CSS attribute not always being grabbed by javascript correctly

I am trying to resize the side bars whenever the image changes.

I have my javascript trying grab the height of the image after it changes

var imgHeight = $('#mainImg').height();
var currImg = 0;

var imagesSet = ["1.jpg", "2.jpg", "3.jpg", "4.jpg", "5.jpg", "6.jpg"];
var imageLoc = "images/zalman/"

$('#bttnRight').click(function(){
  nextImg();
  imgHeight = $('#mainImg').height();
  resizeBttn();
});

function nextImg(){
  currImg++;
  if(currImg>=imagesSet.length){
      currImg=0;
  }
  $('#mainImg').attr("src",imageLoc + imagesSet[currImg]);
}

function resizeBttn() {
  $(document).ready(function() {
        $('#bttnLeft').css("height",imgHeight);
        $('#bttnLeft').css("bottom",imgHeight/2-5);
  });
  $(document).ready(function() {
        $('#bttnRight').css("height",imgHeight);
        $('#bttnRight').css("bottom",imgHeight/2-5);
  });
}

for some reason, it doesn’t always grab the height at the correct time and the side bars will stay at the previous height.

Below I have a JSFiddle that should be working the way my setup is.

Please excuse any inconsistencies and inefficiencies, I am learning.

Just seems weird that it would sometimes grab the height and sometimes not.

I will also be attaching an image of what I see sometimes from the JSfiddle.

I will also attach an image of what I see on my site I am actually writing.

https://jsfiddle.net/6bewkuo5/6/

Answer

The reason is because the resizeBttn code is firing before the image has actually finished downloading and loading into the DOM. I made these changes in your fiddle:

var imgHeight = $('#mainImg').height();
var currImg = 0;

var imagesSet = ["https://www.avalonwinery.com/wp-content/uploads/2013/12/200x300.gif","https://images.sftcdn.net/images/t_app-logo-xl,f_auto/p/ce2ece60-9b32-11e6-95ab-00163ed833e7/1578981868/the-test-fun-for-friends-logo.png", "https://hiveconnect.co.za/wp-content/uploads/2018/05/800x600.png", "https://upload.wikimedia.org/wikipedia/commons/b/b5/800x600_Wallpaper_Blue_Sky.png"];
var imageLoc = "images/zalman/"


$(document).ready(function() {
  resizeBttn();
});

$( window ).resize(function() {
  /* imgHeight = $('#mainImg').height() */; // commented out; we do this in resizeBttn now
  resizeBttn();
});

$('#bttnLeft').click(function(){
  prevImg();
  /* imgHeight = $('#mainImg').height() */; // commented out; we do this in resizeBttn now
  /* resizeBttn() */; // we do this as an `onload` to the image now
});

$('#bttnRight').click(function(){
  nextImg();
  /* imgHeight = $('#mainImg').height() */; // commented out; we do this in resizeBttn now
  /* resizeBttn() */; // we do this as an `onload` to the image now
});
function nextImg(){
  currImg++;
  if(currImg>=imagesSet.length){
      currImg=0;
  }
  $('#mainImg').attr("src",imagesSet[currImg]);
}

function prevImg(){
  currImg--;
  if(currImg<0){
      currImg=imagesSet.length-1;
  }
  $('#mainImg').attr("src",imagesSet[currImg]);
}

function resizeBttn() {
  imgHeight = $('#mainImg').height()
  // removed superfluous doc.ready
  $('#bttnLeft').css("height",imgHeight);
  $('#bttnLeft').css("bottom",imgHeight/2-5);
  $('#bttnRight').css("height",imgHeight);
  $('#bttnRight').css("bottom",imgHeight/2-5);
}

And then rewrote your <img /> tag to call resizeBttn on onload:

<img id="mainImg" src="https://www.avalonwinery.com/wp-content/uploads/2013/12/200x300.gif" onload="resizeBttn()"/>

You can see this in action in this fiddle.

Also, a few additional notes on your code, at a glance:

• You have some invalid HTML; you’re going to want to run that through an HTML validator and fix it, because sometimes it is fine, but sometimes it can lead to all sorts of strange behavior.
• You’re playing fast and l0ose with global variables in your JS that get set in different functions; it might work OK while the script is small, but as things scale it can quickly become difficult to maintain
• You should really avoid abusing the onclick to get link-like behavior from <li> elements; it can impact SEO as well as accessibility. I’d recommend simply using an anchor element inside or outside the <li>
• I’d recommend taking a close look at this answer by user camaulay; he makes an excellent point that this may not require JS at all– if a more elegant solution exists w/ CSS it is probably going to be more performant and maintainable.