[web2py] how to set js variable from session variable?

[weheh]

I imagine someone has asked this but couldn't find it from my searches. 
Anyway, I want to set a javascript variable to the current session variable 
at time of execution of the javascript, not at time when it's loaded. So I 
can't just do:

<script>
    ... blah ...
    function() {
        if ( certain_condition = True ) {
            var myjsvar={{=session.myvar}};
            alert(myjsvar);
        }
    }
    ... blah ...

</script>

because that will not be correct when the function executes.

I tried an ajax call to a controller callback, that in turn returns 
"$('#myjsvar').val('%d');" % session.myvar, based loosely on a thread I 
read here, but that didn't come close to working, either.

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