HTML5

  void select();
           attribute unsigned long selectionStart;
           attribute unsigned long selectionEnd;
           attribute DOMString selectionDirection;

  void setRangeText(DOMString replacement);
  void setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode);

  void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction = "preserve");

enum SelectionMode {
  'select',
  'start',
  'end',
  'preserve',
};

When these methods and attributes are used with input elements while they don't apply, they must throw an InvalidStateError exception. Otherwise, they must act as described below.

For input elements, these methods and attributes must operate on the element's value. For textarea elements, these methods and attributes must operate on the element's raw value.

Where possible, user interface features for changing the text selection in input and textarea elements must be implemented in terms of the DOM API described in this section, so that, e.g., all the same events fire.

The selections of input and textarea elements have a direction, which is either forward, backward, or none. This direction is set when the user manipulates the selection. The exact meaning of the selection direction depends on the platform.

On Windows, the direction indicates the position of the caret relative to the selection: a forward selection has the caret at the end of the selection and a backward selection has the caret at the start of the selection. Windows has no none direction. On Mac, the direction indicates which end of the selection is affected when the user adjusts the size of the selection using the arrow keys with the Shift modifier: the forward direction means the end of the selection is modified, and the backwards direction means the start of the selection is modified. The none direction is the default on Mac, it indicates that no particular direction has yet been selected. The user sets the direction implicitly when first adjusting the selection, based on which directional arrow key was used.

The select() method must cause the contents of the text field to be fully selected, with the selection direction being none, if the platform support selections with the direction none, or otherwise forward. The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

The selectionStart attribute must, on getting, return the offset (in logical order) to the character that immediately follows the start of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the new value as the first argument; the current value of the selectionEnd attribute as the second argument, unless the current value of the selectionEnd is less than the new value, in which case the second argument must also be the new value; and the current value of the selectionDirection as the third argument.

The selectionEnd attribute must, on getting, return the offset (in logical order) to the character that immediately follows the end of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the new value as the second argument, and the current value of the selectionDirection as the third argument.

The selectionDirection attribute must, on getting, return the string corresponding to the current selection direction: if the direction is forward, "forward"; if the direction is backward, "backward"; and otherwise, "none".

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the current value of the selectionEnd attribute as the second argument, and the new value as the third argument.

The setSelectionRange(start, end, direction) method must set the selection of the text field to the sequence of characters starting with the character at the startth position (in logical order) and ending with the character at the (end-1)th position. Arguments greater than the length of the value of the text field must be treated as pointing at the end of the text field. If end is less than or equal to start then the start of the selection and the end of the selection must both be placed immediately before the character with offset end. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end. The direction of the selection must be set to backward if direction is a case-sensitive match for the string "backward", forward if direction is a case-sensitive match for the string "forward" or if the platform does not support selections with the direction none, and none otherwise (including if the argument is omitted). The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

The setRangeText(replacement, start, end, selectMode) method must run the following steps:

1. If the method has only one argument, then let start and end have the values of the selectionStart attribute and the selectionEnd attribute respectively.

   Otherwise, let start, end have the values of the second and third arguments respectively.

2. If start is greater than end, then throw an IndexSizeError exception and abort these steps.

3. If either start or end are greater than the length of the value of the text field, then set them to the length of the value of the text field.

4. Let selection start be the current value of the selectionStart attribute.

5. Let selection end be the current value of the selectionEnd attribute.

6. If start is less than end, delete the sequence of characters starting with the character at the selection startth position (in logical order) and ending with the character at the (selection end-1)th position.

7. Insert the value of the first argument into the text of the value of the text field, immediately before the startth character.

8. Run the appropriate set of substeps from the following list:

   If the fourth argument's value is "select"

   Let selection start be start.

   Let selection end be end.

   If the fourth argument's value is "start"

   Let selection start and selection end be start.

   If the fourth argument's value is "end"

   Let selection start and selection end be end.

   If the fourth argument's value is "preserve", or if the argument was omitted

   1. Let old length be end minus start.

   2. Let new length be the length of the value of the first argument (the newly inserted text).

   3. Let delta be new length minus old length.

   4. If selection start is greater than end, then increment it by delta. (If delta is negative, i.e. the new text is shorter than the old text, then this will decrease the value of selection start.)

      Otherwise: if selection start is greater than start, then set it to start. (This snaps the start of the selection to the start of the new text if it was in the middle of the text that it replaced.)

   5. If selection end is greater than end, then increment it by delta in the same way.

      Otherwise: if selection end is greater than start, then set it to end. (This snaps the end of the selection to the end of the new text if it was in the middle of the text that it replaced.)

9. Set the selection of the text field to the sequence of characters starting with the character at the selection startth position (in logical order) and ending with the character at the (selection end-1)th position. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end. The direction of the selection must be set to forward if the platform does not support selections with the direction none, and none otherwise.

10. Queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

All elements to which this API applies have either a selection or a text entry cursor position at all times (even for elements that are not being rendered). User agents should follow platform conventions to determine their initial state.

var selectionText = control.value.substring(control.selectionStart, control.selectionEnd);

var oldStart = control.selectionStart;
var oldEnd = control.selectionEnd;
var oldDirection = control.selectionDirection;
var prefix = "http://";
control.value = prefix + control.value;
control.setSelectionRange(oldStart + prefix.length, oldEnd + prefix.length, oldDirection);