This file:
<html><script>
document.write("Hello world!<br>")
llength=5
document.write ("the length is: ",llength,"<br>")
width=3
document.write ("the width is: ",width,"<br>")
area=llength*width
document.write ("the area is: ",area,"<br>")
</script></html>

will generate this output if you open it in a browser:

More JS