Alert dialog from <webview> was blocked

more Clarification on @Xan Answer:

you need to listen to the dialog event from the webview ,Please read the comments in the code to understand better , again, I am using nwjs, so you can implement similar version in your language:

    //lets listen to alert dom and enable it     webview.addEventListener('dialog',function(e){   //message type    messageType = e.messageType;   messageText = e.messageText;   DialogController = e.dialog;   //lets checki if alert   if(messageType == 'alert'){     window.alert(messageText);   }//emd if    //if confirm   else if(messageType == 'confirm'){    //confirm    var confirm =  window.confirm(messageText);    //get confirm bool and send to controller    if(confirm == true){       //if true send okay to browser       DialogController.ok();    }else{      //send cancel with to send false false      DialogController.cancel();    }//end if   }//end if confirm  //lastly if its prompt   else if(messageType == 'prompt'){     //get user Input      promptInput = window.prompt(messageText);     //if null , then means the user clicked cancel      if(promptInput == null){          //tell browser to cancel           DialogController.cancel();     }else{         //feed browser with input data          DialogController.ok(promptInput);     }  }//end if prompt});//end dialog test 

A webview cannot show those by default.

You need to catch the dialog event, show your own UI for it (remember, Apps can't use alert and friends, so a <dialog> is a good option) and then pass the response back with DialogController