Function to check if matchMedia is supported; if so, use it, otherwise, use JS window width calculation (fixes issues with -webkit browsers returning incorrect window widths due to scrollbars)

matchmediacheck.js

function getviewport() {
	var win = window,
		matchMediaSupported = (win.matchMedia || win.msMatchMedia),
		viewport;
	
	if (matchMediaSupported) {
		if (win.matchMedia('screen and (min-width: 641px)').matches) {
			viewport = 'large';
		} else {
			viewport = 'small';
		}
	} else if (!matchMediaSupported && win.width > 640) {
		viewport = 'large';
	} else {
		viewport = 'small';
	}
	
	return viewport;
}