JS – Scrivere in un campo di testo premendo un bottone

<script>
function addTesto(sml) {
var obj=document.prova.testo;
if (document.all) {
obj.focus();
var sel=document.selection;
var rng=sel.createRange();
rng.colapse;
rng.text=sml;
} else if(obj.selectionEnd){
var lng=obj.textLength;
var from=obj.selectionStart;
var to=obj.selectionEnd;
obj.value=obj.value.substring(0,from)+sml+obj.valu e.substring(to,lng)
} else obj.value+=sml;
obj.focus();
}

</script>
<form name=”prova”>
<textarea name=”testo”></textarea>
<input type=”button” value=”:)” onclick=”addTesto(‘Testo’)” />
</form>