If you don't have to support IE9 you can just use window.matchMedia() (MDN documentation).
function checkPosition() {
if (window.matchMedia('(max-width: 767px)').matches) {
//...
} else {
//...
}
}
window.matchMedia is fully consistent with the CSS media queries and the browser support is quite good: http://caniuse.com/#feat=matchmedia
If you have to support more browsers you can use Modernizr's mq method, it supports all browsers that understand media queries in CSS.
if (Modernizr.mq('(max-width: 767px)')) {
//...
} else {
//...
}