window.onload and window.onbeforeunload

Question

Why I have to use window.onload to run this code and why if I put

window.onbeforeunload = function () {alert("Wait")}

image-slider stops working?
https://code.sololearn.com/WQ7iM3i5PF5j/?ref=app

Answer 1

Your question contains two parts:

A) Why window.onload?
A1) You can consider the following demo an endorsement to ShortCode's answer:
https://code.sololearn.com/WriFNOoR188j/?ref=app
A2) There are two more ways:
A2. 1) putting the JS scripts at end of body.
A2. 2) define function main then onload = main
B) onbeforeunload fails because your browser bans spammy script.

Answer 2

Because If your window hasnt load, your Element returns null.
var myImg = document.getElementById("curImg");

document.getElementById("btnPrev").addEventListener("click", goPrev);
document.getElementById("btnNext").addEventListener("click", goNext);

Answer 3

Glad that we are of help ☺️
Please try these out.
Am looking forward to seeing your codes in hot today 😉

Answer 4

Thank you for your answers.
It's really helpful, especially this post: https://www.sololearn.com/Discuss/1546397/?ref=app