The javascript code:

function blah()
{
    var blahInterval = setInterval(function(){ 
        if( some_global_variable == 5 )
            clearInterval(blahInterval);
    }, 1000);
}

I have this code with multiple variants, and it is going to be called gazillion times in a NodeJS script. Shall I delete blahInterval; after clearInterval?

Is this a Memory leak? Or the GC will clean it once setInerval function goes out of scope?

The GC should clean it when it goes out of scope. As long as the if condition can be false to clear the interval. I would not consider it a memory leak.