window.onload = function(){
var sample = document.getElementById("test");
sample.value = "damedayo";
}
~~~~
<textarea id = "test"></textarea>
<!DOCTYPE html>
<html>
<head>
<script>
function test() {
var text = document.getElementById("test").selectionStart;
alert(text);
}
</script>
</head>
<body>
<input type="button" value="押して" onclick="test();"><br>
<textarea id="test">テキストエリアです。</textarea>
</body>
</html>
