JAVASCRIPT

Initialise state in class constructor

class App extends React.Component {
  constructor(props) {
    // Required step: always call the parent class' constructor
    super(props);

    // Set the state directly. Use props if necessary.
    this.state = {
      counter: 0,
    }
  }

  render() {
    // whatever you like
  }
}

react set initial state without constructor

import React, { Component } from "react";

class Counter extends Component {
	// you can either initialize state inside constructor
  	constructor() {
		super();
		this.state = {
			count: 1,
			tags: ["tag1", "tag2", "tag3"],
		};
	}
  	// or initialize the state as class field declaration
  	state = {
		count: 1,
		tags: ["tag1", "tag2", "tag3"],
	};
}