为什么 useState() 没有设置默认值？

Question

I have created following reactComponent. From what I understand useState([]); should set the comments variable to an array. But it does not. I get an exeption that I Try to run.map() on an object. What I have to do is Object.values(comments) to make it work but I don't understand why. My api definetly returns an array of comment objects.

import React, { useState, useEffect } from 'react';
import axios from 'axios';

export default ({ postId }) => {
  const [comments, setComments] = useState([]);

  const fetchData = async () => {
    const res = await axios.get(
      `http://localhost:4001/posts/${postId}/comments`
    );

    setComments(res.data);
  };

  useEffect(() => {
    fetchData();
  }, []);

  const renderedComments = comments.map(comment => {
    return <li key={comment.id}>{comment.content}</li>;
  });

  return <ul>{renderedComments}</ul>;
};

Answer 1

Your use of state is correct as far as I can tell, the fact that Object.values works on the data handed back to you implies it is definitely either an object or an array, have you run Array.isArray(res.data) as part of your troubleshooting process?

As stated by a commenter above, the most likely scenario is that you are getting an object back from your back end. Other things you can do to confirm its the data handed back to you at fault would be to simply comment out your useEffect and see if it still throws the same exception.