I'm loading elements via AJAX. Some of them are only visible if you scroll down the page.
Is there any way I can know if an element is now in the visible part of the page?

This should do the trick:

function isScrolledIntoView(elem)
{
    var docViewTop = $(window).scrollTop();
    var docViewBottom = docViewTop + $(window).height();

    var elemTop = $(elem).offset().top;
    var elemBottom = elemTop + $(elem).height();

    return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));
}

Simple Utility Function This will allow you to call a utility function that accepts the element you're looking for and if you want the element to be fully in view or partially.

function Utils() {

}

Utils.prototype = {
    constructor: Utils,
    isElementInView: function (element, fullyInView) {
        var pageTop = $(window).scrollTop();
        var pageBottom = pageTop + $(window).height();
        var elementTop = $(element).offset().top;
        var elementBottom = elementTop + $(element).height();

        if (fullyInView === true) {
            return ((pageTop < elementTop) && (pageBottom > elementBottom));
        } else {
            return ((elementTop <= pageBottom) && (elementBottom >= pageTop));
        }
    }
};

var Utils = new Utils();

Usage

var isElementInView = Utils.isElementInView($('#flyout-left-container'), false);

if (isElementInView) {
    console.log('in view');
} else {
    console.log('out of view');
}

I have such a method in my application, but it does not use jQuery:

/* Get the TOP position of a given element. */
function getPositionTop(element){
    var offset = 0;
    while(element) {
        offset += element["offsetTop"];
        element = element.offsetParent;
    }
    return offset;
}

/* Is a given element is visible or not? */
function isElementVisible(eltId) {
    var elt = document.getElementById(eltId);
    if (!elt) {
        // Element not found.
        return false;
    }
    // Get the top and bottom position of the given element.
    var posTop = getPositionTop(elt);
    var posBottom = posTop + elt.offsetHeight;
    // Get the top and bottom position of the *visible* part of the window.
    var visibleTop = document.body.scrollTop;
    var visibleBottom = visibleTop + document.documentElement.offsetHeight;
    return ((posBottom >= visibleTop) && (posTop <= visibleBottom));
}

Edit : This method works well for I.E. (at least version 6). Read the comments for compatibility with FF.

WebResourcesDepot wrote a script to load while scrolling that uses jQuery some time ago. You can view their Live Demo Here. The beef of their functionality was this:

$(window).scroll(function(){
  if  ($(window).scrollTop() == $(document).height() - $(window).height()){
    lastAddedLiveFunc();
  }
});

function lastAddedLiveFunc() { 
  $('div#lastPostsLoader').html('<img src="images/bigLoader.gif">');
  $.post("default.asp?action=getLastPosts&lastPostID="+$(".wrdLatest:last").attr("id"),
    function(data){
        if (data != "") {
          $(".wrdLatest:last").after(data);         
        }
      $('div#lastPostsLoader').empty();
    });
};

function isScrolledIntoView(elem) {
    var docViewTop = $(window).scrollTop(),
        docViewBottom = docViewTop + $(window).height(),
        elemTop = $(elem).offset().top,
     elemBottom = elemTop + $(elem).height();
   //Is more than half of the element visible
   return ((elemTop + ((elemBottom - elemTop)/2)) >= docViewTop && ((elemTop + ((elemBottom - elemTop)/2)) <= docViewBottom));
}

The best method I have found so far is the jQuery appear plugin. Works like a charm.

Mimics a custom "appear" event, which fires when an element scrolls into view or otherwise becomes visible to the user.

$('#foo').appear(function() {
  $(this).text('Hello world');
});

This plugin can be used to prevent unnecessary requests for content that's hidden or outside the viewable area.

jQuery Waypoints plugin goes very nice here.

$('.entry').waypoint(function() {
   alert('You have scrolled to an entry.');
});

There are some examples on the site of the plugin.

There is a plugin for jQuery called inview which adds a new "inview" event.

Here is some code for a jQuery plugin that doesn't use events:

$.extend($.expr[':'],{
    inView: function(a) {
        var st = (document.documentElement.scrollTop || document.body.scrollTop),
            ot = $(a).offset().top,
            wh = (window.innerHeight && window.innerHeight < $(window).height()) ? window.innerHeight : $(window).height();
        return ot > st && ($(a).height() + ot) < (st + wh);
    }
});

(function( $ ) {
    $.fn.inView = function() {
        var st = (document.documentElement.scrollTop || document.body.scrollTop),
        ot = $(this).offset().top,
        wh = (window.innerHeight && window.innerHeight < $(window).height()) ? window.innerHeight : $(window).height();

        return ot > st && ($(this).height() + ot) < (st + wh);
    };
})( jQuery );

I found this in a comment here ( http://remysharp.com/2009/01/26/element-in-view-event-plugin/ ) by a bloke called James

If you want to tweak this for scrolling item within another div,

function isScrolledIntoView (elem, divID) 

{

    var docViewTop = $('#' + divID).scrollTop();


    var docViewBottom = docViewTop + $('#' + divID).height();

    var elemTop = $(elem).offset().top;
    var elemBottom = elemTop + $(elem).height();

    return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop)); 
}

I needed to check visibility in elements inside scrollable DIV container

    //p = DIV container scrollable
    //e = element
    function visible_in_container(p, e) {
        var z = p.getBoundingClientRect();
        var r = e.getBoundingClientRect();

        // Check style visiblilty and off-limits
        return e.style.opacity > 0 && e.style.display !== 'none' &&
               e.style.visibility !== 'hidden' &&
               !(r.top > z.bottom || r.bottom < z.top ||
                 r.left > z.right || r.right < z.left);
    }

Tweeked Scott Dowding's cool function for my requirement- this is used for finding if the element has just scrolled into the screen i.e it's top edge .

function isScrolledIntoView(elem)
{
    var docViewTop = $(window).scrollTop();
    var docViewBottom = docViewTop + $(window).height();
    var elemTop = $(elem).offset().top;
    return ((elemTop <= docViewBottom) && (elemTop >= docViewTop));
}

Modified the accepted answer so that the element has to have it's display property set to something other than "none" to quality as visible.

function isScrolledIntoView(elem) {
   var docViewTop = $(window).scrollTop();
  var docViewBottom = docViewTop + $(window).height();

  var elemTop = $(elem).offset().top;
  var elemBottom = elemTop + $(elem).height();
  var elemDisplayNotNone = $(elem).css("display") !== "none";

  return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop) && elemDisplayNotNone);
}

isScrolledIntoView is a very needful function, so I tried it, it works for elements not heigher than the viewport, but if the element is bigger as the viewport it does not work. To fix this easily change the condition

return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));

to this:

return (docViewBottom >= elemTop && docViewTop <= elemBottom);

See demo here: http://jsfiddle.net/RRSmQ/

Here's my pure JavaScript solution that works if it's hidden inside a scrollable container too.

Demo here (try resizing the window too)

var visibleY = function(el){
  var rect = el.getBoundingClientRect(), top = rect.top, height = rect.height, 
    el = el.parentNode
  // Check if bottom of the element is off the page
  if (rect.bottom < 0) return false
  // Check its within the document viewport
  if (top > document.documentElement.clientHeight) return false
  do {
    rect = el.getBoundingClientRect()
    if (top <= rect.bottom === false) return false
    // Check if the element is out of view due to a container scrolling
    if ((top + height) <= rect.top) return false
    el = el.parentNode
  } while (el != document.body)
  return true
};

EDIT 2016-03-26: I've updated the solution to account for scrolling past the element so it's hidden above the top of the scroll-able container. EDIT 2018-10-08: Updated to handle when scrolled out of view above the screen.

Here is another solution from http://web-profile.com.ua/

<script type="text/javascript">
$.fn.is_on_screen = function(){
    var win = $(window);
    var viewport = {
        top : win.scrollTop(),
        left : win.scrollLeft()
    };
    viewport.right = viewport.left + win.width();
    viewport.bottom = viewport.top + win.height();

    var bounds = this.offset();
    bounds.right = bounds.left + this.outerWidth();
    bounds.bottom = bounds.top + this.outerHeight();

    return (!(viewport.right < bounds.left || viewport.left > bounds.right ||    viewport.bottom < bounds.top || viewport.top > bounds.bottom));
 };

if( $('.target').length > 0 ) { // if target element exists in DOM
    if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
        $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info       
    } else {
        $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
    }
}
$(window).scroll(function(){ // bind window scroll event
if( $('.target').length > 0 ) { // if target element exists in DOM
    if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
        $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info
    } else {
        $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
    }
}
});
</script>

See it in JSFiddle

This answer in Vanilla:

function isScrolledIntoView(el) {
    var rect = el.getBoundingClientRect();
    var elemTop = rect.top;
    var elemBottom = rect.bottom;

    // Only completely visible elements return true:
    var isVisible = (elemTop >= 0) && (elemBottom <= window.innerHeight);
    // Partially visible elements return true:
    //isVisible = elemTop < window.innerHeight && elemBottom >= 0;
    return isVisible;
}

How about

function isInView(elem){
   return $(elem).offset().top - $(window).scrollTop() < $(elem).height() ;
}

After that you can trigger whatever you want once the element is in view like this

$(window).scroll(function(){
   if (isInView($('.classOfDivToCheck')))
      //fire whatever you what 
      dothis();
})

That works for me just fine

Here is a way to achieve the same thing using Mootools, in horizontal, vertical or both.

Element.implement({
inVerticalView: function (full) {
    if (typeOf(full) === "null") {
        full = true;
    }

    if (this.getStyle('display') === 'none') {
        return false;
    }

    // Window Size and Scroll
    var windowScroll = window.getScroll();
    var windowSize = window.getSize();
    // Element Size and Scroll
    var elementPosition = this.getPosition();
    var elementSize = this.getSize();

    // Calculation Variables
    var docViewTop = windowScroll.y;
    var docViewBottom = docViewTop + windowSize.y;
    var elemTop = elementPosition.y;
    var elemBottom = elemTop + elementSize.y;

    if (full) {
        return ((elemBottom >= docViewTop) && (elemTop <= docViewBottom)
            && (elemBottom <= docViewBottom) && (elemTop >= docViewTop) );
    } else {
        return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));
    }
},
inHorizontalView: function(full) {
    if (typeOf(full) === "null") {
        full = true;
    }

    if (this.getStyle('display') === 'none') {
        return false;
    }

    // Window Size and Scroll
    var windowScroll = window.getScroll();
    var windowSize = window.getSize();
    // Element Size and Scroll
    var elementPosition = this.getPosition();
    var elementSize = this.getSize();

    // Calculation Variables
    var docViewLeft = windowScroll.x;
    var docViewRight = docViewLeft + windowSize.x;
    var elemLeft = elementPosition.x;
    var elemRight = elemLeft + elementSize.x;

    if (full) {
        return ((elemRight >= docViewLeft) && (elemLeft <= docViewRight)
            && (elemRight <= docViewRight) && (elemLeft >= docViewLeft) );
    } else {
        return ((elemRight <= docViewRight) && (elemLeft >= docViewLeft));
    }
},
inView: function(full) {
    return this.inHorizontalView(full) && this.inVerticalView(full);
}});

This method will return true if any part of the element is visible on the page. It worked better in my case and may help someone else.

function isOnScreen(element) {
  var elementOffsetTop = element.offset().top;
  var elementHeight = element.height();

  var screenScrollTop = $(window).scrollTop();
  var screenHeight = $(window).height();

  var scrollIsAboveElement = elementOffsetTop + elementHeight - screenScrollTop >= 0;
  var elementIsVisibleOnScreen = screenScrollTop + screenHeight - elementOffsetTop >= 0;

  return scrollIsAboveElement && elementIsVisibleOnScreen;
}

Simple modification for scrollable div (container)

var isScrolledIntoView = function(elem, container) {
    var containerHeight = $(container).height();
    var elemTop = $(elem).position().top;
    var elemBottom = elemTop + $(elem).height();
    return (elemBottom > 0 && elemTop < containerHeight);
}

NOTE: this does not work if the element is larger than the scrollable div.

This considers any padding, border or margin the element has as well as elements larger than the viewport itself.

function inViewport($ele) {
    var lBound = $(window).scrollTop(),
        uBound = lBound + $(window).height(),
        top = $ele.offset().top,
        bottom = top + $ele.outerHeight(true);

    return (top > lBound && top < uBound)
        || (bottom > lBound && bottom < uBound)
        || (lBound >= top && lBound <= bottom)
        || (uBound >= top && uBound <= bottom);
}

To call it use something like this:

var $myElement = $('#my-element'),
    canUserSeeIt = inViewport($myElement);

console.log(canUserSeeIt); // true, if element is visible; false otherwise

I adapted this short jQuery function extension, which you can feel free to use (MIT licence).

/**
 * returns true if an element is visible, with decent performance
 * @param [scope] scope of the render-window instance; default: window
 * @returns {boolean}
 */
jQuery.fn.isOnScreen = function(scope){
    var element = this;
    if(!element){
        return;
    }
    var target = $(element);
    if(target.is(':visible') == false){
        return false;
    }
    scope = $(scope || window);
    var top = scope.scrollTop();
    var bot = top + scope.height();
    var elTop = target.offset().top;
    var elBot = elTop + target.height();

    return ((elBot <= bot) && (elTop >= top));
};

I prefer using jQuery expr

jQuery.extend(jQuery.expr[':'], {  
    inview: function (elem) {
        var t = $(elem);
        var offset = t.offset();
        var win = $(window); 
        var winST = win.scrollTop();
        var elHeight = t.outerHeight(true);

        if ( offset.top > winST - elHeight && offset.top < winST + elHeight + win.height()) {
            return true;    
        }    
        return false;  
    }
});

so you can use it this way

$(".my-elem:inview"); //returns only element that is in view
$(".my-elem").is(":inview"); //check if element is in view
$(".my-elem:inview").length; //check how many elements are in view

You can easly add such code inside scroll event function etc. to check it everytime user will scroll the view.

You can make use of jquery plugin "onScreen" to check if the element is in the current viewport when you scroll. The plugin sets the ":onScreen" of the selector to true when the selector appears on the screen. This is the link for the plugin which you can include in your project. "http://benpickles.github.io/onScreen/jquery.onscreen.min.js"

You can try the below example which works for me.

$(document).scroll(function() {
    if($("#div2").is(':onScreen')) {
        console.log("Element appeared on Screen");
        //do all your stuffs here when element is visible.
    }
    else {
        console.log("Element not on Screen");
        //do all your stuffs here when element is not visible.
    }
});

HTML Code:

<div id="div1" style="width: 400px; height: 1000px; padding-top: 20px; position: relative; top: 45px"></div> <br>
<hr /> <br>
<div id="div2" style="width: 400px; height: 200px"></div>

CSS:

#div1 {
    background-color: red;
}
#div2 {
    background-color: green;
}

I have written a component for the task, designed to handle large numbers of elements extremely fast (to the tune of <10ms for 1000 elements on a slow mobile).

It works with every type of scroll container you have access to – window, HTML elements, embedded iframe, spawned child window – and is very flexible in what it detects (full or partial visibility, border box or content box, custom tolerance zone, etc).

A huge, mostly auto-generated test suite ensures that it works as advertised, cross-browser.

Give it a shot if you like: jQuery.isInView. Otherwise, you might find inspiration in the source code, e.g. here.

An example based off of this answer to check if an element is 75% visible (i.e. less than 25% of it is off of the screen).

function isScrolledIntoView(el) {
  // check for 75% visible
  var percentVisible = 0.75;
  var elemTop = el.getBoundingClientRect().top;
  var elemBottom = el.getBoundingClientRect().bottom;
  var elemHeight = el.getBoundingClientRect().height;
  var overhang = elemHeight * (1 - percentVisible);

  var isVisible = (elemTop >= -overhang) && (elemBottom <= window.innerHeight + overhang);
  return isVisible;
}

I was looking for a way to see if the element is going to come into view soon, so by extending the snippets above i managed to do it. thought i would leave this here just in case it will help someone

elm = is the element you want to check is in the view

scrollElement = you can pass window or a parent element that has a scroll

offset = if you want it to fire when the element is 200px away before its in the screen then pass 200

function isScrolledIntoView(elem, scrollElement, offset)
        {
            var $elem = $(elem);
            var $window = $(scrollElement);
            var docViewTop = $window.scrollTop();
            var docViewBottom = docViewTop + $window.height();
            var elemTop = $elem.offset().top;
            var elemBottom = elemTop + $elem.height();
            
            return (((elemBottom+offset) >= docViewBottom) && ((elemTop-offset) <= docViewTop)) || (((elemBottom-offset) <= docViewBottom) && ((elemTop+offset) >= docViewTop));
        }

</div>

Most answers here don't take into account that an element can also be hidden because it is scrolled out of view of a div, not only of the whole page.

To cover that possibility, you basically have to check if the element is positioned inside the bounds of each of its parents.

This solution does exactly that:

function(element, percentX, percentY){
    var tolerance = 0.01;   //needed because the rects returned by getBoundingClientRect provide the position up to 10 decimals
    if(percentX == null){
        percentX = 100;
    }
    if(percentY == null){
        percentY = 100;
    }

    var elementRect = element.getBoundingClientRect();
    var parentRects = [];

    while(element.parentElement != null){
        parentRects.push(element.parentElement.getBoundingClientRect());
        element = element.parentElement;
    }

    var visibleInAllParents = parentRects.every(function(parentRect){
        var visiblePixelX = Math.min(elementRect.right, parentRect.right) - Math.max(elementRect.left, parentRect.left);
        var visiblePixelY = Math.min(elementRect.bottom, parentRect.bottom) - Math.max(elementRect.top, parentRect.top);
        var visiblePercentageX = visiblePixelX / elementRect.width * 100;
        var visiblePercentageY = visiblePixelY / elementRect.height * 100;
        return visiblePercentageX + tolerance > percentX && visiblePercentageY + tolerance > percentY;
    });
    return visibleInAllParents;
};

It also lets you specify to what percentage it has to be visible in each direction.
It doesn't cover the possibility that it may be hidden due to other factors, like display: hidden.

This should work in all major browsers, since it only uses getBoundingClientRect. I personally tested it in Chrome and Internet Explorer 11.

Plain vanilla to check if element (el) is visible in scrollable div (holder)

function isElementVisible (el, holder) {
  holder = holder || document.body
  const { top, bottom, height } = el.getBoundingClientRect()
  const holderRect = holder.getBoundingClientRect()

  return top <= holderRect.top
    ? holderRect.top - top <= height
    : bottom - holderRect.bottom <= height
},

Usage with jQuery:

var el = $('tr:last').get(0);
var holder = $('table').get(0);
isVisible =  isScrolledIntoView(el, holder);

Building off of this great answer, you can simplify it a little further using ES2015+:

function isScrolledIntoView(el) {
  const { top, bottom } = el.getBoundingClientRect()
  return top >= 0 && bottom <= window.innerHeight
}

If you don't care about the top going out of the window and just care that the bottom has been viewed, this can be simplified to

function isSeen(el) {
  return el.getBoundingClientRect().bottom <= window.innerHeight
}

or even the one-liner

const isSeen = el => el.getBoundingClientRect().bottom <= window.innerHeight

Using the new IntersectionObserver API it's very easy and efficient to determine if an element visible in the viewport. By using an observer, it eliminates the need to attach a scroll event and manually checking on the event callback.

// this is the target which is observed
var target = document.querySelector('div');

// configure the intersection observer instance
var intersectionObserverOptions = {
  root: null,
  rootMargin: '150px',
  threshold: 1.0
}
    
var observer = new IntersectionObserver(onIntersection, intersectionObserverOptions);

// provice the observer with a target
observer.observe(target);

function onIntersection(entries){
  entries.forEach(entry => {
    console.clear();
    console.log(entry.intersectionRatio)
    target.classList.toggle('visible', entry.intersectionRatio > 0);
    
    // Are we in viewport?
    if (entry.intersectionRatio > 0) {
      // Stop watching 
      // observer.unobserve(entry.target);
    }
  });
}

.box{ width:100px; height:100px; background:red; margin:1000px; }
.box.visible{ background:green; }

Scroll both Vertically & Horizontally...
<div class='box'></div>

View browsers support table (not supported in IE/Safari)

</div>