How to get the caret column (not pixels) position in a textarea, in characters, from the start?

How do you get the caret position in a <textarea> using JavaScript?

For example: This is| a text

This should return 7.

How would you get it to return the strings surrounding the cursor / selection?

E.g.: 'This is', '', ' a text'.

If the word “is” is highlighted, then it would return 'This ', 'is', ' a text'.