Mongoose optional search query parameters?

Build up your query object programmatically:

var query = {
    player: 'player'
};

if (obj.action) {
    query.action = obj.action;
}

Entry.find(query).exec(function(err, res){
    console.log(res);
});

With ES6, you can do this with a splat & ternary like this:

Entry.find({
  player: obj.player,
  ...obj.action ? { action: obj.action } : {}
})
.exec(function(err, res){
    console.log(res);
});