js get element by attribute

//HTML
//<div id="elem" data-id="4hJ3s"></div>

var elem = document.getElementById("elem");
elem.getAttribute("data-id"); // "4hJ3s"

<div id="id_of_the_element" data-attribute_name="foo"></div>

var elem = document.getElementById("id_of_the_element");
var attribute_value = elem.getAttribute("data-attribute_name");

//OR with jquery...
var attribute_value = $('#id_of_the_element').attr('data-attribute_name');
//OR...
var attribute_value = $('#id_of_the_element').data('attribute_name');

document.querySelectorAll('[data-foo="value"]');

document.querySelectorAll('[rel="prev"]')[0].innerHTML = "Older posts";

//find first element with "someAttr" attribute
document.querySelector('[someAttr]')

//find all elements with "someAttr" attribute
document.querySelectorAll('[someAttr]')