javascript - In reactjs and nextjs constructor getting Reference Error: localstorage is not defined

Question

i make system jsonwebtoken in reactjs and use nextjs. i find problem when i run the code in browser that is localStorage is not defined.

this is my code in file AuthStudentContext.js

import React from 'react'
import axios from 'axios'

const axiosReq = axios.create()
const AuthStudentContext = React.createContext()

export class AuthStudentContextProvider extends React.Component {

    constructor() {
        super()
        this.state = {
            students: [],
            student: localStorage.getItem('student') || {},
            token: localStorage.getItem('token') || "",
            isLoggedIn: (localStorage.getItem('student' == null)) ? false : true
        }
    }

    login = (credentials) => {
        return axiosReq.post("http://localhost:4000/api/login", credentials)
            .then(response => {
                const { token } = response.data
                localStorage.setItem("token", token)

                this.setState({
                    token,
                    isLoggedIn: true
                })

                return console.log(response)
            })
    }

and show error localStorage is not defined

Answer 1

On the constructor as well as componentWillMount lifecycle hooks, the server is still rendering the component. On the other hand, localStorage exists as part of the browser's Window global, thus you can only use it when the component is rendered. Therefore you can only access localStorage on the componentDidMount lifecycle hook. Instead of calling localStorage on the constructor, you can define an empty state, and update the state on componentDidMount when you can start to call localStorage.

constructor() { 
  super()
  this.state = {
    students: [],
    student: undefined
    token: undefined,
    isLoggedIn: undefined
  };
}

componentDidMount() {
  this.login();
  this.setState({
    student: localStorage.getItem('student') || {},
    token: localStorage.getItem('token') || "",
    isLoggedIn: (localStorage.getItem('student' == null)) ? false : true
  });
}