componentDidMount.jsx

import React from "react";
import ReactDOM from "react-dom";

class Post extends React.Component {
  state = {
    loading: true,
    post: null
  };
  componentDidMount() {
    const { postId } = this.props;
    fetch("https://jsonplaceholder.typicode.com/posts/" + postId)
      .then(response => response.json())
      .then(post => this.setState({ post, loading: false }));
  }
  render() {
    const { loading, post } = this.state;
    if (loading) {
      return <div>Loading...</div>;
    }
    return (
      <div>
        <h1>{post.title}</h1>
        <p>{post.body}</p>
      </div>
    );
  }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<Post postId={1} />, rootElement);