Just a JS function to check if the viewport matches the guessed bootstrap grid option.

isViewport.js

/*
 *    Just a JS function to check if the viewport matches the guessed bootstrap grid option.
 *    It uses the modern matchMedia object passing the same media query that bootstrap use, in both v3 or 4.
 *    Otherwise falls back using the standard window object.
 *    I often saw using the screen object for the same purpose,
 *    but it behaves strangely in extended desktop layouts with multiple monitors.
 *    Note that window.innerWidth instead, should not be used from within frames.
 */

function isViewport(bsSelector)
{
  BSversion = 3;
  // http://getbootstrap.com/css/#grid-options
  // https://v4-alpha.getbootstrap.com/layout/grid/#grid-options

  width = {
    xs: { min: 0, max: (BSversion < 4 ? 768 : 544) },
    sm: { min: (BSversion < 4 ? 768 : 544), max: (BSversion < 4 ? 992 : 768) },
    md: { min: (BSversion < 4 ? 992 : 768), max: (BSversion < 4 ? 1200 : 992) },
    lg: { min: (BSversion < 4 ? 1200 : 992), max: (BSversion < 4 ? 0 : 1200) }
  };
  if( BSversion > 3 )
    width['xl'] = { min: 1200, max: 0 };
  largest = BSversion < 4 ? 'lg' : 'xl';

  if(  typeof width[bsSelector] === 'undefined' )
    return false;

  if( window.matchMedia )
  {
    mq = window.matchMedia(
      ( width[bsSelector].min > 0 ? '(min-width: '+ width[bsSelector].min +'px)' : '' ) +
      ( bsSelector != 'xs' && bsSelector != largest ? ' and ' : '' ) +
      ( width[bsSelector].max > 0 ? '(max-width: '+ width[bsSelector].max +'px)' : '' )
    );
    return mq.matches;
  }
  else
  {  // fallback
    // http://www.w3schools.com/jsref/prop_win_innerheight.asp
    currentWidth = window.innerWidth ||
      document.documentElement.clientWidth ||
      document.body.clientWidth;

    switch( bsSelector ){
      case 'xs':
        if(currentWidth < width[bsSelector].max)
          return true;
      break;
      case largest:
        if(currentWidth > width[bsSelector].min)
          return true;
      break;
      default:
        if(  currentWidth > width[bsSelector].min &&
          currentWidth < width[bsSelector].max )
          return true;
      break;
    }
    return false;
  }
}