对象作为 React 子项无效 - firebase 数据库，axios

Question

我正在编写一个教程，其中我使用 firebase 创建我的后端内容，然后使用 React 创建我的前端。 我收到错误消息，我认为这与我的代码行有关： this.state.screams.map((scream) => <Scream scream={scream} />) 。 我不确定如何修复它。 任何有关如何编写此内容的帮助将不胜感激。

我在 jsfiddle 上写了我的代码。 谢谢！

https://jsfiddle.net/cvizena/8r3vpkdz/3/

// Scream.js
import React, { Component } from 'react';
import withStyles from '@material-ui/core/styles/withStyles';
import Link from 'react-router-dom/Link';

// MUI Stuff
import Card from '@material-ui/core/Card';
import CardContent from '@material-ui/core/CardContent';
import CardMedia from '@material-ui/core/CardMedia';
import Typorgraphy from '@material-ui/core/Typography';
import Typography from '@material-ui/core/Typography';

const styles = {
  card: {
    display: 'flex',
    marginBottom: 20
  },
  image: {
    minWidth: 200
  },
  content: {
    padding: 25,
    objectFit: 'cover'
  }
};

class Scream extends Component {
  render() {
    const {
      classes,
      scream: {
        body,
        createdAt,
        userImage,
        userHandle,
        screamId,
        likeCount,
        commentCount
      }
    } = this.props;
    return (
      <Card className={classes.card}>
        <CardMedia
          image={userImage}
          title="Profile image"
          className={classes.image}/>
        <CardContent class={classes.content}>
          <Typorgraphy variant="h5">{userHandle}</Typorgraphy>
          <Typography variant="body2" color="textSecondary">{createdAt}
          </Typography>
          <Typography variant="body1">{body}</Typography>
        </CardContent>
      </Card>
    );
  }
}


// home.js

import React, { Component } from 'react';
import axios from 'axios';
import Grid from '@material-ui/core/Grid';

import Scream from '../components/Scream';

class home extends Component {
  state = {
    screams: null
  };
  componentDidMount() {
    axios
      .get('/screams')
      .then((res) => {
        console.log(res.data);
        this.setState({
          screams: res.data
        });
      })
      .catch((err) => console.log(err));
  }
  render() {
    let recentScreamsMarkup = this.state.screams ? (
      this.state.screams.map((scream) => <Scream scream={scream} />)
    ) : (
      <p>Loading...</p>
    );
    return (
      <Grid container spacing={16}>
        <Grid item sm={8} xs={12}>
          {recentScreamsMarkup}
        </Grid>
        <Grid item sm={4} xs={12}>
          <p>Profile...</p>
        </Grid>
      </Grid>
    );
  }
}

Answer 1

jsx代码应该在return语句里面，试试这段代码

  render() {
    return this.state.screams ? (
      <Grid container spacing={16}>
        <Grid item sm={8} xs={12}>
          {this.state.screams.map((scream) => <Scream scream={scream} />}
        </Grid>
        <Grid item sm={4} xs={12}>
          <p>Profile...</p>
        </Grid>
      </Grid>
    ) : (
      <p>Loading...</p>
    );
  }

或者您也可以将 recentScreamsMarkup 分配放在渲染之外 function

Answer 2

发生错误是因为您在 this.state.screams 上使用 map - 将 this.state.screams 分配给一个变量，然后对该变量调用 map。

 let screamsVariable = this.state.screams
    let recentScreamsMarkup = screamsVariable ? (
      screamsVariable.map((scream) => <Scream scream={scream} />)
    ) : (
      <p>Loading...</p>
    );

Answer 3

一些善良的灵魂刚刚帮助我并向我解释了问题：

“createdAt 的值是一个 object，带有错误消息描述的键。你必须告诉 React 如何呈现它。一个简单的解锁方法如下所示：

新日期 (createdAt._seconds * 1000).toString()"

谢谢你们的帮助！