How to show pop-up then user close page use js

Question

I have broblem.

Situation - user first time visited in site. User want close page (not go anover page, only close). Site have pop up comprising offer discounts.

I need show pop up befor user left site.

Answer 1

Use onbeforeunload method.

window.onbeforeunload = function(){
    return 'Your message here.';
};