TextChanged event is not triggered when I am typing inside a Multiline textbox

I have a multi-line textbox CommentsTxtBox. I want to limit the number of letters inside this textbox to only 400 letters. I put the following code to show the number of left letters that can be written into another textbox that shown as the user is writing the text. The problem is that the event is not triggered:
Protected Sub CommentsTxtBox_TextChanged(sender As Object, e As System.EventArgs) Handles CommentsTxtBox.TextChanged CounterTextBox.Text = CommentsLenght - Len(CommentsTxtBox.Text) End Sub So I need to write a client side javascript. I want to assign this javascript to the CommentsTxtBox. Whenevent there is a keyUp or KeyDown I want to show the length of the text inside the CommentsTxtBox to  another textbox countertextbox.