Is There a Way That I Can Check If a Data Attribute Exists

Is there a way that I can check if a data attribute exists?

if ($("#dataTable").data('timer')) {
  ...
}

NOTE this only returns true if the data attribute is not empty string or a "falsey" value e.g. 0 or false.

If you want to check for the existence of the data attribute, even if empty, do this:

if (typeof $("#dataTable").data('timer') !== 'undefined') {
  ...
}

How to check if data attribute exist with plain javascript?

Element.getAttribute returns null or empty string if the attribute does not exist.

You'd use Element.hasAttribute:

if (!object.hasAttribute("data-example-param")) {
    // data attribute doesn't exist
}

or Element.dataset (see also: in operator):

if (!("exampleParam" in object.dataset)) {
    // data attribute doesn't exist
}

or even

if (!object.getAttribute("data-example-param")) {
    // data attribute doesn't exist or is empty
}

Check if data attribute exists without a value

Through jQuery you can use the .is(selector) method.

So if you set the selector to an attribute one, you can do your check

$(function() {  $('div').click(function() {    if ($(this).is('[data-specs]')) {      console.log('has specs');    } else {      console.log('no specs');    }  });});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><div data-specs>Specs</div>

checking to see if data attribute has a value or not

First as pointed out in comments, you are missing an equal sign so you are not doing a comparison. Second issue is that if there is no data attribute it is undefined, not null.

if ($('#myImg').data('tablet')===undefined) {

Find if HTML data attribute exists in any ancestor elements

I want to see if there is a more efficient, more eloquent, or easier-to-code way of doing this.

There certainly is. Meet closest()

let ancestor = baseElement.closest('[html-attribute]');
let attr_val = ancestor ? ancestor.getAttribute('html-attribute') : null;

closest() takes a selector, just like querySelector and similar methods.

Check for existence of specific data attribute value

try this:

if ($('.item[data-index="' + indexVal + '" ]').length > 0){

}

Jquery check element exists by data attribute matching only the start of a specific value

here is how

if ($('[data-attr^="123"]').length > 0) { 
 //do something
}

Javascript - On click, check if element has an attribute with value

Use e.target.getAttribute to get the attribute value of the target element. Then compare that with the value of the attribute you expect in the if condition:

document.body.addEventListener('click', function(e){    console.log(e.target);    if(e.target.getAttribute('data-a-target') === "popout-chat-button"){        console.log('popout button');    }});

<button data-a-target="popout-chat-button" data-test-selector="popout-button">Popout</button>

Is There a Way That I Can Check If a Data Attribute Exists