Twitter Bootstrap - how to detect when media queries starts

Question

which is the fastest and easy way to fire when bootstrap-responsive.css media queries go in action?

go in action = when you resize window to mobile width and site is

Answer 1

Excellent Tip, @falsarella!

For those who like this sort of thing to not affect their actual markup the following works:

$(function(){
...
var mqClasses = ["visible-xs", "visible-sm", "visible-md", "visible-lg"];
var mq = $("<span id='mqDetector' style='visibility:hidden'></span>").appendTo($("body"));
$.each(mqClasses, function(idx, val) {
    mq.append("<span class='" + val + "'></span>");
});
function checkMQ() {
    var visible = mq.find(":visible").get(0).className;
    return visible.slice(-2);
};

function checkResize(){
    switch(checkMQ){
      case 'xs' : ...; break;
      case 'sm' : ...; break;
     ...
    }
}
$(window).on('resize', checkResize);
checkResize(); //do it once when page loads.

Answer 2

Simpler

$(window).on('resize', function () {
  if ($('<div class="visible-lg">').css('display')=='block') {
    // Do something for lg
  }
  // ...
});

Answer 3

I came up with an approach that uses window resize event, but relying on Twitter Bootstrap's media queries without hard coding their breakpoints:

<span id="mq-detector">
    <span class="visible-xs"></span>
    <span class="visible-sm"></span>
    <span class="visible-md"></span>
    <span class="visible-lg"></span>
</span>

---

#mq-detector {
    visibility: hidden;
}

---

var currMqIdx = undefined;
var mqDetector = $("#mq-detector");
var mqSelectors = [
    mqDetector.find(".visible-xs"),
    mqDetector.find(".visible-sm"),
    mqDetector.find(".visible-md"),
    mqDetector.find(".visible-lg")
];

var checkForResize = function() {
    for (var i = 0; i <= mqSelectors.length; i++) {
        if (mqSelectors[i].is(":visible")) {
            if (currMqIdx != i) {
                currMqIdx = i;
                console.log(mqSelectors[i].attr("class"));
            }
            break;
        }
    }
};

$(window).on('resize', checkForResize);

checkForResize();

Answer 4

Improving the excellent @falsarella answer, here is a shorter version that works with Bootstrap 4 :

CSS :

#mq-detector {
    visibility: hidden;
}

HTML :

<div id="mq-detector">
    <span data-mq="xs" class="d-block d-sm-none"></span>
    <span data-mq="sm" class="d-none d-sm-block d-md-none"></span>
    <span data-mq="md" class="d-none d-md-block d-lg-none"></span>
    <span data-mq="lg" class="d-none d-lg-block d-xl-none"></span>
    <span data-mq="xl" class="d-none d-xl-block"></span>
</div>

JAVASCRIPT :

//Define function that returns the currently used media query
function getBsMq(){
    var currMq;
    var mqDetector = $('#mq-detector [data-mq]');
    mqDetector.each(function(i){
        if ($(this).is(':visible')) {
            currMq = $(this).attr('data-mq');
        }
    });
    return currMq;
}


//Call the function and get the currently used media query
alert(getBsMq());