React: what happens to a function when component stop being rendered

Question

I have a function defined in a component, it start and keeps running when the component is mounted. What happens to the function when the component stops being rendered or dismount?

class MyComponent extends React.Component<> {
  _count = () => {
    console.log('a second passed');
    setTimeout(1000, this._count);
  }

  componentDidMount() {
    _count();
  }
}

Answer 1

The timer will keep running unless it gets cleared, so you will want to clear it in componentWillUnmount which is used to "perform any necessary cleanup...such as invalidating timers":

class MyComponent extends React.Component {
  _count = () => {
    console.log('a second passed');
    this.countTimer = setTimeout(this._count, 1000);
  }

  componentDidMount() {
    _count();
  }

  componentWillUnmount() {
    clearTimeout(this.countTimer);
  }
}