/**
 * 动态限定用户只能在field里输入maxLength个字符
 * 
 * @param field		a textarea element, specially comments textarea
 * @param maxLength		max number of characters allowed 
 * @return	
 */
function limitCommentLength(field, maxLength) {
	if(field.value.length > maxLength) {
		field.value = field.value.substring(0, maxLength);
	}
}

/**
 * 用于限定课程详细页的评论输入长度
 */
function limitCommentForm() {
	var textareaElem = document.commentForm.elements['comment.content'];
	limitCommentLength(textareaElem, 256);
}