Javascript doesn’t really have the notion of a named constant, or an immutable property of an object. (Note that I’m not talking about ES5 here.)
You can declare globals with a simple var declaration in the global scope, like outside any function in a script included by a web page:
<script>
var EXACTLY_ONE = 1;
Then your code can use that constant of course, though it’s not really “constant” because the value can be changed (the property updated, in other words).
edit — this is an ancient answer to an ancient question. In 2019, there’s the const declaration that’s supported just about everywhere. However note that like let, const scoping is different from var scoping.