This is a collection of Vanilla JavaScript equivalents to jQuery methods.

vanilla.js

/**
 * This is a collection of Vanilla JavaScript 
 * equivalents to jQuery methods.
 *
 * @author Rhythm Shahriar <[email protected]>
 * @link http://rhythmshahriar.com
 */

/**----------------------------------------------------------------------
 *                          DOM selectors
 *----------------------------------------------------------------------*/
document.querySelectorAll('.item'); /* $('.item'); */

/*----------- Specific -----------*/
var parent = document.getElementById('parent');
var child  = parent.getElementsByClassName('child');

/**----------------------------------------------------------------------
 *                         DOM traversing
 *----------------------------------------------------------------------*/
document.querySelector('.item').parentNode; /* $('.item').parentNode(); */
document.querySelector('.item').children; /* $('.item').children(); */
document.querySelector('.item').firstElementChild; /* $('.item').children().first(); */
document.querySelector('.item').lastElementChild; /* $('.item').children().last(); */
document.querySelector('.item').nextElementSibling; /* $('.item').next(); */
document.querySelector('.item').previousElementSibling; /* $('.item').prev();*/


/**----------------------------------------------------------------------
 *                        DOM manipulation
 *----------------------------------------------------------------------*/
/*----------- Append -----------*/
var node = document.createElement('p'); /* $('.item').append('<p>Text</p>');*/
node.appendChild(document.createTextNode('Text'));
document.querySelector('.item').appendChild(node);


/*----------- Before & after -----------*/
document.querySelector('.item').insertAdjacentHTML('beforebegin', '<p>Text</p>'); /* $('.item').before('<p>Text</p>'); */
document.querySelector('.item').insertAdjacentHTML('afterend', '<p>Text</p>'); /*$('.item').after('<p>Text</p>'); */


/*----------- Manage HTML content -----------*/
document.querySelector('.item').innerHTML = null; /* $('.item').empty(); */
document.querySelector('.item').innerHTML = '<p>Text</p>'; /* $('.item').html('<p>Text</p>'); */


/*----------- Manage text content -----------*/
document.querySelector('.item').textContent; /* $('.item').text(); */
document.querySelector('.item').textContent = 'Text'; /* $('.item').text('Text'); */

/*----------- Remove HTML element -----------*/
var element = document.querySelector('.item'); /* $('.item').remove();*/
element.parentNode.removeChild(element);


/**----------------------------------------------------------------------
 *                        Check if element is empty
 *----------------------------------------------------------------------*/
if (!document.querySelector('.item').hasChildNodes()); /* if ($('.item').is(':empty')); */


/**----------------------------------------------------------------------
 *                        HTML attributes
 *----------------------------------------------------------------------*/
/*----------- Managing attributes -----------*/
document.querySelector('.home').getAttribute('href'); /* $('.home').attr('href'); */
document.querySelector('.home').setAttribute('href', 'http://rhythmshahriar.com'); /*$('.home').attr('href', 'http://rhythmshahriar.com'); */

/*----------- Managing attributes -----------*/
document.querySelector('img').naturalWidth; /* $('.item').addClass('active'); */
document.querySelector('img').naturalHeight; /* $('img').naturalHeight; */


/**----------------------------------------------------------------------
 *                        Managing classes
 *----------------------------------------------------------------------*/
document.querySelector('.item').classList.add('active'); /* $('.item').addClass('active'); */
document.querySelector('.item').classList.remove('active'); /* $('.item').removeClass('active'); */
document.querySelector('.item').classList.toggle('active'); /* $('.item').toggleClass('active'); */
document.querySelector('.item').classList.contains('active'); /* $('.item').hasClass('active'); */

/*----------- Managing attributes -----------*/
document.querySelector('.item').className = 'active';
document.querySelector('.item').className += ' active';

/*----------- Easiest :: Helper function -----------*/
function hasClass(el, cl) {
    return el.className && new RegExp("(\\s|^)" + cl + "(\\s|$)").test(el.className);
}

function addClass(el, cl) {
    if (!hasClass(el, cl)) { el.className += ' ' + cl; }
}

function removeClass(el, cl) {
    var reg = new RegExp("(\\s|^)" + cl + "(\\s|$)");
    el.className = el.className.replace(reg, " ").replace(/(^\s*)|(\s*$)/g,"");
}

function toggleClass(el, cl) {
    if (hasClass(el, cl)) { removeClass(el, cl); } else { addClass(el, cl); }
}

/*----------- Call :: Helper function -----------*/
hasClass(document.querySelector('.item'), 'active');   /* $('.item').hasClass('active'); */
addClass(document.querySelector('.item'), 'active');  /* $('.item').addClass('active'); */
removeClass(document.querySelector('.item'), 'active'); /* $('.item').removeClass('active'); */
toggleClass(document.querySelector('.item'), 'active'); /* $('.item').toggleClass('active'); */


/**----------------------------------------------------------------------
 *                        Styles
 *----------------------------------------------------------------------*/
/*----------- Change style -----------*/
document.querySelector('.item').style.display = 'none'; /* $('.item').hide();  */
document.querySelector('.item').style.display = ''; /*  $('.item').show(); */
document.querySelector('.item').style.borderWidth = '1px'; /*  $('.item').css('border-width', '1px'); */

/*----------- Get style -----------*/
var element = document.querySelector('.item'); /* $('.item').css(display); */
getComputedStyle(element)[display];

/*----------- Get position -----------*/
document.querySelector('.item').offsetLeft; /* $('.item').position().left;  */
document.querySelector('.item').offsetTop; /* $('.item').position().top;  */


/**----------------------------------------------------------------------
 *                        Events
 *----------------------------------------------------------------------*/
/*----------- Click -----------*/
/* $('.item').on('click', function(e) { ... });  */
var elements = document.querySelectorAll('.item');
for (var i = 0; i < elements.length; i++) {
    elements[i].addEventListener('click', function(e) {
        //...
    }, false);
}

/*----------- Ready -----------*/
/* $(document).ready(function(){ ... }); */
document.addEventListener('DOMContentLoaded', function() {
    //...
}, false);

/*----------- Custom event -----------*/
/* $('.item').trigger('foo'); */
var element = document.querySelector('.item');
var foo = new Event('customEvent');
element.addEventListener('customEvent', function (e) {
    //...
}, false);
element.dispatchEvent(foo);


/**----------------------------------------------------------------------
 *                        Helpers
 *----------------------------------------------------------------------*/
/*----------- Each -----------*/
/* $('.item').each(function(i, el){ ... }); */
var elements = document.querySelectorAll('.item');
for (var i = 0; i < elements.length; i++) {
    //...
}

/**----------------------------------------------------------------------
 *                        Trim whitespace
 *----------------------------------------------------------------------*/
/* $.trim(' string   '); */
var string = ' string   ';
string.trim();