what is the proper way to set up an if statement to handle document.getElementsByClassName("class_name") undefined?

Question

var prev = document.getElementsByClassName("swiper-slide swiper-slide-prev");
var next = document.getElementsByClassName("swiper-slide swiper-slide-next");

alert(prev[0].className===undefined);

For example:

So where I have the alert. What gets returned when document.getElementsByClassName("swiper-slide swiper-slide-prev") does not find a element with the class name swiper-slide swiper-slide-prev? How would you handle this in a if statement?

Answer 1

You can simply use length of the object to check if something was found by your selector.

var prev = document.getElementsByClassName("swiper-slide swiper-slide-prev");
var next = document.getElementsByClassName("swiper-slide swiper-slide-next");

if( !prev.length ) {
    alert("no prev");
}

if( !next.length ) {
    alert("no next");
}

<div class="swiper-slide swiper-slide-next"></div>

Answer 2

document.getElementsByClassName returns an array of found elements. If non was found it is empty.

so you can just check if(prev.length === 0)

Btw. you can easily try this one out in your browser.

Answer 3

The getElementsByClassName return an ARRAY and you can know if you have an element with the length of the array. Here is an example:

var prev = document.getElementsByClassName("swiper-slide swiper-slide-prev");
var next = document.getElementsByClassName("swiper-slide swiper-slide-next");

if(prev.length > 0 ){
   alert(prev[0].className);
}

Obviously this element have a class name because you take it with the by the class name.

I hope that this helps you!