[Solved]-Get default state value by receiving prop data - React-Reactjs

Without the full code, we can't tell what's wrong but it is obvious that following_status comes asynchronously to the component and that's why is not accessible right away in the constructor.

To somehow fix it you can detect if props have changed and reset state accordingly in componentDidUpdate.

class RelationButton extends React.Component {

  constructor(props){
    super(props);
    console.log(props.user.following_status) #  undefined!!!

    this.state = {
      following_status: props.user.following_status
    }
  }

  componentDidUpdate(prevProps) {
    if(prevProps.user.following_status !== this.props.user.following_status) {
      this.setState({ following_status: this.props.user.following_status })
    }
  }

  render() {
     // you forgot about return statements :
     if (this.state.following_status == 'following') {
       return <UnFollowBtn/>
     } else {
       return <FollowBtn/>
     }
  }
}