递归回调

Question

所以我试着编写一个获取params的函数并调用DynamoDB的scan方法，该方法返回分页结果，并且我将limit作为输入从params开始，所以我不想递归调用我的扫描函数来获取所有数据，而是希望获得尽可能多的limit数据作为参数。我尝试了以下代码块，但似乎我的逻辑不正确！你能帮我解决一下我做错的事吗？

function scan(params, total, callback) {
    dynamo.scan(params, function(err, data) {
        if (err) {
            callback(err, data);
        } else {
            if (((!params.Limit || (params.Limit && total.length < params.Limit)))
                    && data.Items && data.LastEvaluatedKey) {
                params.ExclusiveStartKey = data.LastEvaluatedKey;
                (function(data) {
                    scan(params, data.Items.concat(data.Items), function(err, result) {
                        data.Items = data.Items.concat(result.Items);
                        if (result.LastEvaluatedKey)
                            data.LastEvaluatedKey = result.LastEvaluatedKey;
                        callback(err, data);
                    });
                } (data));
            } else {
                callback(err, total); //Here it says undefined is not a function
            }
        }
    });
}

通过以下代码致电：

//params is stuffed with needed parameters for scan method of DynamoDB
scan(params,[],function(err, data){
    //Processing data here
})

Answer 1

以下是我编写的一些示例代码，请告诉我它是否适合您的用例，或者您是否有任何问题。

＆＃13;

＆＃13;

/**
  * This method will scan continue to scan until
  *     1) There are no more items (LastEvaluatedKey is null)
  *     2) We have retrieved N items from paginated scan
  * If we find more than N, we take the first N items.
  **/
function scanWithItemLimit(params, n, callback, items) {
    db.scan(params, function(err, data) {
        if (err) {
            callback(err);
        } else {
            var leftover = n - data.Items.length;
            var itemsCopy = items ? items : [];
            if (leftover > 0) {
                itemsCopy = itemsCopy.concat(data.Items);
                var paramsCopy = params;
                paramsCopy.ExclusiveStartKey = data.LastEvaluatedKey;
                if (params.ExclusiveStartKey) {
                    scanWithItemLimit(paramsCopy, leftover, callback, itemsCopy);
                } else {
                    callback(err, itemsCopy);
                }
            } else {
                if (n != 0) {
                    for (var i = 0; i < n; i++) {
                        itemsCopy.push(data.Items[i]);
                    }
                }
                callback(err, itemsCopy);
            }
        }
    });
}

var pfunc = function(err, items) { console.log(err, items); };
params = {TableName: "Table"};
scanWithItemLimit(params, 5, pfunc);

＆＃13;

＆＃13;

＆＃13;