Can JSON contain undefined anywhere?

No. JSON uses null for "no value". JSON.stringify drops object keys whose value is undefined and writes null for undefined items in an array.

What does the Repair button do with it?

Repair turns the text into null. That parses, but your code most likely expected an object, so fix the value where it comes from.

Should I wrap every JSON.parse in try/catch?

Wrap the ones that read data you do not control: storage, URLs, network responses. Log the raw value in the catch block; a catch that quietly returns a default hides the bug that saved the bad value.

"undefined" is not valid JSON: What It Means and How to Fix

Why it happens

JSON.parse() converts its argument to a string, and String(undefined) is the word undefined, which JSON does not have. null is different: JSON.parse(null) returns null, so reading a localStorage key that was never set is safe. Common sources, in rough order:

1. The text “undefined” was saved earlier. Storage APIs convert values to strings, and JSON.stringify(undefined) returns undefined, not a string, so the word itself gets stored. A React effect that saves state before it loads does exactly this:

import { useEffect, useState } from 'react';

function App() {
  const [session, setSession] = useState();   // undefined until loaded

  useEffect(() => {
    localStorage.setItem('session', JSON.stringify(session));   // first run stores "undefined"
  }, [session]);
  // ...
}

On the next page load, JSON.parse(localStorage.getItem('session')) throws.

2. A field or variable is missing. The API renamed payload to data, req.body is undefined because no body parser runs, or a Lambda test event has no body, so JSON.parse(event.body) gets undefined.

3. An environment variable is not set. JSON.parse(process.env.FEATURE_FLAGS) works locally and fails in CI or Docker. In Vite, client code only sees variables whose names start with VITE_; any other name reads as undefined.

4. A tool variable does not exist. Postman’s pm.environment.get('session') returns undefined for a missing variable. In RPG Maker MV and MZ, PluginManager.parameters('Name') returns an empty object when Name does not match the plugin’s file name, so every parameter is undefined.

If undefined sits inside a larger text, as in {"total": undefined}, Chrome reports Unexpected token 'u', "{"total": undefined}" is not valid JSON instead. That JSON was built by joining strings; use JSON.stringify.

How to fix “undefined” is not valid JSON

Log the value before the parse, console.log(typeof raw, raw), then fix its source.

localStorage and React

Never store undefined, and treat an old bad value as empty:

function saveSession(session) {
  if (session === undefined) localStorage.removeItem('session');
  else localStorage.setItem('session', JSON.stringify(session));
}

function loadSession() {
  const saved = localStorage.getItem('session');
  if (saved === null || saved === 'undefined') return null;
  return JSON.parse(saved);
}

In the component: useState(loadSession) and useEffect(() => saveSession(session), [session]).

Environment variables

Default optional settings, and fail clearly on required ones:

const flags = JSON.parse(process.env.FEATURE_FLAGS ?? '{}');

const raw = process.env.DB_CONFIG;
if (raw === undefined) throw new Error('DB_CONFIG is not set');
const dbConfig = JSON.parse(raw);

A variable that is set but empty gives Unexpected end of JSON input instead.

Request bodies

In Express, req.body stays undefined until a body parser runs. Add app.use(express.json()) and use req.body as an object, without JSON.parse; parsing it again gives “[object Object]” is not valid JSON.

Postman

const raw = pm.environment.get('session');
const session = raw ? JSON.parse(raw) : {};
pm.environment.set('session', JSON.stringify(session));

Environment values are strings, so store objects with JSON.stringify, as on the last line.

RPG Maker plugins

Keep the plugin’s file name and the name passed to PluginManager.parameters() identical.

Same error in other languages

Python: json.loads(None) raises TypeError: the JSON object must be str, bytes or bytearray, not NoneType.
Safari words this case Unexpected identifier "undefined". Its other common JSON message, Unrecognized token ‘<’, means HTML arrived instead.

To check a stored or logged value, paste it into the JSON validator. Other parse errors: common JSON errors.

"undefined" is not valid JSON

Find it in your JSON