如何在组件安装之前获取数据？

Question

I am trying to make a fetch from a local server in Node, but my problem is that when I call the render function of the component the array users from the state seems to be empty and therefore does not render the users on the screen as I would like, the weird thing is that the console.log(users) I have inside the fetch does bring me the data, but when I do it on the render does not bring me anything:

this is the code so far:

import React, { PureComponent } from "react";
import Nav from "./Nav";
import { IState, IProps } from "./Interfaces";

class Home extends PureComponent<IProps, IState> {
  constructor(props: IProps) {
    super(props);

    this.state = {
      users: [],
      books: []
    };
  }

  getUsers = async () => {
    const response = await fetch(`http://localhost:5000/users`, {
      headers: {
        "Content-Type": "application/json"
      }
    });
    const users = await response.json();
    for (let user of users) {
      this.state.users.push(user);
    }
    console.log(this.state.users);
  };

  getBooks = async (id: number) => {
    const token =
      "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6MSwidXNlciI6Implc3VzIiwiaXNBZG1pbiI6dHJ1ZSwiaWF0IjoxNTc2NjgzNTkwfQ.1FWmtj-fCsqSza_pwfewIpp3zQ50BxDagRTvrh5x3cU";
    const response = await fetch(`http://localhost:5000/bookUser/${id}/books`, {
      headers: {
        Authorization: `Bearer ${token}`,
        "Content-Type": "application/json"
      }
    });
    const books = await response.json();
    this.setState({ books });
  };

  onUserSelection = (id: number) => this.getBooks(id);

  componentDidlMount() {
    this.getUsers();
  }

  render() {
    const { users } = this.state;
    console.log(this.state.users);
    return (
      <div>
        <Nav username={this.props.username} />
        <h1>Hello {this.props.username}</h1>
        <table>
          <tbody>
            <tr>
              <th>ID</th>
              <th>Name</th>
            </tr>
            {users.map(u => (
              <tr
                key={u.user_id}
                onClick={() => this.onUserSelection(u.user_id)}
              >
                <td>{u.user_id}</td>
                {console.log(u.user_id)}
                <td>{u.username}</td>
              </tr>
            ))}
          </tbody>
        </table>
      </div>
    );
  }
}

export default Home;

Answer 1

As previously mentioned in Sameer's comment, you want to avoid using push when setting state. Doing this will not trigger a re-render.

Instead of the this.state.users.push(user) you're doing, replace that (and the console.log following it) with this:

this.setState(prevState => ({
  users: [...prevState.users, user]
}, console.log("updated users", this.state.users)));

A few things different here.

The first argument in setState takes in a function. There is an argument exposed in that function which is your previous state (prevState). Using the spread operator you can create a new array with the old users and all the new ones.

Secondly, due to the asynchronous nature of setState, you can't expect to run a console.log() right after a setState function. Instead, you can pass a callback function (console.log in this instance) as the second argument to setState & it will be fired when completed.

Answer 2

Use setState to populate users in state

this.setState({
users: users
})

and in render before map, do this

{
    users.length>0 &&
    users.map(u => ( <tr key={u.user_id} onClick={() => this.onUserSelection(u.user_id)} > <td>{u.user_id}</td> {console.log(u.user_id)} <td>{u.username}</td> </tr> ))
}