如何迭代 reactjs 数组对象？ 无法读取未定义的属性“地图”

Question

I'm getting this error

Uncaught TypeError: Cannot read property 'map' of undefined

when I try iterate array of object as below. When I console.log first iteration throwing empty array and after that return array of object. I did checking before map array of object as in code. Did I miss anything?

ShipmentDetail.tsx

export class ShipmentDetail extends Component<any, ShipmentInterface> {
  constructor(props: any) {
    super(props);

    this.state = {
      detail: [] as any[]
    };
  }

  componentDidMount() {
    this.getShipmentDetail();
  }

  getShipmentDetail() {
    let { params } = this.props.match;
    params = params.id;
    axios
      .get(`http://localhost:3001/shipments/${params}`)
      .then((response: any) => {
        this.setState({ detail: response.data });
      });
  }
  render() {
    return (
      <Table>
        <TableHead>
          <TableRow>
            <TableCell>type</TableCell>
            <TableCell>description</TableCell>
            <TableCell>volume</TableCell>
          </TableRow>
        </TableHead>
        <TableBody>
          {this.state.detail &&
            this.state.detail.cargo.map((row: any) => {
              <TableRow>
                <TableCell component="th" scope="row">
                  wewe
                </TableCell>
                <TableCell>{row.description}</TableCell>
                <TableCell>{row.volume}</TableCell>
              </TableRow>;
            })}
        </TableBody>
      </Table>
    );
  }
}

export default ShipmentDetail;

Here is sample response from http://localhost:3001/shipments/${params}

Sample.json

{
  "id": "S1000",
  "name": "T-shirts(Summer2018) from Shanghai to Hamburg",
  "cargo": [
    {
      "type": "Fabric",
      "description": "1000 Blue T-shirts",
      "volume": "2"
    },
    {
      "type": "Fabric",
      "description": "2000 Green T-shirts",
      "volume": "3"
    }
  ],
  "mode": "sea",
  "type": "FCL",
  "destination": "Saarbrücker Str. 38, 10405 Berlin",
  "origin": "Shanghai Port",
  "services": [
    {
      "type": "customs"
    }
  ]
}

Answer 1

You set this.state.detail to [] which is truthy value, so this.state.detail returns true , so map will fire on undefined

Solution is to set detail to null in constructor

EDIT: you can set it to any falsey value such as 0 (zero), "" empty string, null , undefined , false , NaN but I'd prefer null

Answer 2

Detail in your state is an array, it should be an object. So during the first render empty array does not have cargo, so it is undefined and thus map is not found