如何处理 在textarea中？

$("textarea").keydown(function(e) {    if(e.keyCode === 9) { // tab was pressed        // get caret position/selection        var start = this.selectionStart;        var end = this.selectionEnd;        var $this = $(this);        var value = $this.val();        // set textarea value to: text before caret + tab + text after caret        $this.val(value.substring(0, start)         + "t"         + value.substring(end));        // put caret at right position again (add one for the tab)        this.selectionStart = this.selectionEnd = start + 1;        // prevent the focus lose        e.preventDefault();    }});