Common JSON syntax errors

Trailing commas are probably the most common JSON error. They happen when the final property or array item ends with a comma. JavaScript allows trailing commas in many contexts, but JSON does not.

Fixing this is simple: remove the comma after the last item. If the same data came from a JavaScript object, check for other JavaScript-only syntax too.

JSON requires double quotes around string values and object keys. Single quotes, smart quotes, and unquoted keys are invalid. This often appears when copying from JavaScript examples or rich text documents.

Replace single quotes with double quotes and make sure every key is quoted. Be careful with automated replacements if string values contain apostrophes.

Standard JSON does not allow comments. It also does not allow undefined, NaN, Infinity, functions, dates, or regular expressions as native values. Use null or strings when a value needs to be represented in JSON.

If you need comments in a config file, consider whether the file is actually JSONC, YAML, or another format rather than JSON.

Unclosed objects, unclosed arrays, and broken escape sequences can be harder to spot in dense JSON. Formatting helps only after the document is valid, so use a validator first. Parser positions often point near the issue.

Inside strings, backslashes must form valid escape sequences. File paths, regular expression-like text, and copied command strings are common sources of escaping mistakes.