document.write writes html.
"string" can be a variable or a literal.
Multiple strings to write can be handled by one document.write statement, the strings separated by commas or plus signs.
Code:
<script type="text/javascript">
//<![CDATA[
document.write("This is a literal", "<br><br>");
firstName = "John";
lastName = "Smith";
document.write("This uses commas for concatenation: ", firstName, " ", lastName, "<br>");
document.write("This uses plus signs for concatenation: " + firstName + " " + lastName + "<br>");
//]]>
</script>
Output: