How to add filter: alpha using javascript ?

for opacity i use this code. It's work good.

```
<div id="5">test</div>
<script>
document.getElementById("5").style.opacity = "0.5";
</script>

```

And i tried to use with `filter: alpha` like this but not work, how can i do that ?

for me 

```
<div id="5">test</div>
<script>
document.getElementById("5").style.filter = "alpha(opacity=30)";
</script>

```

but not work.