Five Reasons JSON.parse Fails (and Where They Come From)

Trailing commas, single quotes, comments, unquoted keys and NaN, with the exact error for each.

By the TurantTools Dev team · Updated October 27, 2026 · 3 min read

JSON is stricter than JavaScript. Five habits from JS code cause most "Unexpected token" errors. Here's what our formatter reports for each.

Problem	Input	Error
Trailing comma	{"a": 1, "b": 2,}	Trailing comma: remove the comma before }. (line 1, column 16)
Single quotes	{'a': 1}	Keys must use double quotes ("), not single quotes ('). (line 1, column 2)
Comment	{"a": 1 // note⏎}	Expected , or } after a value, found "/". Is a comma missing? (line 1, column 9)
Unquoted key	{a: 1}	Keys must be in double quotes. Found "a". (line 1, column 2)
NaN value	{"a": NaN}	Unexpected word starting with "N". Text values need double quotes; only true, false and null are allowed without them. (line 1, column 7)

Where they come from

Config files like tsconfig.json and VS Code settings accept comments and trailing commas (JSONC), so habits leak into real JSON.
Copying a JavaScript object from the console gives single quotes and unquoted keys.
Serialising NaN or Infinity needs a decision: JSON.stringify turns them into null.