Q418: Why should not call setState in componentWillUnmount? - React & Modern Frontend (445 Qs)

You should not call setState() in componentWillUnmount() because once a component instance is unmounted, it will never be mounted again.

Code Example

tsx

import React, { Component } from 'react';

class TimerListener extends Component {
  private timerId: any = null;

  componentDidMount() {
    this.timerId = setInterval(() => {
      this.forceUpdate();
    }, 1000);
  }

  componentWillUnmount() {
    // ❌ NEVER call setState here: component is already unmounted,
    // so no re-render occurs and React flags a memory leak.
    // this.setState({ active: false });

    // ✅ Clean up resources, intervals, subscriptions, and sockets:
    clearInterval(this.timerId);
  }

  render() {
    return <div>Active Timer</div>;
  }
}