How to detect responsive breakpoints of Twitter Bootstrap 3 using JavaScript?

If you don't have specific needs you can just do this:

if ($(window).width() < 768) {
    // do something for small screens
}
else if ($(window).width() >= 768 &&  $(window).width() <= 992) {
    // do something for medium screens
}
else if ($(window).width() > 992 &&  $(window).width() <= 1200) {
    // do something for big screens
}
else  {
    // do something for huge screens
}

Edit: I don't see why you should use another js library when you can do this just with jQuery already included in your Bootstrap project.

Detect responsive breakpoint of Twitter Bootstrap 4.1.x using JavaScript

The Bootstrap v.4.0.0 (and the latest version Bootstrap 4.1.x) introduced the updated grid options, so the old concept on detection may not directly be applied (see the migration instructions):

• Added a new sm grid tier below 768px for more granular control. We now have xs, sm, md, lg, and xl;
• xs grid classes have been modified to not require the infix.

I written the small utility function that respects an updated grid class names and a new grid tier:

/**
 * Detect the current active responsive breakpoint in Bootstrap
 * @returns {string}
 * @author farside {@link https://stackoverflow.com/users/4354249/farside}
 */
function getResponsiveBreakpoint() {
    var envs = {xs:"d-none", sm:"d-sm-none", md:"d-md-none", lg:"d-lg-none", xl:"d-xl-none"};
    var env = "";

    var $el = $("<div>");
    $el.appendTo($("body"));

    for (var i = Object.keys(envs).length - 1; i >= 0; i--) {
        env = Object.keys(envs)[i];
        $el.addClass(envs[env]);
        if ($el.is(":hidden")) {
            break; // env detected
        }
    }
    $el.remove();
    return env;
};

Detect responsive breakpoint of Bootstrap v4-beta using JavaScript

The Bootstrap v4-alpha and Bootstrap v4-beta had different approach on grid breakpoints, so here's the legacy way of achieving the same:

/**
 * Detect and return the current active responsive breakpoint in Bootstrap
 * @returns {string}
 * @author farside {@link https://stackoverflow.com/users/4354249/farside}
 */
function getResponsiveBreakpoint() {
    var envs = ["xs", "sm", "md", "lg"];
    var env = "";

    var $el = $("<div>");
    $el.appendTo($("body"));

    for (var i = envs.length - 1; i >= 0; i--) {
        env = envs[i];
        $el.addClass("d-" + env + "-none");;
        if ($el.is(":hidden")) {
            break; // env detected
        }
    }
    $el.remove();
    return env;
}

I think it would be useful, as it's easy to integrate to any project. It uses native responsive display classes of the Bootstrap itself.

Edit: This library is now available through Bower and NPM. See github repo for details.

UPDATED ANSWER:

• Live example: CodePen
• Latest version: Github repository
• Don't like Bootstrap? Check: Foundation demo and Custom framework demos
• Have a problem? Open an issue

Disclaimer: I'm the author.

Here's a few things you can do using the latest version (Responsive Bootstrap Toolkit 2.5.0):

// Wrap everything in an IIFE
(function($, viewport){

    // Executes only in XS breakpoint
    if( viewport.is('xs') ) {
        // ...
    }

    // Executes in SM, MD and LG breakpoints
    if( viewport.is('>=sm') ) {
        // ...
    }

    // Executes in XS and SM breakpoints
    if( viewport.is('<md') ) {
        // ...
    }

    // Execute only after document has fully loaded
    $(document).ready(function() {
        if( viewport.is('xs') ) {
            // ...
        }
    });

    // Execute code each time window size changes
    $(window).resize(
        viewport.changed(function() {
            if( viewport.is('xs') ) {
                // ...
            }
        })
    ); 

})(jQuery, ResponsiveBootstrapToolkit);

As of version 2.3.0, you don't need the four <div> elements mentioned below.

---

ORIGINAL ANSWER:

I don't think you need any huge script or library for that. It's a fairly simple task.

Insert the following elements just before </body>:

<div class="device-xs visible-xs"></div>
<div class="device-sm visible-sm"></div>
<div class="device-md visible-md"></div>
<div class="device-lg visible-lg"></div>

These 4 divs allow you check for currently active breakpoint. For an easy JS detection, use the following function:

function isBreakpoint( alias ) {
    return $('.device-' + alias).is(':visible');
}

Now to perform a certain action only on the smallest breakpoint you could use:

if( isBreakpoint('xs') ) {
    $('.someClass').css('property', 'value');
}

Detecting changes after DOM ready is also fairly simple. All you need is a lightweight window resize listener like this one:

var waitForFinalEvent = function () {
      var b = {};
      return function (c, d, a) {
        a || (a = "I am a banana!");
        b[a] && clearTimeout(b[a]);
        b[a] = setTimeout(c, d)
      }
    }();

var fullDateString = new Date();

Once you're equipped with it, you can start listening for changes and execute breakpoint-specific functions like so:

$(window).resize(function () {
    waitForFinalEvent(function(){

        if( isBreakpoint('xs') ) {
            $('.someClass').css('property', 'value');
        }

    }, 300, fullDateString.getTime())
});