未捕获的语法错误：JSON 中的意外令牌 u 在 JSON.parse 的 position 0 处（<anonymous> ) at./src/context/AuthContext.js</anonymous>

Question

i was developing a booking app using MERN STACK,can you please help me in this error

So the error comes in this part of Code in AuthContext.js file

const INITIAL_STATE = {
  user: JSON.parse(localStorage.getItem("user")) || null,
  loading: false,
  error: null,
}

I used the useEffect

export const AuthContextProvider=({children})=>{
  const [state,dispatch]=useReducer(AuthReducer,INITIAL_STATE);

  useEffect(() => {
    localStorage.setItem("user",JSON.stringify(state.user))
  },[state.user]);

and the error which comes

Uncaught SyntaxError: Unexpected token u in JSON at position 0 at JSON.parse () at./src/context/AuthContext.js

Answer 1

localStorage.get returns undefined which cannot be parsed by JSON.parse. I usually use a function like this which catches the error and returns the value itself as a fallback.

function parse(value){
   try {
      return JSON.parse(value)
   } catch(_) {
      return value
   }
}

Answer 2

Error message indicates that issue is within JSON.parse which is used in INITIAL_STATE .

You can add guard, something like this:

const userLs = localStorage.getItem("user");
const INITIAL_STATE={
  user: userLs ? JSON.parse(userLs) : null,
  loading:false,
  error:null,
}

Answer 3

You should always remember two things when getting an item from your localStorage and parsing it:

1. You should always handle the case that localStorage does not have the item you're trying to get. If the item is not there, your localStorage.getItem() call will return undefined .
2. You should never pass an invalid JSON object into a JSON.parse() call. undefined is not a valid JSON item. So in your case, you should separate out the localStorage.getItem() call and probably store it in a variable, and then, only if you get a valid JSON back, pass the variable into your JSON.parse() call.

The other answers here have given you example ways of doing this.

If you still get the same error after trying the above, then your stored JSON value maybe corrupted. It's hard to say why without looking at the actual returned item from the localStorage . You could probably delete the item from your localStorage and start the authentication from scratch, to see if that fixes the issue.

Answer 4

You are actually not passing the details that you want store in the Local Storage. Please pass the user details as an object(inside curly bracket) in the backend.

Hint: please reconstruct your data then pass