I've recently started learning React and I'm a little bit confused.

Please see the following codepen or the code snippet below.

class App extends React.Component {

  constructor(props) {
    super(props);
    this.state = {
      sessions: [],
      sessionSelected: null
    }
    this.addSession = this.addSession.bind(this);
    this.switchSession = this.switchSession.bind(this);
  }

  addSession() {
    let sessions = this.state.sessions;
    let id = (sessions.length)
    let title = "Session " + id;
    let session = <Session title={title} index={id + 1} />;
    sessions.push(session);
    this.setState({
      sessions: sessions,
      sessionSelected: id
    });
  }

  switchSession(id) {
    this.setState({
      sessionSelected: id
    });
  }

  render() {
    return (
      <div>
        <button onClick={this.addSession} >+ Add Session</button>
        <div>{this.state.sessions[this.state.sessionSelected]}</div>
        <div className="switchers">
          {this.state.sessions.map((x, i) => {
            return <SessionSwitcher index={i + 1} onClick={() => { this.switchSession(i) }} />;
          })}
        </div>
      </div>
    );
  }
}

class Session extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    }
    this.startTimer = this.startTimer.bind(this);
    this.count = this.count.bind(this);
  }
  startTimer() {
    setInterval(this.count, 1000);
  }
  count() {
    this.setState({
      count: this.state.count + 1
    });
  }
  render() {
    return (
      <div>
        <h1>{this.props.title}</h1>
        <p>{this.state.count}</p>
        <button onClick={this.startTimer}>Start Timer</button>
      </div>
    )
  }
}

class SessionSwitcher extends React.Component {
  render() {
    return (
      <div className="switcher" onClick={this.props.onClick}>
        <span>{this.props.index}</span>
      </div>
    )
  }
}

ReactDOM.render(
  <App />,
  document.querySelector('#app')
);

I want to be able to trigger multiple timers within multiple ponents.

For some reason when I click the start timer in one ponent, it triggers it for the other ponents too.

Can someone explain to me what I am doing wrong?

I've recently started learning React and I'm a little bit confused.

Please see the following codepen or the code snippet below.

class App extends React.Component {

  constructor(props) {
    super(props);
    this.state = {
      sessions: [],
      sessionSelected: null
    }
    this.addSession = this.addSession.bind(this);
    this.switchSession = this.switchSession.bind(this);
  }

  addSession() {
    let sessions = this.state.sessions;
    let id = (sessions.length)
    let title = "Session " + id;
    let session = <Session title={title} index={id + 1} />;
    sessions.push(session);
    this.setState({
      sessions: sessions,
      sessionSelected: id
    });
  }

  switchSession(id) {
    this.setState({
      sessionSelected: id
    });
  }

  render() {
    return (
      <div>
        <button onClick={this.addSession} >+ Add Session</button>
        <div>{this.state.sessions[this.state.sessionSelected]}</div>
        <div className="switchers">
          {this.state.sessions.map((x, i) => {
            return <SessionSwitcher index={i + 1} onClick={() => { this.switchSession(i) }} />;
          })}
        </div>
      </div>
    );
  }
}

class Session extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    }
    this.startTimer = this.startTimer.bind(this);
    this.count = this.count.bind(this);
  }
  startTimer() {
    setInterval(this.count, 1000);
  }
  count() {
    this.setState({
      count: this.state.count + 1
    });
  }
  render() {
    return (
      <div>
        <h1>{this.props.title}</h1>
        <p>{this.state.count}</p>
        <button onClick={this.startTimer}>Start Timer</button>
      </div>
    )
  }
}

class SessionSwitcher extends React.Component {
  render() {
    return (
      <div className="switcher" onClick={this.props.onClick}>
        <span>{this.props.index}</span>
      </div>
    )
  }
}

ReactDOM.render(
  <App />,
  document.querySelector('#app')
);

I want to be able to trigger multiple timers within multiple ponents.

For some reason when I click the start timer in one ponent, it triggers it for the other ponents too.

Can someone explain to me what I am doing wrong?

• javascript
• reactjs

1 Answer 1

Here are a couple of things you could change to get a more predictable experience:

1. Rather than storing JSX in state, which is a plex object, store just the id, and render JSX according to that id.

2. Try not to mutate state in place (as with pushing onto an array that's already in state).

3. If you want to have persistent timers across ponents, regardless of which ones are currently selected, be sure not to unmount them when they're deselected.

Here are the relevant updated parts of your <App/> ponent (note that these are more quick fixes than best practices):

  addSession() {
    const id = this.state.sessions.length;
    this.setState( state => ({
      // treat your state as immutable
      sessions: state.sessions.concat( id ),
      sessionSelected: id
    }));
  }

  // don't unmount when switching ponents, just hide
  render() {
    return (
      <div>
        <button onClick={this.addSession} >+ Add Session</button>
        <div>
          { this.state.sessions.map( session => 
            <div style={{ display: session === this.state.sessionSelected ? 'block' : 'none' }}>
              <Session title={"Session " + session} />
            </div>
          )}
        </div>
        <div className="switchers">
          {this.state.sessions.map((x, i) => {
            return <SessionSwitcher index={i + 1} onClick={() => { this.switchSession(i) }} />;
          })}
        </div>
      </div>
    );
  }

Try it here: https://codepen.io/glortho/pen/ZrLMda?editors=0011