gistfile1.md

function getStylePropValInNum (el, prop) {
	if (!prop || typeof prop !== 'string' || !el || el.nodeType !== 1) return 0;
	
	var reg = /(\d*)px/, matchResult, result;
	matchResult = el.style[prop].match(reg);
	
	if (matchResult && matchResult.length === 2) {
		result = +matchResult[1];
	}
	
	return isNaN(result) ? 0 : result;
}

getStylePropValByNum(document.querySelectorAll('div')[0], 'marginTop');