Constructor getting called twice React Component

Question

The constructor of my react component is getting called twice but I can't figure out why. I am using react-redux to store the language of my app. I have a function that set the default language base on browser's language. LoginPage is the first component that gets render so I called my function in its constructor. Basically what it does is compare and dispatch an action. When I checked my state with redux developer tools I saw that it was getting dispatched twice. I printed dummy data in the constructor and it gets printed twice too.

LoginPage.js

import React from 'react';
import {connect} from 'react-redux';
import {startLogin} from '../actions/auth';
import {setLanguage} from '../actions/lang';

export class LoginPage extends React.Component{

  constructor(props){
    super(props);
    this.setDefaultLanguage();
    console.log('i am constructor');
  }

  changeLanguage = (e) => {
    const lan = e.target.value;
    this.props.setLanguage(lan);
  };

  setDefaultLanguage = () => {
    const defaultLanguage = navigator.language || navigator.userLanguage || 'en-US';

    if(defaultLanguage == 'es'){
      this.props.setLanguage(defaultLanguage);
    }else{
      this.props.setLanguage('en');
    }
  }

  render(){
    return(
      <div className="box-layout">
        <div className="box-layout__box">
          <h1 className="box-layout__title">Expensify</h1>
          <p>It\'s time to get your expenses under control.</p>
          <button className="button" onClick={this.props.startLogin}>Log in with google</button>
          <select className="select" onChange={this.changeLanguage}>
            <option value="en">English</option>
            <option value="es">Español</option>
          </select>
        </div>
      </div>
    )
  };
}

const mapDispatchToProps = (dispatch) => ({
  startLogin: () => dispatch(startLogin()),
  setLanguage: (language) => dispatch(setLanguage(language))
});

export default connect(undefined, mapDispatchToProps)( LoginPage);

App.js

import React from 'react';
import ReactDom from 'react-dom';
import {Router, Route, Switch} from 'react-router-dom';
import createHistory from 'history/createBrowserHistory';
import ExpenseDashBoardPage from '../components/ExpenseDashBoardPage';
import AddExpensePage from '../components/AddExpensePage';
import EditExpensePage from '../components/EditExpensePage';
import NotFoundPage from '../components/NotFoundPage';
import LoginPage from '../components/LoginPage';
import PrivateRoute from './PrivateRoute';
import PublicRoute from './PublicRoute';

export const history = createHistory();

const AppRouter = () => (
  <Router history={history}>
    <div>
      <Switch>
        <PublicRoute path="/" component={LoginPage} exact={true} />
        <PrivateRoute path="/dashboard" component={ExpenseDashBoardPage} />
        <PrivateRoute path="/create" component={AddExpensePage} />
        <PrivateRoute path="/edit/:id" component={EditExpensePage} />
        <Route component={NotFoundPage} />
      </Switch>
    </div>

  </Router>
)

export default AppRouter;

Answer 1

Yep. It’s intentional and documented here. It only happens in development mode.

https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects

Answer 2

I was having the same issue because I was redirecting the user inside my public routes. So it was rendering the component twice. In case you are using

history.push('/');

This is going to render the component twice.