SOURCE

function floor(number, significance) {
	var result = "#VALUE";
	  if (!(number > 0 && significance > 0) && !(number < 0 && significance < 0)) {
	    return "NUM!";
	  }

	  significance = Math.abs(significance);
	  var precision = -Math.floor(Math.log(significance) / Math.log(10));
	result = ROUND(Math.floor(Math.abs(number) / significance) * significance, precision);
	  if (number >= 0) {
	    result = result;
	  } else {
	    result = -result;
	  }
	document.write(result);
	};
function ROUND(number,digits){
	return Math.round(number * Math.pow(10, digits)) / Math.pow(10, digits);
}
floor(34,5);