Checking div visibility using js

Checking div visibility using js

I have two divs (soru and yanit) in a div (alan). When someone clicks on
the screen, the other invisible div must be visible. At the begining,
"yanit" div is invisible and "soru" div is visible.
My code is below but doesn't work. How can I do this?
<script>
window.onload= function(){
var divSoru = document.getElementById("soru");
var divYanit = document.getElementById("yanit");
document.getElementById("alan").addEventListener('click', containerClick,
false);
function containerClick(){
if(divSoru.style.visibility=='visible'){
divSoru.style.visibility=='hidden';
divYanit.style.visibility='visible';
}
if(divSoru.style.visibility=='hidden'){
divYanit.style.visibility='hidden';
divSoru.style.visibility=='visible';
}
}
}
</script>