wait for a node callback function

Question

     var existingTokenValidated = new Boolean(false);

if($.cookie("sessionToken") != undefined)
{
    ValidateToken($.cookie("sessionToken"),function(result){ //wait here
        if(result == 'true')
        {
            existingTokenValidated = true;      
        }
        
    });
}

if(!existingTokenValidated)
{
    GetLoginToken(function(result)
    {
        if(result == 'true' && $.cookie("sessionToken") != undefined){
            callback('true');
        }
        else if(result == 'false'){
            callback('false');
        }
    });     
}
else{
        callback('true');
}

I am i beginner to nodes js, want to wait for Validate token to finish as it does in synchronous function call. Currently for a solution i am calling GetLoginTOken twice in my code want to do in the above way.