if ($(selector).length > 0) {
    // Do something
}

if ($(selector).length)

jQuery.fn.exists = function(){ return this.length > 0; }

if ($(selector).exists()) {
    // Do something
}

jQuery.fn.exists = function(){return ($(this).length > 0);}
if ($(selector).exists()) { }

jQuery.exists = function(selector) {return ($(selector).length > 0);}
if ($.exists(selector)) { }

if ( $('#myDiv').length ) { /* Do something */ }

if ( $('#myDiv')[0] ) { /* Do something */ }

// if element exists
if($('selector').length){ /* do something */ }

// if element does not exist
if(!$('selector').length){ /* do something */ }

if (document.getElementById('element_id')) {
    // Do something
}

if ($(selector)[0]) { ... }

if ($(selector).is('*')) {
  // Do something
}

;;(function($) {
    if (!$.exist) {
        $.extend({
            exist: function() {
                var ele, cbmExist, cbmNotExist;
                if (arguments.length) {
                    for (x in arguments) {
                        switch (typeof arguments[x]) {
                            case 'function':
                                if (typeof cbmExist == "undefined") cbmExist = arguments[x];
                                else cbmNotExist = arguments[x];
                                break;
                            case 'object':
                                if (arguments[x] instanceof jQuery) ele = arguments[x];
                                else {
                                    var obj = arguments[x];
                                    for (y in obj) {
                                        if (typeof obj[y] == 'function') {
                                            if (typeof cbmExist == "undefined") cbmExist = obj[y];
                                            else cbmNotExist = obj[y];
                                        }
                                        if (typeof obj[y] == 'object' && obj[y] instanceof jQuery) ele = obj[y];
                                        if (typeof obj[y] == 'string') ele = $(obj[y]);
                                    }
                                }
                                break;
                            case 'string':
                                ele = $(arguments[x]);
                                break;
                        }
                    }
                }

                if (typeof cbmExist == 'function') {
                    var exist =  ele.length > 0 ? true : false;
                    if (exist) {
                        return ele.each(function(i) { cbmExist.apply(this, [exist, ele, i]); });
                    }
                    else if (typeof cbmNotExist == 'function') {
                        cbmNotExist.apply(ele, [exist, ele]);
                        return ele;
                    }
                    else {
                        if (ele.length <= 1) return ele.length > 0 ? true : false;
                        else return ele.length;
                    }
                }
                else {
                    if (ele.length <= 1) return ele.length > 0 ? true : false;
                    else return ele.length;
                }

                return false;
            }
        });
        $.fn.extend({
            exist: function() {
                var args = [$(this)];
                if (arguments.length) for (x in arguments) args.push(arguments[x]);
                return $.exist.apply($, args);
            }
        });
    }
})(jQuery);

if ($.exist('#eleID')) {    /*    DO WORK    */ }        //    param as STRING
if ($.exist($('#eleID'))) { /*    DO WORK    */ }        //    param as jQuery OBJECT
if ($('#eleID').exist()) {  /*    DO WORK    */ }        //    enduced on jQuery OBJECT

$.exist('#eleID', function() {            //    param is STRING && CALLBACK METHOD
    /*    DO WORK    */
    /*    This will ONLY fire if the element EXIST    */
}, function() {            //    param is STRING && CALLBACK METHOD
    /*    DO WORK    */
    /*    This will ONLY fire if the element DOES NOT EXIST    */
})

$('#eleID').exist(function() {            //    enduced on jQuery OBJECT with CALLBACK METHOD
    /*    DO WORK    */
    /*    This will ONLY fire if the element EXIST    */
})

$.exist({                        //    param is OBJECT containing 2 key|value pairs: element = STRING, callback = METHOD
    element: '#eleID',
    callback: function() {
        /*    DO WORK    */
        /*    This will ONLY fire if the element EXIST    */
    }
})

$.fn.exists = $.fn.exists || function() {
  return !!(this.length && (this[0] instanceof HTMLDocument || this[0] instanceof HTMLElement));
}

$(1980).exists(); //return false
$([1,2,3]).exists(); //return false
$({name: 'stackoverflow', url: 'http://www.stackoverflow.com'}).exists(); //return false
$([{nodeName: 'foo'}]).exists() // returns false
$('div').exists(); //return true
$('.header').exists(); //return true
$(document).exists(); //return true
$('body').exists(); //return true

if(document.getElementById("myElement")) {
    //Do something...
}

jQuery.fn.extend({
    exists: function() { return this.length }
});

if($(selector).exists()){/*do something*/}

function elementIfExists(selector){  //named this way on purpose, see below
    return document.querySelector(selector);
}
/* usage: */
var myelement = elementIfExists("#myid") || myfallbackelement;

var myel=elementIfExists("#myid");
// now we are using a reference to the element which will linger after removal
myel.getParentNode.removeChild(myel);
console.log(elementIfExists("#myid")); /* null */
console.log(myel); /* giant table lingering around detached from document */
myel=null; /* now it can be garbage collected */

/* locally scoped myel gets garbage collected even with the break; */
for (var myel; myel = elementIfExist(sel); myel.getParentNode.removeChild(myel))
    if (myel == myblacklistedel) break;

function elementExists(selector){
    return !!document.querySelector(selector);
}
/* usage: */
var hastables = elementExists("table");  /* will be true or false */
if (hastables){
    /* insert css style sheet for our pretty tables */
}
setTimeOut(function (){if (hastables && !elementExists("#mytablecss"))
                           alert("bad table layouts");},3000);

var $target = $({});
console.log($target, $target.length);

// Console output:
// -------------------------------------
// [▼ Object              ] 1
//    ► __proto__: Object

if ($.isEmptyObject(selector) || !$(selector).length) {
    throw new Error('Unable to work with the given selector.');
}

$.isEmptyObject('hello') // FALSE in Chrome and TRUE in IE

// These by Id
if( $('#elementid').length > 0){
  // Element is Present
}else{
  // Element is not Present
}

// These by Class
if( $('.elementClass').length > 0){
  // Element is Present
}else{
  // Element is not Present
}

$(selector).length && //Do something

if ($("#myDiv").length) {
    $("#myDiv").show();
}

$("#myDiv").show();

jQuery.fn.exists = function(){return !!this.length};

if ($(selector).exists()) {
    // the element exists, now what?...
}

if (!!$(selector)[0]) // do stuff

$.fn.exists = function() {
    return $.contains( document.documentElement, this[0] );
}

function isExists(selector){
  return document.querySelectorAll(selector).length>0;
}

// Checks if an object exists.
// Usage:
//
//     $(selector).exists()
//
// Or:
//
//     $(selector).exists(anotherSelector);
jQuery.fn.exists = function(selector) {
    return selector ? this.find(selector).length : this.length;
};

$.fn.exists = function(callback) {
    var self = this;
    var wrapper = (function(){
            function notExists () {}

            notExists.prototype.otherwise = function(fallback){
                if (!self.length) {
                    fallback.call();
                }
            };

            return new notExists;
        })();

    if(self.length) {
        callback.call();
    }

    return wrapper;
}

$("#elem").exists(function(){
    alert ("it exists");
}).otherwise(function(){
    alert ("it doesn't exist");
});

$.fn.exists = (callback) ->
    exists = @length
    callback.call() if exists
    new class
       otherwise: (fallback) ->
            fallback.call() if not exists

if(document.querySelector('.a-class')) {
  // do something
}

function exists(selector) {
    return $(selector).length;
}

if (exists(selector)) {
    // do something
}

$.fn.ifExists = function(fn) {
      if (this.length) {
        $(fn(this));
      }
    };
    $("#element").ifExists(
      function($this){
        $this.addClass('someClass').animate({marginTop:20},function(){alert('ok')});
      }
    );

$.fn.exist = function(callback) {
    return $(this).each(function () {
        var target = $(this);

        if (this.length > 0 && typeof callback === 'function') {
            callback.call(target);
        }
    });
};

$.fn.exist = function(onExist, onNotExist) {
    return $(this).each(function() {
        var target = $(this);

        if (this.length > 0) {
            if (typeof onExist === 'function') {
                onExist.call(target);
            }
        } else {
            if (typeof onNotExist === 'function') {
                onNotExist.call(target);
            }
        }
    });
};

$('#foo .bar').exist(
    function () {
        // Stuff when '#foo .bar' exists
    },
    function () {
        // Stuff when '#foo .bar' does not exist
    }
);

if($("selector").length){
   //code in the case
}

if ( $('#myDiv').size() > 0 ) { //do something }

if($(selector).length){
  // true if length is not 0
} else {
  // false if length is 0
}