How do I insert a character at the caret with javascript?

Question

I want to insert some special characters at the caret inside textboxes using javascript on a button. How can this be done?

The script needs to find the active textbo

Answer 1

A butchered version of @bmb code in previous answer works well to reposition the cursor at end of inserted characters too:

var lasttext;

function doinsert_ie() {
 var ttInsert = "bla";
 lasttext.focus();
 var sel = document.selection.createRange();
 sel.text = ttInsert;
 sel.select();
}

function doinsert_ff() {
 var oldtext = lasttext.value;
 var curposS = lasttext.selectionStart;
 var curposF = lasttext.selectionEnd;
 pretext = oldtext.substring(0,curposS);
 posttest = oldtext.substring(curposF,oldtext.length);
 var ttInsert='bla';
 lasttext.value = pretext + ttInsert + posttest;
 lasttext.selectionStart=curposS+ttInsert.length;
 lasttext.selectionEnd=curposS+ttInsert.length;
}