Importing CSS Breakpoints Into Javascript - based on https://www.lullabot.com/blog/article/importing-css-breakpoints-javascript

main.js

//
// Get the breakpoint
//

var breakpoint = {};
breakpoint.refreshValue = function () {
    this.value = window.getComputedStyle(document.querySelector('body'), ':before').getPropertyValue('content').replace(/"/g, '');
};

//
// resize handler
//

$(window).resize(function () {
    breakpoint.refreshValue();
}).resize();

//
// In use
//

if (breakpoint.value == 'tablet') {
    console.log('Tablet breakpoint');
}
else {
    console.log('Some other breakpoint');
}

style.css

/**
* These values will not show up in content, but can be 
* queried by JavaScript to know which breakpoint is active.
* Add or remove as many breakpoints as you like.
*/
body:before {
    content: "smartphone";
    display: none; /* Prevent from displaying. */
}
@media (min-width: 700px) {
    body:before {
        content: "tablet";
    }
}
@media (min-width: 1100px) {
    body:before {
        content: "desktop";
    }
}