console.log() 在控制台上打印 undefined

Question

my applicants.js

import React, { useEffect, useState } from "react";
import "./Applicant.css";
import axios from "axios";
import Applicant from "./Applicant";
const URL = "http://localhost:5000/applicants";
const fetchHandler = async () => {
  return await axios.get(URL).then((res) => res.data);
};
const Applicants = () => {
  const [applicants, setApplicant] = useState();
  useEffect(() => {
    fetchHandler().then((data) => setApplicant(data.applicants));
  }, []);
  console.log(applicants);
  return (
    <div>
      <ul>
        {applicants &&
          applicants.map((applicant, i) => (
            <li key={i}>
              <Applicant applicant={applicant} />
            </li>
          ))}
      </ul>
    </div>
  );
};

export default Applicants;

In the console the output is: undefined applicants.js

there is no other error in the console only this one. i'm using react v6

Answer 1

useEffect is working as compomentDidMount with [] (that means passing without any dependency) and you are trying to print before its running at that time state is undefined

Answer 2

setState() runs asynchronously in a queue based system. Therefore if you want to update a state, this update will be piled up on the system queue.

However, you can check if data has been loaded

  const [applicants, setApplicant] = useState();
  useEffect(() => {
    fetchHandler().then((data) => 
       const newApplicants = data.applicants
       setApplicant(data.applicants));
       console.log(newApplicants) // returns data
       console.log(applicants) // returns undefined but in the final state it will contain the data
  }, []);