JavaScript method to make Textarea support tab key_javascript skills

The example in this article describes how JavaScript allows Textarea to support the tab button. Share it with everyone for your reference. The specific implementation method is as follows:

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);
  }
 } 
}

I hope this article will be helpful to everyone’s JavaScript programming design.