问Firebase:从实时数据库自动弹出随机元素

EN

function popRandElemAsynch() {

    var choice = null;
  
    db.ref('list').transaction(

        // repeats with updated list until run without collision
        function( list ) {

            // discard previous repetition choice
            choice = null;

            // edge-case of list emptied during transac repeats 
            if (!list)
                return list;
      
            // choose and remember a random element
            var keys = Object.keys(list);
            choice = keys[ keys.length * Math.random() << 0 ];

            // remove the element
            delete list[choice];
            return list;
        },

        // runs once after above has run for final time
        function() {
        
            // choice is the final uniquely popped element
            // if it is null, list was emptied before collision-free pop
            someFunc(choice);
        },

        // don't trigger premature events from transaction retries
        false
    );
}