javascript - 如何在react状态钩子(Hook)上使用map函数

Question

有人可以解释一下，为什么我无法在 React 状态 Hook 上运行 JavaScript 映射函数？

const [sequenceNames, setSequenceNames] = useState(null);

useEffect(() => {
  fetch('/savedFiles')
    .then(response => response.json())
    .then(data => setSequenceNames(data));
}, []);

const table = sequenceNames.map(name => Sequence(name));

这适用于 for in 循环，但我的 linter 禁止使用 for in。

const table = [];

for (const name in sequenceNames) {
  table.push(Sequence(sequenceNames[name]));
}

当我使用 .map 时，我收到以下错误。

TypeError: Cannot read property 'map' of null
    at main.a21158832f7ed8c55e25.bundle.js:1
    at Bi (main.a21158832f7ed8c55e25.bundle.js:1)
    at main.a21158832f7ed8c55e25.bundle.js:1
    at f (main.a21158832f7ed8c55e25.bundle.js:1)
    at d (main.a21158832f7ed8c55e25.bundle.js:1)
    at main.a21158832f7ed8c55e25.bundle.js:1

即使我的sequenceNames数组不应该为空。

Answer 1

您遇到的问题是您正在设置 sequenceNames null 的初始状态，因此您遇到了竞争条件，第一次尝试时在 sequenceNames 上运行循环，它是 null。您可以将 sequenceNames 初始化为空数组 []，但完成您想要做的事情的最佳方法是采用钩子(Hook)。

import React, { useMemo, useState } from 'react';
import { Sequence } from '.';

export interface DisplayNamesProps {
    id?: number;
}

export function DisplayNames(props: DisplayNamesProps) {
    const { id } = props;
    const [sequenceNames, setSequenceNames] = useState<Sequence[]>([]);

    // Use useEffect with the 2nd param, to guard against `fetch`
    // executing on each render. In this example I use `id` as
    // a var that should be unique for each `fetch` call.
    useEffect(() => {
        fetch(`/savedFiles/${id}`)
            .then(response => response.json())
            .then(data => setSequenceNames(data));
    }, [id]);

    // Create a memoized var `table` that updates when there
    // is a change to `sequenceNames`.
    const table = useMemo(() => {
        return sequenceNames.map(name => Sequence(name))
    }, [sequenceNames]);

    return <div>{table}</div>;
}