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var viewportWidth = window.innerWidth || document.documentElement.clientWidth;
if (viewportWidth > 640) {
	console.log('Wide viewport');
} else {
	console.log('Small viewport');
}

window.addEventListener('resize', function () {
	viewportWidth = window.innerWidth || document.documentElement.clientWidth;
	if (viewportWidth > 640) {
		console.log('Wide viewport');
	} else {
		console.log('Small viewport');
	}
}, false);