If your looking for a way to write JavaScript codes all you need is Notepad and the knowledge to save as an HTML document because JavaScript isnt a full fleged Programming Language its just a Scripting language that is implememnted directly into an html docment.
Heres an Example of how JS is implemented into HTML. This code simply creates a special welcome message which is located inside a fieldset for special effects

<fieldset>
<script type="JavaScript">
var name= prompt("What is your name?","Name")
document.write("Hi "+name+"!")
</script>
</fieldset>
The rest of your page goes here
This code would make a pop up that says What is your name? [Name]
Then after you type it in it would write inside a fieldset
Hi Name!