JavaScript让Textarea支持tab按键的方法

HTMLTextAreaElement.prototype.getCaretPosition = function () {//return the caret position of the textarea return this.selectionStart;};HTMLTextAreaElement.prototype.setCaretPosition = function (position) {//change the caret position of the textarea this.selectionStart = position; this.selectionEnd = position; this.focus();};HTMLTextAreaElement.prototype.hasSelection = function () {//if the textarea has selection then return true if (this.selectionStart == this.selectionEnd) {  return false; } else {  return true; }};HTMLTextAreaElement.prototype.getSelectedText = function () {//return the selection text return this.value.substring(this.selectionStart, this.selectionEnd);};HTMLTextAreaElement.prototype.setSelection = function (start, end) {//change the selection area of the textarea this.selectionStart = start; this.selectionEnd = end; this.focus();};var textarea = document.getElementsByTagName('textarea')[0]; textarea.onkeydown = function(event) { //support tab on textarea if (event.keyCode == 9) { //tab was pressed  var newCaretPosition;  newCaretPosition = textarea.getCaretPosition() + " ".length;  textarea.value = textarea.value.substring(0, textarea.getCaretPosition()) + " " + textarea.value.substring(textarea.getCaretPosition(), textarea.value.length);  textarea.setCaretPosition(newCaretPosition);  return false; } if(event.keyCode == 8){ //backspace  if (textarea.value.substring(textarea.getCaretPosition() - 4, textarea.getCaretPosition()) == " ") {  //it's a tab space   var newCaretPosition;   newCaretPosition = textarea.getCaretPosition() - 3;   textarea.value = textarea.value.substring(0, textarea.getCaretPosition() - 3) + textarea.value.substring(textarea.getCaretPosition(), textarea.value.length);   textarea.setCaretPosition(newCaretPosition);  } } if(event.keyCode == 37){ //left arrow  var newCaretPosition;  if (textarea.value.substring(textarea.getCaretPosition() - 4, textarea.getCaretPosition()) == " ") {  //it's a tab space   newCaretPosition = textarea.getCaretPosition() - 3;   textarea.setCaretPosition(newCaretPosition);  }  } if(event.keyCode == 39){ //right arrow  var newCaretPosition;  if (textarea.value.substring(textarea.getCaretPosition() + 4, textarea.getCaretPosition()) == " ") {  //it's a tab space   newCaretPosition = textarea.getCaretPosition() + 3;   textarea.setCaretPosition(newCaretPosition);  } } }