The Mystery of "Unexpected Token u" in JSON — Solved

You refresh your app expecting everything to work perfectly, but your console shows: SyntaxError: Unexpected token u in JSON at position 0. The error looks cryptic. What is "u"? Why position 0? And why does JSON even care about the letter u?

This is actually one of the most common and easily fixable JavaScript errors — once you know what the "u" represents.

What Does the "u" Actually Mean?

The "u" stands for "undefined". Here is what happens step by step:

Your variable has the value undefined — either it was never set, or an API returned nothing.
You call JSON.parse(myVariable).
JavaScript converts undefined into the string "undefined" to pass it to the parser.
The JSON parser reads the first character — the letter u — and immediately throws because valid JSON can only start with {, [, ", a number, true, false, or null.

So the full error message "Unexpected token u in JSON at position 0" is the parser telling you: "I got the string 'undefined' and I cannot parse it."

Common Situations That Cause This Error

Missing localStorage key

localStorage.getItem() returns null for keys that don't exist. Parsing null also triggers this error.

Failed fetch response

The API returned an error status or empty body, and the variable was never populated with real data before parsing.

Typo in variable name

A misspelled variable name evaluates to undefined in JavaScript without throwing an error — until you try to parse it.

Missing await

Forgetting await on an async call means you pass a Promise object to JSON.parse() instead of the resolved value.

Reproducing the Error

// Variable never assigned — undefined
let myData;
JSON.parse(myData); // ❌ SyntaxError: Unexpected token u

// localStorage key doesn't exist — null
const saved = localStorage.getItem('settings');
JSON.parse(saved); // ❌ SyntaxError: Unexpected token n (for null)

// Missing await
async function fetchUser() {
  const res = fetch('/api/user'); // ❌ missing await — res is a Promise
  return JSON.parse(res);        // ❌ parsing [object Promise]
}

The Fix: Guard Before You Parse

// ✅ Check for undefined and null before parsing
let myData = localStorage.getItem('settings');
if (myData) {
  const parsed = JSON.parse(myData);
  console.log(parsed);
}

// ✅ With await and error handling
async function fetchUser() {
  try {
    const res = await fetch('/api/user');
    if (!res.ok) throw new Error('Network error: ' + res.status);

    const text = await res.text();
    if (!text) return null; // empty response guard

    return JSON.parse(text);
  } catch (err) {
    console.error('Failed to fetch user:', err.message);
    return null;
  }
}

localStorage Pattern That Never Fails

function loadSettings() {
  try {
    const raw = localStorage.getItem('appSettings');
    return raw ? JSON.parse(raw) : {}; // default to empty object
  } catch {
    return {}; // malformed JSON — return safe default
  }
}

function saveSettings(settings) {
  localStorage.setItem('appSettings', JSON.stringify(settings));
}

This pattern handles all three failure modes: key not found (returns null), malformed JSON (throws), and valid JSON (parses correctly).

Quick mental checklist when you see "unexpected token u":

Log the variable right before JSON.parse() — is it actually a string?
Check if you are missing await on an async call.
Check if the API or localStorage key can return nothing.
Add a guard: if (data) { JSON.parse(data) }.

Validate JSON Before You Parse It

Paste your JSON response into our free validator to confirm it is valid before putting it in your code.

Open JSON Validator

Once you understand that "unexpected token u" always means "you tried to parse undefined", this error loses all its mystery. The fix is almost always the same: add a guard, add an await, or handle the case where data has not arrived yet.