Component Lifecycle componentDidMount 

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import { Component } from "react";
import { withRouter } from "react-router-dom";

class Details extends Component {
  constructor() {
    super();
    this.state = { loading: true };
  }

  async componentDidMount() {
    const res = await fetch(
      `http://pets-v2.dev-apis.com/pets?id=${this.props.match.params.id}`
    );
    const json = await res.json();
    this.setState(Object.assign({ loading: false }, json.pets[0]));
  }

  render() {
    console.log(this.state);

    if (this.state.loading) {
      return <h2>loading … </h2>;
    }

    const { animal, breed, city, state, description, name } = this.state;

    return (
      <div className="details">
        <div>
          <h1>{name}</h1>
          <h2>{`${animal} — ${breed} — ${city}, ${state}`}</h2>
          <button>Adopt {name}</button>
          <p>{description}</p>
        </div>
      </div>
    );
  }
}

export default withRouter(Details);

To replace the ctr

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# so we can replace the constructor in React.Component class
npm i -D @babel/plugin-proposal-class-properties@7.13.0 @babel/preset-env@7.13.5 @babel/eslint-parser@7.13.4

Then replace

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constructor() {
  super();
  this.state = { loading: true };
}

With

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state = { loading: true };