reactjs - useState not updating for some reason?

Question

when I try to get some data from my backend API using axios, and set the state after I've gotten the result for some reason the state is not updated and when I try to use the state it will only show me an empty array. but what's so interesting is that when I console.log(res.data) it will show me my array of lists with no problem, so I guess the problem is with the setCategories() state function. What am I doing wrong?

const Home = (props) => {
  const [categories, setCategories] = useState([]);
  useEffect(() => {
    getCats();
  }, []);

  const getCats = async () => {
    const data = await axios.get(`${myUrl}/allItems`, {
      withCredentials: true,
    });
    const cats = await data.data;
    console.log(cats); //this one works perfectly
    setCategories(cats);
    console.log(categories) //this one doesn'nt work which means the setState didn't work
  };


  return (
    <>
      <div className="card-div mt-5">
        {categories.map((cat) => {
          <li>{cat.name}</li>;
        })}
      </div>
    </>
  );
};

Answer 1

the state is set asynchronously, so the data is not updated instantly. that's why you are not getting the output on console.log(categories) right after setCategories(cats);

here is a small example of asynchronous behaviour of useState state update:

Link to working example: stackblitz

import React, { useEffect, useState } from "react";
import "./style.css";
import axios from "axios";
const url = "https://jsonplaceholder.typicode.com/users";
export default function App() {
  const [users, setUsers] = useState([]);

  useEffect(() => {
    axios.get(url).then(result => {
      console.log("1. when data is fetched sucessfully: ", result.data);
      setUsers(result.data);
      console.log("2. Just after setting state: ", users);
    });
  }, []);

  // secons useEffect for logging out upadated todos useState
  useEffect(() => {
    console.log("todos upadated: ", users);
  }, [users]);
  return (
    <div>
      <h1>Hello StackBlitz!</h1>
      <p>Start editing to see some magic happen :)</p>
      {users.map(user => (
        <p>{user.name}</p>
      ))}
    </div>
  );
}

Here is what is happening in the above example:

You can see the flow of data fetching and async update of state.