Javascript statements are terminated by a semicolon. Or you can omit the semicolon and let the parser determine where the end of a statement is.
Including semicolons helps prevent errors.
The first line of code has no semicolon. It leaves the parser to decide.
var sum = a + b //valid even without a semicolon - not recommended var diff = a - b; //valid - preferred
<!DOCTYPE HTML>
<html>
<body>
<script type="text/javascript">
document.writeln("This is a statement");
document.writeln("This is also a statement");
</script>
</body>
</html>