React Typescript - Argument of type is not assignable to parameter of type

Question

I have a demo here

It's a React app using Typescript and hooks to capture entries into form that are simple displayed below.

Here in Stackblitz it works but locally I am using VS code and I get an error for setUser(userData); the error is

const userData: {     username: string;     password: string;     prevState: null; } Argument of type '{ username: string; password: string; prevState: null; }' is not assignable to parameter of type '(prevState: null) => null'.   Type '{ username: string; password: string; prevState: null; }' provides no match for the signature '(prevState: null): null'.ts(2345) 

How can I fox this typescript error

Answer 1

const [user, setUser] = useState(null); 

Since you havn't given this a type, typescript has to try to infer it. It sees you passed in a null, so it assumes this state is (and always will be) null. Instead, you need to specify the type, as in:

interface UserData {   username: string;   password: string;   prevState: null }  //... const [user, setUser] = useState<UserData | null>(null); 

Answer 2

If you fetching data from API you need this:

import React, { useState, useEffect } from 'react' import axios from 'axios';  const MyComponent = () => { const [data, setData] = useState<any | null>(null); const fetchData = () => { (...) //Axios.get() or async if u await data. setData(fetchData) } } useEffect(() =>{     fetchData() }, [])