sw.js

var SLOW_TIME = 3000;

this.addEventListener( 'install', function () {
  console.log('Installed service worker');
} );

this.addEventListener( 'fetch', function(event) {
  var url = event.request.url;

  if ( url.indexOf( 'blocking' ) === -1) {
  return;
  }

  var promise = Promise.race( [
    new Promise( ( resolve, reject) => setTimeout(
      () => reject( new Response( 'Request killed!' ) ),
      SLOW_TIME
    ) ),
    fetch( event.request ),
  ] );

  event.respondWith( promise );
} );

console.log('Hello from sw.js');

Thank you.