jQuery实现在textarea指定位置插入字符或表情的方法

(function($){
    $.fn.extend({
        insertAtCaret: function(myValue){
            var $t=$(this)[0];
            if (document.selection) {
                this.focus();
                sel = document.selection.createRange();
                sel.text = myValue;
                this.focus();
            }
            else
                if ($t.selectionStart || $t.selectionStart == '0') {
                    var startPos = $t.selectionStart;
                    var endPos = $t.selectionEnd;
                    var scrollTop = $t.scrollTop;
                    $t.value = $t.value.substring(0, startPos) + myValue + $t.value.substring(endPos, $t.value.length);
                    this.focus();
                    $t.selectionStart = startPos + myValue.length;
                    $t.selectionEnd = startPos + myValue.length;
                    $t.scrollTop = scrollTop;
                }
                else {
                    this.value += myValue;
                    this.focus();
                }
        }
    }) 
})(jQuery);

$("#textareaId").insertAtCaret("新表情");