处理 React useEffect 钩子中的响应状态 - 生命周期问题？

Question

I'm currently rendering some data by fetching from API within the useEffect hook.

I have a searchValue variable which holds the value the user inputs on the home page search field. This is passed down to this component .

I have set searchValue in the useEffect dependencies so that when it changes it will run the effect again.

When it mounts, it seems to be skipping the !res.ok if statement. I have logged the res.ok to the console outside of this if statement and with each character inputted into the searchValue it always comes back true . The response status is always 200 after a character press so it never runs the !res.ok if statement block. Something must be wrong with my understanding of the lifecycle or asynchronous calls.

I am getting the error: countriesData.map is not a function - so it is trying to render the map on invalid data however this should not even return since I have if (error) written first within the Content function. If this is matched it should return the error message instead, but this is skipped and the countriesData is updated with invalid data. My code is below, please help! I feel like I'm missing something simple and misunderstanding some fundamentals of React!

import { useEffect, useState } from 'react'
import CountryCard from '../CountryCard'
import { countries } from './CountriesList.module.css'

const CountriesList = ({ searchValue }) => {
    const [error, setError] = useState(false)
    const [loading, setLoading] = useState(true)
    const [countriesData, setCountriesData] = useState([])

    useEffect(() => {
        const getCountries = async () => {
            let res = await fetch('https://restcountries.com/v2/all')
            if (searchValue !== '') {
                res = await fetch(
                    `https://restcountries.com/v2/name/${searchValue}`
                )
            }

            if (!res.ok) {
                console.log(res)
                return setError(true)
            }

            const data = await res.json()
            setCountriesData(data)
            setLoading(false)
        }

        getCountries()

        // return () => setLoading(true)
    }, [searchValue])

    const Content = () => {
        if (error) {
            return <div>Error: please check the country you have typed</div>
        }

        if (loading) {
            return <div>Loading...</div>
        }

        return (
            <section className={`${countries} columns is-multiline`}>
                {countriesData.map((country) => (
                    <CountryCard {...country} key={country.name} />
                ))}
            </section>
        )
    }

    return <Content />
}

export default CountriesList

Answer 1

I think you must be mistaken about the shape of countriesData . You initialise it to an empty array, so .map will work on that. Then the only place you change it is setCountriesData(data) - so this must not be an array.

Answer 2

If you are sure that you are setting array to countriesData, then you can fix by just adding Optional chaining syntax.

replace countriesData.map with

countriesData?.map

Optional chaining