[jQuery] How can I return some value in jquery's callback function?

[Freshow]

it didn't work when I 'return true' or 'return false', is there some
way to get it?
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var url='http://localhost/coudou/check/register.php?email=' + str;
        $.getJSON(url, function(res){
                if(res.registed){
                        return true;  // not work
                }else{
                        return false; // not work
                }
        });