"[object Object]" is not valid JSON

Parsing data that’s already been parsed

This is the most common cause. response.json() doesn’t return JSON text; it reads the text and gives you back the parsed object. Axios’s response.data does the same. So if you then call JSON.parse on the result, you’re parsing twice:

const response = await fetch("/api/user");
const user = await response.json();   // already an object

JSON.parse(user);                     // "[object Object]" is not valid JSON
console.log(user.name);               // just use it

Saving or sending an object without stringifying it

With localStorage, the error shows up later, when you read the data back. It only stores text, so saving an object stores the string [object Object] and the data is lost. Stringify when you save, and clear out the bad entry once:

// Wrong: stores the text "[object Object]"
localStorage.setItem("user", user);

// Right
localStorage.setItem("user", JSON.stringify(user));
const saved = JSON.parse(localStorage.getItem("user") ?? "null");

The same thing happens if you pass an object as a request body. The server receives the text [object Object], and it’s the server’s JSON parser that fails:

await fetch("/api/user", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify(user),   // not body: user
});

Where the undefined comes from

Usually it’s a property that doesn’t exist, like a misspelled response.body, or a function that forgot to return anything. A missing localStorage key won’t cause it: getItem returns null, and JSON.parse(null) returns null without an error. If a value might legitimately be missing, check before you parse: text ? JSON.parse(text) : null.

The same error elsewhere