Jquery check if element is visible in viewport

You can write a jQuery function like this to determine if an element is in the viewport.

Include this somewhere after jQuery is included:

$.fn.isInViewport = function() {
    var elementTop = $(this).offset().top;
    var elementBottom = elementTop + $(this).outerHeight();

    var viewportTop = $(window).scrollTop();
    var viewportBottom = viewportTop + $(window).height();

    return elementBottom > viewportTop && elementTop < viewportBottom;
};

Sample usage:

$(window).on('resize scroll', function() {
    if ($('#Something').isInViewport()) {
        // do something
    } else {
        // do something else
    }
});

Note that this only checks the top and bottom positions of elements, it doesn't check if an element is outside of the viewport horizontally.

Check if element is visible in viewport using jquery:

First determine the top and bottom positions of the element. Then determine the position of the viewport's bottom (relative to the top of your page) by adding the scroll position to the viewport height.

If the bottom position of the viewport is greater than the element's top position AND the top position of the viewport is less than the element's bottom position, the element is in the viewport (at least partially). In simpler terms, when any part of the element is between the top and bottom bounds of your viewport, the element is visible on your screen.

Now you can write an if/else statement, where the if statement only runs when the above condition is met.

The code below executes what was explained above:

// this function runs every time you are scrolling

$(window).scroll(function() {
    var top_of_element = $("#element").offset().top;
    var bottom_of_element = $("#element").offset().top + $("#element").outerHeight();
    var bottom_of_screen = $(window).scrollTop() + $(window).innerHeight();
    var top_of_screen = $(window).scrollTop();

    if ((bottom_of_screen > top_of_element) && (top_of_screen < bottom_of_element)){
        // the element is visible, do something
    } else {
        // the element is not visible, do something else
    }
});

This answer is a summary of what Chris Bier and Andy were discussing below. I hope it helps anyone else who comes across this question while doing research like I did. I also used an answer to the following question to formulate my answer: Show Div when scroll position.

According to the documentation for that plugin, .visible() returns a boolean indicating if the element is visible. So you'd use it like this:

if ($('#element').visible(true)) {
    // The element is visible, do something
} else {
    // The element is NOT visible, do something else
}