用Jquery让textarea支持自动缩进

这个代码的作用是让textarea也能输入tab，并可以像代码编辑器一样能自动缩进。

废话不多说了，直接上Demo：

https://codepen.io/Wu23333/pen/MWwYmmK

var textarea_id = "text";
 (function($, undefined) {
         $.fn.getCursorPosition = function() {
         var el = $(this).get(0);
         var pos = 0;
         if ('selectionStart' in el) {
             pos = el.selectionStart;
         }else
             if ('selection' in document) {
                 el.focus();
                 var Sel = document.selection.createRange();
                 var SelLength = document.selection.createRange().text.length;
                 Sel.moveStart('character', -el.value.length);
                 pos = Sel.text.length - SelLength;
             }
         return pos;
     }
 })(jQuery);
 $(document).keydown(function(e){
     text=document.getElementById(textarea_id);
     if(e.which == 9){
         var cursor_pos = $(text).getCursorPosition();
         $(text).val($(text).val().slice(0,cursor_pos) + '\t' + $(text).val().slice(cursor_pos));
         text.focus();
         text.setSelectionRange(cursor_pos+1, cursor_pos+1);
         return false;
     }
     if(e.which == 13){
         var cursor_pos = $(text).getCursorPosition();
         var notelines = $(text).val().slice(0,cursor_pos).split('\n');
         var listline = notelines[notelines.length-1];
         var n = 0;
         while(listline[n] == '\t'){
             n+=1;
         }
         $(text).val($(text).val().slice(0, cursor_pos)+'\n'+$(text).val().slice(cursor_pos));
         for (i=n; i>0; i--){
             $(text).val($(text).val().slice(0, cursor_pos+1)+'\t'+$(text).val().slice(cursor_pos+1));
         }
         text.focus();
         text.setSelectionRange(cursor_pos + n+1, cursor_pos + n+1);
         return false;
     }
 });