Edit in JSFiddle

var jcnt = function (str, num, lst) {
	len = 0;
	for (i=0; i<str.length; i++) {
		if (escape(str.charAt(i)).substring(0, 2) == "%u") {
			len++;
		}
		len++;
		
		if (len >= num) {
			str = str.substring(0,i+1)+lst;
			break;
		}
	}
	return str;
}

var str = "あいうえおabcdeかきくけこ12345";
alert(jcnt(str,13,"..."));