Preventing Transitions in react router 4

App.js

import React from 'react';

class App extends Component {
  this.unblock = this.props.history.block(nextLocation => this.getShouldBlockTransition(nextLocation));

 getShouldBlockTransition = nextLocation => {
    // shouldBlock is a boolean which represent if history should block/not to block.
    // If set to true, the user wont be able to exit, and on false he will.
   return shouldBlock;
  }
 
 render() {
   ...
 }
}