What is the role of constructor() in React components?

Implement a constructor in your React component when,

• Initializing local state by assigning an object to this.state.
• Binding event handler methods to an instance.

The constructor for a React component is called before it is mounted. When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement. Otherwise, this.props will be undefined in the constructor, which can lead to bugs.

constructor(props) {
  super(props);
  // Don't call this.setState() here!
  this.state = { counter: 0 };
  this.handleClick = this.handleClick.bind(this);
}

Avoid introducing any side-effects or subscriptions in the constructor. For those use cases, use componentDidMount() instead.

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