React 一直在说。map 不是 function 并且我尝试过的没有任何工作？

Question

I have a component that gets listings from and API using axios. When the home page first loads all the listings are retrieved and load fine, but when I go to say the listing detail page and back to the home page I get a 'listings.map is not a function' error, when I refresh the page I do not get the error and the listings load.

Here is the component(tried using a loading boolean but did nothing):

import React, { Component } from 'react';
import { connect } from 'react-redux';
import { getListings } from '../../actions/listings';
import { Link as RouteLink } from 'react-router-dom';
import PropTypes from 'prop-types';
//material ui
import {
  Container,
  Typography,
  Button,
  Card,
  CardActions,
  CardActionArea,
  CardContent,
  CardMedia,
  Grid,
  Link,
} from '@material-ui/core';
import { withStyles } from '@material-ui/core/styles';

const useStyles = {
  listings: {
    marginTop: '100px',
    backgroundColor: 'white',
  },
  title: {
    marginBottom: '15px',
  },
  listImg: {
    width: '250px',
    height: '250px',
  },
  card: {
    width: '275px',
    height: '300px',
  },
  media: {
    height: 140,
  },
};

class Listings extends Component {
  static propTypes = {
    listings: PropTypes.array.isRequired,
    isLoading: PropTypes.bool,
  };

  componentDidMount() {
    this.props.getListings();
  }

  render() {
    const { classes, listings } = this.props;
    return (
      <>
        <Container maxWidth='lg' className={classes.listings}>
          <Typography align='center' variant='h1' className={classes.title}>
            Recent Listings
          </Typography>
          <Grid container justify='center' spacing={8}>
            {listings &&
              listings.map((listing, index) => (
                <Grid key={index} item>
                  <Link component={RouteLink} to={`/listing/${listing._id}`}>
                    <Card className={classes.card}>
                      <CardActionArea>
                        <CardMedia
                          className={classes.media}
                          title='Contemplative Reptile'
                          image={listing.imgUrls[0]}
                        />
                        <CardContent>
                          <Typography gutterBottom variant='h6' component='h2'>
                            {listing.title}
                          </Typography>
                          <Typography
                            variant='body2'
                            color='textSecondary'
                            component='p'
                          >
                            {listing.price} /night
                          </Typography>
                        </CardContent>
                      </CardActionArea>
                      <CardActions>
                        <Button size='small' color='primary'>
                          BOOK
                        </Button>
                      </CardActions>
                    </Card>
                  </Link>
                </Grid>
              ))}
          </Grid>
        </Container>
      </>
    );
  }
}

//sets component props to the app state
const mapStateToProps = (state) => ({
  listings: state.listings.listings,
  isLoading: state.listings.isLoading,
});

//withStyles is used for material-ui styles (using class components)
export default connect(mapStateToProps, { getListings })(
  withStyles(useStyles)(Listings)
);

here is the getListings action:

export const getListings = () => (dispatch) => {
  axiosInstance
    .get('/listings')
    .then((res) => {
      dispatch({
        type: GET_LISTINGS,
        payload: res.data,
      });
    })
    .catch((err) => console.log(err));
};

and the reducer:

const initialState = {
  listings: [],
  isLoading: true,
};

export default function (state = initialState, action) {
  switch (action.type) {
    case GET_LISTINGS:
      return {
        ...state,
        listings: action.payload,
        isLoading: false,
      };
    case GET_LISTING:
      return {
        ...state,
        listings: action.payload,
        isLoading: false,
      };
    case CREATE_LISTING:
      return {
        ...state,
        listings: [...state.listings, action.payload],
      };
    case CLEAR_LISTINGS:
      return {
        ...state,
        listings: [],
      };
    default:
      return state;
  }
}

Just don't get how it does work, but then doesn't at times. Listings are declared as an array also in initialState.

Answer 1

If your listings variable type is

Object

then you have to map like this:

const object1 = {
  a: 'somestring',
  b: 42
};

for (const [key, value] of Object.entries(object1)) {
  console.log(`${key}: ${value}`);
}

// expected output:
// "a: somestring"
// "b: 42"
// order is not guaranteed

On the other hand if your variable is

Array

then:

const numbers = [2, 4, 6, 8];
const squares = numbers.map(number => number * numbers);

console.log(squares);
// output: Array [4, 16, 36, 64]