MongoDb aggregation $match error : "Arguments must be aggregate pipeline operators"

Pipeline stages are separate BSON documents in the array:

games.aggregate([
                { $match: { 'game_user_id' : '12345789' } },
                { $group: {
                    _id: '$id',
                    game_total: { $sum: '$game_amount'}, 
                    game_total_profit: { $sum: '$game_profit'}}
                }}
]).exec(function ( e, d ) {
    console.log( d )            
});

So the Array or [] bracket notation in JavaScript means it expects a "list" to be provided. This means a list of "documents" which are generally specified in JSON notation with {} braces.