Parsing JSON in JavaScript

Quick answer: JavaScript has JSON built in: JSON.parse() turns a JSON string into a value, and JSON.stringify() turns a value into a JSON string. With fetch, call await res.json() to do the parse for you. The pitfalls are dates, big numbers, and undefined — none of which survive a round-trip untouched. Debug a failing parse with the JSON Debugger.

const obj = JSON.parse('{"name":"Ada","age":36}');   // → { name: "Ada", age: 36 }
const str = JSON.stringify(obj);                      // → '{"name":"Ada","age":36}'

JSON.stringify(obj, null, 2);   // pretty-print with 2-space indent

JSON.parse only accepts a string. A frequent bug is passing an object that's already parsed — it gets coerced to "[object Object]" and throws Unexpected token o. If you already have an object, you don't need to parse it.

With fetch, res.json() reads the body and parses it:

const res = await fetch("/api/users");
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const data = await res.json();   // already a JS object

Two classic failures here:

• Calling .json() on an error page throws Unexpected token < because the body is HTML, not JSON. Check res.ok and the Content-Type first — see Unexpected Token in JSON.
• Calling .json() on an empty 204 response throws Unexpected end of JSON input. Read .text() first and parse only if non-empty — see Unexpected End of JSON Input.

JSON.stringify quietly transforms or drops some values:

• Date → ISO string. JSON.stringify(new Date()) emits "2026-06-25T14:30:00.000Z". Parsing it back gives a string, not a Date — you must re-hydrate it. See Dates in JSON.
• undefined, functions, symbols are dropped from objects (and become null in arrays). JSON has no undefined — see null vs missing.
• NaN and Infinity → null.
• BigInt throws — JSON.stringify(1n) raises a TypeError. Convert big integers to strings; see Big Numbers in JSON.

Both methods take an optional function to customize the transform:

// reviver: re-hydrate ISO date strings into Date objects
const data = JSON.parse(text, (key, value) =>
  key === "createdAt" ? new Date(value) : value
);

// replacer: drop or transform fields on the way out
JSON.stringify(obj, (key, value) =>
  key === "password" ? undefined : value
);

The replacer is a clean way to strip sensitive fields before sending. For the underlying serialization rules, see Parse and Stringify, and wrap parsing of untrusted input in try/catch since invalid JSON throws a SyntaxError.

Do I need to call JSON.parse after fetch? No — await res.json() reads and parses the body in one step, returning a JavaScript object. Use JSON.parse only when you have a raw JSON string yourself.

Why does JSON.parse throw "Unexpected token o"? You passed an object instead of a string. JSON.parse stringifies it to "[object Object]" and fails. If you already have an object, skip parsing.

Why did my Date become a string after JSON.stringify? JSON has no date type. stringify calls the date's toJSON() to emit an ISO string, and parse returns that string unchanged. Re-create the Date with a reviver or after parsing.

Why are some properties missing after stringify? undefined, functions, and symbols are dropped from objects during serialization (and turned into null inside arrays), because JSON can't represent them.