constructor()

The constructor method is defined inside of the component class.

The constructor is the right place to initialize state. If you don't initialize state and you don't bind methods, you don't need to implement a constructor for your React component.

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.

import React, { Component } from 'react'

// if you need to access this.props in the constructor
class Panel extends Component {
  constructor(props) {
    super(props)

    this.state = { color: this.props.initColor }
  }
}

import React, { Component } from 'react'

// if you do NOT need to access this.props in the constructor
class Panel extends Component {
  constructor() {
    super()
    this.state = { ledOn: false }
  }
}