON like JavaScript object literals.
Trailing commas
JSON forbids a comma after the last element: `[1,2,]` is invalid.
Comments and unquoted keys
Use a JSON5 parser if you need comments; standard JSON does not allow them.
Fixing errors quickly
Use a JSON validator that reports line and column numbers. Fix the first reported error before re-running — later errors are often cascading. Format valid JSON only after syntax passes.
API integration
When an API returns 400 with invalid JSON body, log the raw response text and validate offline. Copy-paste errors from Postman or curl often introduce invisible characters.
Common mistakes
Trailing comma
Delete the comma after the final array/object item.
Single-quoted strings
Replace `'` string delimiters with `"`.
Undefined or NaN
Use `null` or quoted strings; JSON has no undefined.